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Author Topic: Test Equipment: Oscillocopes  (Read 67845 times)

Offline MarkE

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Re: Test Equipment: Oscillocopes
« Reply #90 on: February 22, 2015, 06:37:50 AM »


Actually the parts list in the manual I have identifies all the tantalums and other electrolytics just as "electrolytic". The page numbers I'm citing are the pages of the .pdf file that I downloaded from the electrotanya link I provided earlier. It's a real pain that the manual itself doesn't have page numbers. Perhaps the easiest thing would be for you to download that manual too so we can refer to page numbers-- I obviously don't have the paper copy like you do. You have the advantage of seeing the good drawings of the boards, the ones in the .pdf file kind of suck. But it is nice to be able to refer to page numbers--- you could look in the .pdf file at the page numbers I cite, and then find the correct page in your paper copy.
The box referring to the tantalum decoupling caps is in the Power Supply troubleshooting flowchart for the low voltage (15, -8, 5 v) supplies. The schematic is the "power supply" tab , marked on the tab #12 in a diamond box. And the box referring to the tantalum decoupling caps is on the tab marked "+15V, +5V and -8V supplies Troubleshooting"
Good on the iron. I'd check the voltage at the testpoint first, (read the box on the troubleshooting chart about how they behave as the scope warms up), then I'd replace the caps if you have the right capacitance values. You can use higher voltage ratings of course, if there is room physically. You may even be able to use other capacitance values as long as they are close: the originals are 20 percent tolerance, really cheap tantalums.
I'd say, let the scope cool completely, then power it up, test the  +5V TP1558 with the DMM right away, then let it warm up to the point where the glitching gets bad and test the TP again to see if there is any difference. Then, regardless of what you see, shut down and replace the caps if you've got suitable replacements. Then repeat the process starting with a cold scope again.

Another thing you might like to try, since the transistors are mostly all in sockets, is to wiggle them or tap on them with a wooden or plastic tapper and see if the display responds in any way. If you find one that does cause a change in the display when you tap on it, shut down the scope, carefully pull the transistor and re-seat it, maybe the it wasn't making a good connection and reseating will fix that sometimes. Lather rinse repeat.
Within reason you can generally go larger with bulk power supply capacitors without a problem.  A 47uF or even 68uF capacitor can be used in place of a 33uF in almost all situations.  What you can't safely do is use a lower voltage rated capacitor.

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Re: Test Equipment: Oscillocopes
« Reply #90 on: February 22, 2015, 06:37:50 AM »

Offline picowatt

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Re: Test Equipment: Oscillocopes
« Reply #91 on: February 22, 2015, 07:38:03 AM »
Just some food for thought.

Whenever troubleshooting any piece of electronic gear, it is always wise to measure the rails first thing, and as expeditiously as possible.  I often pre-attach my scope/voltmeter to supply test points using spring hooks and with the unit's power off and then power the unit on just long enough to get a stable reading.  Power off and then repeat for the next supply test point.  If a supply has gone overvoltage, or is not regulating current, this method may minimize damage.  With regard to the current topic, I agree with TK... measure the rails!  Although tantalums often fail just because that's what they like to do (particularly older gen tants), tantalums do not like overvoltage, or even more so, application of reverse polarity.  So check the rails first thing.     

I just had one of my SG505's in a TM500 rack develop a noise.  Tore it apart and probed the supplies first thing and noted one rail was noisey.  The rail used a 33uf 20V tantalum.  It was one of the old early tants, a big honkin' thing the size of a large peanut M&M with horizontal color stripes (in familiar M&M colors).  There was what looked like a tear dropped shaped solder blob stuck to the side of the tant.  Upon closer inspection, a very small hole had formed in the side of the tant's "candy coating" and the apparently molten at the time metal blob had oozed thru the almost microscopic hole and solidified.  A very light and unfamiliar metal was that blob.  Anyone who has ever worked on electronic equipment that uses tants will have many stories to tell regarding them.  Definitely a love hate thing.

I ordered new tants to recap all my SG505's, but in order to put this unit back in service, I temporarily replaced the tant with a 47uf 35V electrolytic (1/4 the size of the tant) and then tacked a .1uf ceramic to the back of the board in parallel with the electrolytic.  This parallel combination will work well in most applications that use a tant, and some of the newer electrolytics approach or exceed the HF performance of early gen tantalums.  A functioning electrolytic will always perform better than a non-functioning tantalum!

Also, regarding probes.  Yes indeed, expensive probes are nice and sometimes necessary, and I agree with everything MarkE has said about them.  However, when repairing a piece of equipment where one might have to get a bit creative just to get a probe hook attached to a component lead or other measurement point that is not easy to get to, I use the cheap probes.  Most of the time you are looking for a DC level, noise, or just the presence of a specific signal, and the cheap probes are fine for this.  If a probe hook gets bent or broken from somewhat abusive probing, well, it was just a cheap probe, no worries.

Now, if you are looking at a fast rise time digital signal, switching supply, something you cant load, a circuit that is screaming with an unwanted HF oscillation, or you need absolute measurement precision, that is the time to break out the expensive or active probes.  And since you did not use them as your "daily driver", they are always in great shape (and of course you will slow down and drive more carefully when using the expensive probes).

Anyway, like I said, just food for thought...

PW

Offline MarkE

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Re: Test Equipment: Oscillocopes
« Reply #92 on: February 22, 2015, 08:29:14 AM »
Just some food for thought.

Whenever troubleshooting any piece of electronic gear, it is always wise to measure the rails first thing, and as expeditiously as possible.  I often pre-attach my scope/voltmeter to supply test points using spring hooks and with the unit's power off and then power the unit on just long enough to get a stable reading.  Power off and then repeat for the next supply test point.  If a supply has gone overvoltage, or is not regulating current, this method may minimize damage.  With regard to the current topic, I agree with TK... measure the rails!  Although tantalums often fail just because that's what they like to do (particularly older gen tants), tantalums do not like overvoltage, or even more so, application of reverse polarity.  So check the rails first thing.     

I just had one of my SG505's in a TM500 rack develop a noise.  Tore it apart and probed the supplies first thing and noted one rail was noisey.  The rail used a 33uf 20V tantalum.  It was one of the old early tants, a big honkin' thing the size of a large peanut M&M with horizontal color stripes (in familiar M&M colors).  There was what looked like a tear dropped shaped solder blob stuck to the side of the tant.  Upon closer inspection, a very small hole had formed in the side of the tant's "candy coating" and the apparently molten at the time metal blob had oozed thru the almost microscopic hole and solidified.  A very light and unfamiliar metal was that blob.  Anyone who has ever worked on electronic equipment that uses tants will have many stories to tell regarding them.  Definitely a love hate thing.

I ordered new tants to recap all my SG505's, but in order to put this unit back in service, I temporarily replaced the tant with a 47uf 35V electrolytic (1/4 the size of the tant) and then tacked a .1uf ceramic to the back of the board in parallel with the electrolytic.  This parallel combination will work well in most applications that use a tant, and some of the newer electrolytics approach or exceed the HF performance of early gen tantalums.  A functioning electrolytic will always perform better than a non-functioning tantalum!

Also, regarding probes.  Yes indeed, expensive probes are nice and sometimes necessary, and I agree with everything MarkE has said about them.  However, when repairing a piece of equipment where one might have to get a bit creative just to get a probe hook attached to a component lead or other measurement point that is not easy to get to, I use the cheap probes.  Most of the time you are looking for a DC level, noise, or just the presence of a specific signal, and the cheap probes are fine for this.  If a probe hook gets bent or broken from somewhat abusive probing, well, it was just a cheap probe, no worries.

Now, if you are looking at a fast rise time digital signal, switching supply, something you cant load, a circuit that is screaming with an unwanted HF oscillation, or you need absolute measurement precision, that is the time to break out the expensive or active probes.  And since you did not use them as your "daily driver", they are always in great shape (and of course you will slow down and drive more carefully when using the expensive probes).

Anyway, like I said, just food for thought...

PW
One thing to be a bit careful about is that there are a lot of circuits that rely on the ESR of the capacitors for stability.  A really low ESR may break a regulator circuit causing it to oscillate.  Those old "candy wrapper" tantalums tended to have ESRs in the 2-10 Ohm range. 

I like the robustness of passive probes.  A colleague of mine set me up with some really sweet passive probes that I can clip almost anywhere and they perform very politely.  The oscilloscope trace below was snapped with one hooked to a 5V < 1ns risetime CMOS logic circuit.

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Re: Test Equipment: Oscillocopes
« Reply #92 on: February 22, 2015, 08:29:14 AM »
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Offline picowatt

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Re: Test Equipment: Oscillocopes
« Reply #93 on: February 22, 2015, 08:35:53 AM »
One thing to be a bit careful about is that there are a lot of circuits that rely on the ESR of the capacitors for stability.  A really low ESR may break a regulator circuit causing it to oscillate.  Those old "candy wrapper" tantalums tended to have ESRs in the 2-10 Ohm range. 

I like the robustness of passive probes.  A colleague of mine set me up with some really sweet passive probes that I can clip almost anywhere and they perform very politely.  The oscilloscope trace below was snapped with one hooked to a 5V < 1ns risetime CMOS logic circuit.

MarkE,

Yes, but the tants claim to fame when they first arrived was their low inductance/HF performance.  A "one stop shop" for decoupling applications.

What model probe?

PW


Offline MarkE

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Re: Test Equipment: Oscillocopes
« Reply #94 on: February 22, 2015, 09:56:55 AM »
They didn't quite live up to that claim.

My colleague custom made the probes he gave me.  They are quite handy.

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Re: Test Equipment: Oscillocopes
« Reply #94 on: February 22, 2015, 09:56:55 AM »
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Offline Brian516

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Re: Test Equipment: Oscillocopes
« Reply #95 on: February 22, 2015, 04:48:48 PM »
Actually the parts list in the manual I have identifies all the tantalums and other electrolytics just as "electrolytic". The page numbers I'm citing are the pages of the .pdf file that I downloaded from the electrotanya link I provided earlier. It's a real pain that the manual itself doesn't have page numbers. Perhaps the easiest thing would be for you to download that manual too so we can refer to page numbers-- I obviously don't have the paper copy like you do. You have the advantage of seeing the good drawings of the boards, the ones in the .pdf file kind of suck. But it is nice to be able to refer to page numbers--- you could look in the .pdf file at the page numbers I cite, and then find the correct page in your paper copy.
The box referring to the tantalum decoupling caps is in the Power Supply troubleshooting flowchart for the low voltage (15, -8, 5 v) supplies. The schematic is the "power supply" tab , marked on the tab #12 in a diamond box. And the box referring to the tantalum decoupling caps is on the tab marked "+15V, +5V and -8V supplies Troubleshooting"

I downloaded that manual so now we are able to easily toss page numbers around.  If you ever need a higher quality image of anything out of that book, or any of the materials that I have, let me know and I will scan it and put it up on my photobucket.

Quote
Good on the iron. I'd check the voltage at the testpoint first, (read the box on the troubleshooting chart about how they behave as the scope warms up), then I'd replace the caps if you have the right capacitance values. You can use higher voltage ratings of course, if there is room physically. You may even be able to use other capacitance values as long as they are close: the originals are 20 percent tolerance, really cheap tantalums.
I'd say, let the scope cool completely, then power it up, test the  +5V TP1558 with the DMM right away, then let it warm up to the point where the glitching gets bad and test the TP again to see if there is any difference. Then, regardless of what you see, shut down and replace the caps if you've got suitable replacements. Then repeat the process starting with a cold scope again.

All the tantalums I have are in the nF/pF range. The only larger ones I have are molded and are way too big.  I have a huge variety of electrolytics, and also ceramic disc caps.  I could go the temp route of what picowatt was saying, but I'd rather do replacements with exact value/type replacements.  I am going to go thru first off and do what picowatt suggested with pre attaching the dmm to the rail test points and quickly power on to see first of all if the voltages are correct.  If they are, I will let it warm up and test TP1558. again.

Quote
Another thing you might like to try, since the transistors are mostly all in sockets, is to wiggle them or tap on them with a wooden or plastic tapper and see if the display responds in any way. If you find one that does cause a change in the display when you tap on it, shut down the scope, carefully pull the transistor and re-seat it, maybe the it wasn't making a good connection and reseating will fix that sometimes. Lather rinse repeat.

As I was cleaning all the boards, I did exactly that (wiggling and making sure they are fully seated) to all the transistors/fets/round IC's. maybe one of them does still have a bad connection still.   what do you think of the idea of putting a wee bit of dielectric "grease" on the leads? I've used it in a ton of automotive connections and even some household connections such as breakers to ensure a good connection and no corrosion.  I'm  sure you already know that it's not conductive except when it's a very thin layer between contacts.  I already did this with all of the coax connections that I messed with.. the leads off of them had a little bit of a blackened layer formed around them where the connections weren't full contact, so I cleaned them off a little with a light rub with 1000grit sandpaper and a med bristle toothbrush.


Also, I managed to find another bad capacitor.  this time it was under the HV cover of the IC and is a multilayer ceramic. 
C1476 of A9 CRT circuit.  110V regulated. also one that goes to ground.... .1uf 200V +80-20%.
None of my component sites have this exact cap, and ebay doesn't even have them. 

http://i1081.photobucket.com/albums/j347/Brian_Bloom/CAM00167_zpsfvycu68n.jpg
http://i1081.photobucket.com/albums/j347/Brian_Bloom/CAM00166_zpss6er2akh.jpg
(It may be a little hard to tell, but the lead with the "scratch" looking mark above it appears to be split apart in a diagonal manner.  On the side that I can't get the cam in to show, there is a chunk taken out of the lead which looks like it was caused by an arc.  Possible that the cap started to go bad, heated, split the lead, and then the tiny gap created in the lead caused an arc which melted the lead?   If so, this could very well be the source of my perpetual noise, right?  It's in the CRT circuit, and the arcing in the gap would cause a visual noise on screen.)

Here are my options:
(sorry, had to break up the links so they wouldn't expand the page horizontally...)
http://www.digikey.com/product-search/en?pv14=14&
FV=fff40002%2Cfff8000b%2Cc000c%2C340025%2C340026%2C340027%2C340028%2C340029%2C34002b%2C34002c%2C34002d%2C34013c%2C340168%2C34
01a8%2C3401aa%2C3401ac%2C3402c0%2C340364%2C380002%2C38000e%2C1140050&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&
quantity=0&ptm=0&fid=0&pageSize=25

http://www.mouser.com/Passive-Components/Capacitors/Ceramic-Capacitors
/_/N-5g8mZscv7?P=1z0wqusZ1z0wpxxZ1z0wpygZ1z0wq52Z1z0wplvZ1z0wps8Z1z0wplsZ1z0wqu1Z1z0wl0jZ1z0wqtpZ1z0wqg1Z1z0wqtyZ1z0wq34Z1z0wk75Z1yzut9
8Z1z0wxf7Z1z0vvr9Z1z0x8e0Z1z0x8eaZ1z0wx3tZ1z0x86hZ1z0wxoyZ1z0x810Z1z0wdmxZ1yyeqdfZ1z0wd6qZ1yz7fw1Z1z0wdl4Z1z0x86rZ1z0x43wZ1z0wx10Z1z0x8g
xZ1yz7f70Z1z0wx8wZ1z0wf8bZ1z0wl3nZ1yx7yinZ1z0wxeoZ1yyy7xyZ1yzml5fZ1z0x8elZ1z0wxey

http://www.allelectronics.com/make-a-store/item/rm-104c/0.1uf-500v-radial-multi-layer-ceramic-capacitor/1.html

Anyone else know of any possible reliable sites?  Onlinecomponents doesn't have any caps that are close to equivalent.

The second one on Mouser is out of the question.  $23 for a single cap...   and the one on Allelectronics isn't the right tolerance. it's 20%, not -20+80%.
This replacement part is going to dictate where I end up placing my order from.  All of these sites have the two values of tantalums that I need.
If there are any other specific areas of the scope that I should pick up some replacement caps or transistors for, let me know. I already plan to get a few extra tantalums just to have. I'm going to see what the most common values of those this scope uses and pick up some to have.

MarkE - seems like that guy made you some great probes!  Thanks for the pointers.

picowatt - thanks for the input.  I'll definitely try testing the rails with that method first.  Wouldn't wanna burn up even more components if I do have an overvoltage issue somewhere!

Brian
p.s. Sorry I passed out early on you guys last night.

Offline TinselKoala

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Re: Test Equipment: Oscillocopes
« Reply #96 on: February 22, 2015, 05:57:02 PM »
I downloaded that manual so now we are able to easily toss page numbers around.  If you ever need a higher quality image of anything out of that book, or any of the materials that I have, let me know and I will scan it and put it up on my photobucket.
Good, thanks. It would be nice to have some good images of the board layouts. As you can see, the ones in the pdf really are bad and hard to read.
Quote

All the tantalums I have are in the nF/pF range.

That seems odd. They must be very tiny. I think the smallest ones I've got are 100 nF. A 68 uF 16V one is about the size of a large pea. This is the great advantage of tantalums, in addition to their fast response: they are really physically small for the capacitance value.

Quote
The only larger ones I have are molded and are way too big.  I have a huge variety of electrolytics, and also ceramic disc caps.  I could go the temp route of what picowatt was saying, but I'd rather do replacements with exact value/type replacements.  I am going to go thru first off and do what picowatt suggested with pre attaching the dmm to the rail test points and quickly power on to see first of all if the voltages are correct.  If they are, I will let it warm up and test TP1558. again.
Good. You could also test the other power supply test points while you are at it.
Quote
As I was cleaning all the boards, I did exactly that (wiggling and making sure they are fully seated) to all the transistors/fets/round IC's. maybe one of them does still have a bad connection still.   what do you think of the idea of putting a wee bit of dielectric "grease" on the leads? I've used it in a ton of automotive connections and even some household connections such as breakers to ensure a good connection and no corrosion.  I'm  sure you already know that it's not conductive except when it's a very thin layer between contacts.  I already did this with all of the coax connections that I messed with.. the leads off of them had a little bit of a blackened layer formed around them where the connections weren't full contact, so I cleaned them off a little with a light rub with 1000grit sandpaper and a med bristle toothbrush.
The very idea gives me the willies. I would definitely _not_ recommend that you put any kind of compound on anything. Clean it off the coax connectors!
The black oxide layer is silver oxide forming on the silver plating of the connectors. Cleaning carefully can remove this, but you don't want to remove the silver plating by being too aggressive with sandpaper!
Quote


Also, I managed to find another bad capacitor.  this time it was under the HV cover of the IC and is a multilayer ceramic. 
C1476 of A9 CRT circuit.  110V regulated. also one that goes to ground.... .1uf 200V +80-20%.
None of my component sites have this exact cap, and ebay doesn't even have them. 

http://i1081.photobucket.com/albums/j347/Brian_Bloom/CAM00167_zpsfvycu68n.jpg
http://i1081.photobucket.com/albums/j347/Brian_Bloom/CAM00166_zpss6er2akh.jpg
(It may be a little hard to tell, but the lead with the "scratch" looking mark above it appears to be split apart in a diagonal manner.  On the side that I can't get the cam in to show, there is a chunk taken out of the lead which looks like it was caused by an arc.  Possible that the cap started to go bad, heated, split the lead, and then the tiny gap created in the lead caused an arc which melted the lead?   If so, this could very well be the source of my perpetual noise, right?  It's in the CRT circuit, and the arcing in the gap would cause a visual noise on screen.)
I have been thinking all along that the regularity of the glitch and the steady rate no matter the scope settings seemed like an arcing problem. You may have found the trouble.
Quote
Here are my options:
(sorry, had to break up the links so they wouldn't expand the page horizontally...)
http://www.digikey.com/product-search/en?pv14=14&
FV=fff40002%2Cfff8000b%2Cc000c%2C340025%2C340026%2C340027%2C340028%2C340029%2C34002b%2C34002c%2C34002d%2C34013c%2C340168%2C34
01a8%2C3401aa%2C3401ac%2C3402c0%2C340364%2C380002%2C38000e%2C1140050&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&
quantity=0&ptm=0&fid=0&pageSize=25

http://www.mouser.com/Passive-Components/Capacitors/Ceramic-Capacitors
/_/N-5g8mZscv7?P=1z0wqusZ1z0wpxxZ1z0wpygZ1z0wq52Z1z0wplvZ1z0wps8Z1z0wplsZ1z0wqu1Z1z0wl0jZ1z0wqtpZ1z0wqg1Z1z0wqtyZ1z0wq34Z1z0wk75Z1yzut9
8Z1z0wxf7Z1z0vvr9Z1z0x8e0Z1z0x8eaZ1z0wx3tZ1z0x86hZ1z0wxoyZ1z0x810Z1z0wdmxZ1yyeqdfZ1z0wd6qZ1yz7fw1Z1z0wdl4Z1z0x86rZ1z0x43wZ1z0wx10Z1z0x8g
xZ1yz7f70Z1z0wx8wZ1z0wf8bZ1z0wl3nZ1yx7yinZ1z0wxeoZ1yyy7xyZ1yzml5fZ1z0x8elZ1z0wxey

http://www.allelectronics.com/make-a-store/item/rm-104c/0.1uf-500v-radial-multi-layer-ceramic-capacitor/1.html

Anyone else know of any possible reliable sites?  Onlinecomponents doesn't have any caps that are close to equivalent.

The second one on Mouser is out of the question.  $23 for a single cap...   and the one on Allelectronics isn't the right tolerance. it's 20%, not -20+80%.
This replacement part is going to dictate where I end up placing my order from.  All of these sites have the two values of tantalums that I need.
The one from Allelectronics should be fine. Don't worry about the tolerance, you are now dealing with modern components that will be much closer to the printed value than those ancient ones in the scope. C1476 appears in the schematic on page 228 of the .pdf SM and appears to be an ordinary decoupling cap for the Emitter of Q1478. This should be a "nothing special" capacitor and the one from Allelectronics will work perfectly here.
The Q1478 transistor is a PNP silicon, and 2n5401 is the generic cross for that one, should you need to replace it. This is a HV silicon amplifier and the NTE equivalent is NTE288. Cheap and easy to find if you need to replace it. Check that associated resistor R1476 also for off-value.
http://www.reprise.com/host/tektronix/reference/find_transistor.asp
Quote
If there are any other specific areas of the scope that I should pick up some replacement caps or transistors for, let me know. I already plan to get a few extra tantalums just to have. I'm going to see what the most common values of those this scope uses and pick up some to have.

MarkE - seems like that guy made you some great probes!  Thanks for the pointers.

picowatt - thanks for the input.  I'll definitely try testing the rails with that method first.  Wouldn't wanna burn up even more components if I do have an overvoltage issue somewhere!

Brian
p.s. Sorry I passed out early on you guys last night.

This is where a Variac can come in handy. For general troubleshooting reference: if you have a device that is blowing the main fuse, for example, you remove the fuse and put an ammeter across the fuse contacts, then use the Variac to slowly run up the line input voltage from zero to your local mains voltage. If the current draw indicated on the ammeter approaches 90 percent of the fuse value.. STOP!
If you suspect a severe power supply fault, you can do the same thing, while monitoring the suspect power supply with a voltmeter. Again, if the output voltage of the monitored supply begins to look like it is going to exceed the spec value + tolerance, then STOP increasing input voltage from the Variac.

However I don't think your problem fits in this category. The scope seems to be working, basically, at least so far. Once these suspect caps are replaced we can go further into the various features and functions to see if there is anything else wrong.

I never did see what the CH2 trace from the calibrator looks like. I'm presuming it is also mostly OK but with the same glitching as you've shown on the CH1 trace.

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Re: Test Equipment: Oscillocopes
« Reply #96 on: February 22, 2015, 05:57:02 PM »
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Offline picowatt

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Re: Test Equipment: Oscillocopes
« Reply #97 on: February 22, 2015, 06:44:42 PM »
Brian,

I have not been following this thread much, and I don't even know what model 'scope you are working on.

That said, I looked at the jpg's you posted last of a few caps and saw none that were visually concerning.  The tant with the crack at the base/side is not necessarily an indication that it is bad.  During manufacture, one lead is attached to the bottom of a pellet and the other lead runs up the side to the top of the pellet and then the whole thing is dipped.  What I see in the image posted in your last post is a tant with the lead going up the side slightly separated near the base with the encapsulation slightly cracked.  If it is not shorted (which you can test with an ohmmeter) keep it in mind but move on for now.

The next jpg I looked at was of an orangish brown cap that looked more like a poly cap than a multilayer ceramic.  Unless you have a manual that specifies it as a multilayer ceramic, I would assume it is a poly cap.  Again, I see no visual evidence that would lead me to believe the cap is bad.  There is a bit of cracking of the encapsulation around one lead, but this is not uncommon nor an indication that it is bad.  In another image, I saw another poly looking cap with a slight scratch near one lead that again did not indicate the cap was bad.  I suspect the scratch was from someone probing the lead of that cap that might have scratched it with a probe tip (previous repair attempt?).

Keep in mind that although resistors typically look bad when they have been thermally stressed (except for the flameproof variety) with signs of heat damage to the resistor, PCB, etc, failed caps often show no outwardly visual clues as to their status (other than the rather "dramatic" failures). 

Now, you talked about a ceramic cap with one lead burnt open with signs of arcing.  That is a major concern.  First, you need to look at the schematic and determine what that cap was there for.  Is it a supply decoupling cap or is it in the horizontal/HV oscillator section?  Although it is possible that the cap "just went bad", it is also possible that the cap saw huge amounts of AC current or an over voltage that shorted the cap and then blew the lead open.  For example, if the cap is a supply decoupler, it is possible a failed regulator, or another failed cap upstream of this blown cap, caused an over voltage or large AC oscillation to appear on the failed cap prior to failure.  If that is the case, simply replacing that cap will not fix the initial problem that caused that cap to fail. 

As I said, you will need to know what that failed cap's function was in the circuit to begin to do a forensic analysis of why it failed.  As well, you will need to know what DC and AC voltage appeared across it.  Unfortunately, to be able to "see" what is going on at that caps point in the circuit, you will need a 'scope.

With any used piece of electronic equipment, it is critical that the supplies be checked very first thing.  Both for "quantity" and "quality".  A supply rail might measure correctly with a DC voltmeter but a check with a 'scope might reveal noise or oscillations riding on that DC that you would not see without using a 'scope.  Also keep in mind that when probing around universal supplies (switchers) there are different grounds (commons) to contend with, some of which are mains connected and usually indicated on the schematic.  One must be very careful when working on the primary side of these supplies, particularly without the use of an isolation transformer at the AC line input.

Repairing this 'scope might be a bit much for you to chew on at this time.  It is a shame that you cannot find a functioning used scope or even a low cost USB scope to get you more acquainted with 'scopes in general (and to use in repairing this and future used equipment)

PW

 

 

Offline picowatt

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Re: Test Equipment: Oscillocopes
« Reply #98 on: February 22, 2015, 07:01:12 PM »
The words "sandpaper" and "oscilloscope", when used together, makes all that is me "cringe"...

Similarly, "grease" and "oscilloscope" do not go well together, although in reference to switch ball detents, there may be some utility. 

"Sandpaper" and "silver plating" also invoke a very disagreeable response within me.

PW

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Re: Test Equipment: Oscillocopes
« Reply #98 on: February 22, 2015, 07:01:12 PM »
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Offline MileHigh

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Re: Test Equipment: Oscillocopes
« Reply #99 on: February 22, 2015, 07:15:59 PM »
Great post PW.  I also did not think the caps looked like they had any problems.  One suggestion for Brian when he checks the rails is to put the multimeter on DC and then on AC.  I believe that the multimeter on AC would pick up any glitching on a supply rail within limits.  However, careful because some "cruder" DC supply rails may have some AC on them which is normal.  I apologize because I am flying blind and haven't looked at the schematic.

In my days we would sprinkle tantalum caps around a board for the bulk decoupling and then put a 0.1 uF ceramic at the power pin for the chips.  I have to confess I didn't even know that they were also good for HF decoupling, all I knew was that it was just standard practice to sprinkle them around.

It's interesting (fun?) to look at the "decoupling and regulation war zone" around a motherboard CPU socket these days.  I am 25 years out of the design game which is 100 years in non-tech time.

Offline Brian516

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Re: Test Equipment: Oscillocopes
« Reply #100 on: February 22, 2015, 07:27:58 PM »
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Good, thanks. It would be nice to have some good images of the board layouts. As you can see, the ones in the pdf really are bad and hard to read.

When I change the zoom to 100%, everything seems perfectly readable.  If you don't have anything visible showing your % zoom, try the "view" menu. Depending on your program, it might be under zoom or magnification, and turn it up some until it's readable.  If you are using Adobe Reader, go to the "View" menu, -"show/hide" - "toolbar items" - "select & zoom" and then check "zoom value" and the two above it.  That'll add it to the toolbar so you can change the zoom much easier. :)
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That seems odd. They must be very tiny. I think the smallest ones I've got are 100 nF. A 68 uF 16V one is about the size of a large pea. This is the great advantage of tantalums, in addition to their fast response: they are really physically small for the capacitance value.
Yep they are pretty tiny.   

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The very idea gives me the willies. I would definitely _not_ recommend that you put any kind of compound on anything. Clean it off the coax connectors!
The black oxide layer is silver oxide forming on the silver plating of the connectors. Cleaning carefully can remove this, but you don't want to remove the silver plating by being too aggressive with sandpaper!

Will do, sir. Will clean it all off with alcohol.  I didn't rub the sandpaper on it much at all, I didn't even take off all the silver oxide layer. I just rubbed it gently around the contact one time to clean off any debris.

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I have been thinking all along that the regularity of the glitch and the steady rate no matter the scope settings seemed like an arcing problem. You may have found the trouble.  The one from Allelectronics should be fine. Don't worry about the tolerance, you are now dealing with modern components that will be much closer to the printed value than those ancient ones in the scope. C1476 appears in the schematic on page 228 of the .pdf SM and appears to be an ordinary decoupling cap for the Emitter of Q1478. This should be a "nothing special" capacitor and the one from Allelectronics will work perfectly here.
Well now that I know that it doesn't need to be anything special, I will check my boxes for a quick replacement! I most likely have one of those, as I remember vividly having quite a large number of .1uf caps of that type.  lets just hope that I have at least one that's 200V or more! Then if that was the problem this whole time, I can just get that one done and over with, leave the other two for now, and give 'er a shot!   of course I'm still going to check the rail voltages before and after.
One thing I did notice, which would go hand in hand with this bad cap:    after it warmed up the first time, it started with the noise. I let it cool for a while and then powered it back up, and it was still there.   Therefore saying the damage happened when it warmed up the first time round, and didn't go away, so this little cap arcing issue that melted the lead could very well be my problem. let's hope it's my only problem, and the other two caps are just cosmetic issues! (I'll still replace them, though. If it was a part that was more than a dollar and was only cosmetic, I would leave it. But I can get 2 or 3 good quality tant caps for a dollar....)

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The Q1478 transistor is a PNP silicon, and 2n5401 is the generic cross for that one, should you need to replace it. This is a HV silicon amplifier and the NTE equivalent is NTE288. Cheap and easy to find if you need to replace it. Check that associated resistor R1476 also for off-value.
Will checking the Hfe on a DMM actually tell me if a transistor is bad?  I know sometimes I may not be able to find the datasheet for the original cap, but the generics should still have close to the same Hfe range, right?  I will def check the resistor as well.  I checked a bunch on the preamp board and at the inputs, and they were fine.
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http://www.reprise.com/host/tektronix/reference/find_transistor.asp
That is a great tool to have!  thanks for pointing me to it! 

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This is where a Variac can come in handy. For general troubleshooting reference: if you have a device that is blowing the main fuse, for example, you remove the fuse and put an ammeter across the fuse contacts, then use the Variac to slowly run up the line input voltage from zero to your local mains voltage. If the current draw indicated on the ammeter approaches 90 percent of the fuse value.. STOP!
If you suspect a severe power supply fault, you can do the same thing, while monitoring the suspect power supply with a voltmeter. Again, if the output voltage of the monitored supply begins to look like it is going to exceed the spec value + tolerance, then STOP increasing input voltage from the Variac.

However I don't think your problem fits in this category. The scope seems to be working, basically, at least so far. Once these suspect caps are replaced we can go further into the various features and functions to see if there is anything else wrong.
Well unfortunately I do not have a variac yet.  Closest thing I have is a dimmer switch, and there's no way I'm going to use that on any equipment! haha  different when using for some oddball experiment, like I did for the mini Kapanadze before I had a clue...

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I never did see what the CH2 trace from the calibrator looks like. I'm presuming it is also mostly OK but with the same glitching as you've shown on the CH1 trace.
If the power supply rails are OK, or if I need to replace that cap before running it, and have a replacement, I will make another vid with both CH1 and CH2.  Rob, the guy I got it from, said that there was something minor wrong with CH2.. and I remember that the trace was almost flat (had no vertical amplitude) when I first turned it on before I did any cleaning or anything to it.  Remember? I should have but didnt make a vid of it, but that was when I had a ton of noise from the switches, and the trace was also very, very dull, even when I had the intensity turned way up. ( don't know why it was like that, but whatever cleaning I did fixed it, cuz in the vids I made it was nice and bright.)

If for some reason your copy of the manual is unable to be zoomed in on, let me know and I will scan the diagrams that we are dealing with for you.

Brian

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Re: Test Equipment: Oscillocopes
« Reply #100 on: February 22, 2015, 07:27:58 PM »
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Offline MileHigh

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Re: Test Equipment: Oscillocopes
« Reply #101 on: February 22, 2015, 07:39:38 PM »
Going back to the scope, there seems to be a problem with the raster scan.  So there must be test points for the horizontal and vertical rasters.  I am using a very liberal interpretation of the term "raster."  More specifically, there may be a three test points for the vertical raster.  The first one would be the DC vertical bias signal.  Then next one would be the input signal.  And the third one would be the DC bias plus the input signal.  Presumably the DC bias signal goes "off screen" during the horizontal blanking interval.  I don't know the guts of scopes enough to know if the beam is modulated off during the horizontal blanking interval also.  If it is there may be a separate test point for that, and that would form part of the "raster signal matrix."

When the scope is giving a good display at the slower time base, you should be able to scope (I realize you need a second scope to do this) something akin to a "TV signal" for the two rasters.  I am going to assume that during the horizontal flyback interval the vertical raster is supposed to be "down and off the screen."   Then when the time base is faster, something is corrupting the regular raster waveforms.  It looks like there may be a problem during the blanking interval.

If you did indeed observe a corruption in the raster waveforms, then you could trace the signal and follow it back through the schematic.

Higher raster speeds presumably mean more stress on semiconductor components.  Perhaps there is a transistor somewhere in the signal path that starts to croak from the stress above a certain frequency.

Offline Brian516

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Re: Test Equipment: Oscillocopes
« Reply #102 on: February 22, 2015, 07:41:20 PM »
PW and MH -

I know that you can't really see it, even with the close up of that .1uf 200V cap, but from the other side and various angles I can easily see that there is a split in the lead and a chunk melted out of the lead.   I tried to get a pic of it using some small inspection mirrors but I just can't get the angle. 
Either way that cap is getting replaced, and so is at least one of the cosmetically damaged tantalums, regardless of whether it is bad or not.  I'm perfectly capable of soldering in tight places.   

I am still just learning the ropes of equipment repair, so I appreciate all the pointers and pointing out what the proper order of operations and methods are for checking a piece of equipment. I will definitely be hooking a DMM up to the V-rail test point and ground and powering it on for a min, then letting it cool for a few min,  and checking the next point, doing so with both DC and AC settings. 

Yes, sandpaper and equipment don't really go together. I know not to even bother now. However, just for an FYI, it was 1000grit sandpaper, which feels more like a piece of paper than it does sandpaper.   I know not to touch the PCB or any actual components with it, as it would remove the protective layers.  All I use for cleaning is IPA, and Deoxit where absolutely necessary, like inside of pots or switches that aren't exposed to where I can easily get a camel hair paintbrush and IPA in, and just spraying with IPA won't do the trick.  I've also gotten the pointer from MH about using a tiny drop of WD40 and letting it sit for a couple days. What wasn't mentioned, and what I would likely do following these couple of days and a couple minutes of use to make sure it's done its job, is clean off the WD with some IPA.

Brian

Offline MileHigh

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Re: Test Equipment: Oscillocopes
« Reply #103 on: February 22, 2015, 07:46:54 PM »
Quote
the other two caps are just cosmetic issues! (I'll still replace them, though)

I would strongly advise against that.  If there are little trim pots all over the boards in your scope that typically means that the boards had to be tuned and aligned with the trimpots as part of the manufacturing process.  Changing a cap could adversely affect the tuning and alignment of your scope.  I am being cautious and very generic in my statement.

Offline picowatt

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Re: Test Equipment: Oscillocopes
« Reply #104 on: February 22, 2015, 07:53:05 PM »
Great post PW.  I also did not think the caps looked like they had any problems.  One suggestion for Brian when he checks the rails is to put the multimeter on DC and then on AC.  I believe that the multimeter on AC would pick up any glitching on a supply rail within limits.  However, careful because some "cruder" DC supply rails may have some AC on them which is normal.  I apologize because I am flying blind and haven't looked at the schematic.

In my days we would sprinkle tantalum caps around a board for the bulk decoupling and then put a 0.1 uF ceramic at the power pin for the chips.  I have to confess I didn't even know that they ware also good for HF decoupling, all I knew was that it was just standard practice to sprinkle them around.

It's interesting (fun?) to look at the "decoupling and regulation war zone" around a motherboard CPU socket these days.  I am 25 years out of the design game which is 100 years in non-tech time.

MH,

The AC voltmeter idea would be fine for rails, but if the issue is with the switcher or the sweep/HV supply, I'd want a decent probe and 'scope.

A bit off topic, but my favorite tantalum story is with regard to a PA mixing console a friend owned.

About 20 years ago or so, I visited this friend while out of town on business.  He was bummed because he had a 3 or 4 year old low cost, but nice looking, 28X4 PA mixer with a shorted +18V rail.  He took it in for repair but was told that due to the low cost method used to construct the board (all channels on one massive motherboard) it was impossible to isolate which of the literally hundreds of tants used at every IC for decoupling was shorting the rail.  He was told all tants would have to be changed at a cost more than the board was worth.  The shorted cap made the supply go into current limit so the entire rail was down at all measurement points.  Were I at home, I would have connected a constant current supply to the rail feed and used a comparator probe or a 6 digit voltmeter to follow the short (using the PC traces as a CSR and noting polarity).

Long story short, we took the board apart (every knob and control nut had to be removed), unplugged the supply from the motherboard and using a 12V gel cell (i.e., a high current source) and clip leads he had laying around, I attached the gel cell to the +18V rail for a few seconds and then BANG!  There went the bad cap!  The ugliest smelling/tasting curly trails of black "something" wafted downward as the room slowly cleared of smoke.  We replaced that single bad (and now exploded/mostly missing) cap and reassembled the board.  A thorough test showed all preamps, EQ's, etc, to be working just fine.  He was still using it eight years later when I was on another out of town trip.

PW

 

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