Test Equipment: Oscillocopes

[Brian516]

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.

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.

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.

[TinselKoala]

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.

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.

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.

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!

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.

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

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.

[picowatt]

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



 

[picowatt]

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

[MileHigh]

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.