Test Equipment: Oscillocopes

[Pirate88179]

Especially DC HV, since at least AC will throw you away from it, instead of getting stuck to it. 

Brian

I am no expert but this is not correct from what I was taught while working on 3 phase 240 volts machinery.  I was taught that you never grab anything...you use the back of your hand so in case you get zapped, and your hand closes involuntarily, you are not clinging to a hv source.  Now, maybe this is only true for 3 phase circuits but...I don't think so.  This saved my life once when working on some equipment (before lock-outs were invented) with the breaker off and, a note saying to leave it off.  Well, someone was trying to figure out why their machine was not working and...yes...turned it on.  I did get bit but, I was careful not to be in a position to have my hand close around the wire.  I was also taught to use one hand only, that way the charge does not go across your heart.

Off-topic I suppose.  Sorry.

Don't worry...your scope was made with quality components and meant to last.  It could easily be as simple as one of those caps.  You will get it figured out and then you will feel very proud for doing so.  I know that I would.

Bill

[TinselKoala]

I found one on the vertical preamp that has a hairline crack in it, and one on the IC board in the Power supply +5V rail.
The one on the +5V rail is number C1559.  It's split apart by one of the leads. It looks to have been caused by heat, from the component itself though since there are no heat marks around it, the fan works, and everything else in the area is in great shape.

The one on the vertical preamp board only has a hairline crack on the top of it. I thought it was just the paint but i ran my fingernail over it and there is definitely a crack.
It is part number C161 on board A3.  It is also part of a +5V section of the board, interestingly enough.  both caps go to ground.

Could this possibly be part of a bigger issue, since they are both on +5V? 

Yes, this could be a problem. The pic you posted... yes, that cap sure does have a crack in it. This could be bad or not, but I'd replace it anyhow. That's C1559, a 33uF 10V tantalum, and appears in the schematic on P. 240. (I'm guessing you have the same copy of the SM that I have, so page numbers should match.)

The other one C161 is on P. 174, board A3, and is a 6.8 uF 35V tantalum. I'd replace it too if it looks cracked.

Yes, if these caps are bad they could indeed be causing problems. At the very least the one in the 5V PS will either pull that supply down if it is shorted or increase the ripple if it is open.

Tantalum caps are polarized electrolytics with the _positive_ lead marked, instead of the negative like most electrolytics. Obviously you have to get them in the right way round, with negative to ground.  Tek advises cleaning out the plated thru-holes in the doublesided circuitboards by using a toothpick once the component has been removed. This will assure that you don't damage the traces on either side of the board. Melt the solder and stick the toothpick in there and rotate it and it will leave a nice clean hole for the new part. You can usually get away with just heating and soldering on one side (the component side in this case) of the board. The solder will flow thru the holes and make good contact.

But be prepared for it not to make any difference, it is possible that these caps are still good even if cracked! Replace them anyhow. Do you have a local component supplier? If not, DigiKey is great, it usually takes 3 or 4 days from ordering on-line to delivery to my mailbox.

I presume you have a gentle soldering iron, 35 Watts or so with a fine tip. 

Before you do anything, you could check TP1558 which is right in there near the C1559 cap and see if you get the 5 V on your DMM and if this changes as the scope warms up. Also look at the box in the troubleshooting flowchart on the right side of the page on page 248, referring to the tantalum decoupling capacitors.....


(ETA: It's too bad you don't live next door to me... I just checked my tantalum stash and I have both needed values in there....  But I'm not sure about the voltage rating of my 6.8 uF ones, they are just color coded not marked. The 33uf 10V one is marked though. So best order from DigiKey or hit up your local parts guy. )

[Brian516]

I am no expert but this is not correct from what I was taught while working on 3 phase 240 volts machinery.  I was taught that you never grab anything...you use the back of your hand so in case you get zapped, and your hand closes involuntarily, you are not clinging to a hv source.  Now, maybe this is only true for 3 phase circuits but...I don't think so.  This saved my life once when working on some equipment (before lock-outs were invented) with the breaker off and, a note saying to leave it off.  Well, someone was trying to figure out why their machine was not working and...yes...turned it on.  I did get bit but, I was careful not to be in a position to have my hand close around the wire.  I was also taught to use one hand only, that way the charge does not go across your heart.

Off-topic I suppose.  Sorry.

Don't worry...your scope was made with quality components and meant to last.  It could easily be as simple as one of those caps.  You will get it figured out and then you will feel very proud for doing so.  I know that I would.

Bill

I believe we are both correct.

If the AC source you are handling is something like a wire, your hand would close around it from the reflex of getting shocked, which would in a way hold you to it, but if you get zapped by AC say, on the tip of your finger, it will blow you back, and if it's strong enough, possibly blow off your finger and a toe or two if that's where it decided to leave your body at.  I know someone that lost two fingers and two toes from HV AC.    I could be wrong here, but I believe we are both correct.

Brian

[Brian516]

Yes, this could be a problem. The pic you posted... yes, that cap sure does have a crack in it. This could be bad or not, but I'd replace it anyhow. That's C1559, a 33uF 10V tantalum, and appears in the schematic on P. 240. (I'm guessing you have the same copy of the SM that I have, so page numbers should match.)

The other one C161 is on P. 174, board A3, and is a 6.8 uF 35V tantalum. I'd replace it too if it looks cracked.

Yes, if these caps are bad they could indeed be causing problems. At the very least the one in the 5V PS will either pull that supply down if it is shorted or increase the ripple if it is open.

Tantalum caps are polarized electrolytics with the _positive_ lead marked, instead of the negative like most electrolytics. Obviously you have to get them in the right way round, with negative to ground.  Tek advises cleaning out the plated thru-holes in the doublesided circuitboards by using a toothpick once the component has been removed. This will assure that you don't damage the traces on either side of the board. Melt the solder and stick the toothpick in there and rotate it and it will leave a nice clean hole for the new part. You can usually get away with just heating and soldering on one side (the component side in this case) of the board. The solder will flow thru the holes and make good contact.

But be prepared for it not to make any difference, it is possible that these caps are still good even if cracked! Replace them anyhow. Do you have a local component supplier? If not, DigiKey is great, it usually takes 3 or 4 days from ordering on-line to delivery to my mailbox.

Interestingly enough, my manual, which is for my exact model, and in the correct serial number range, says that the cap on the PSU +5V rail is an electrolytic 33uf 10V..
My manual is the original one that came with the scope when the power company purchased it originally, and has the fold-out schematics and PCB layouts.  It doesn't sound like we are both working with the same copies.  Mine doesn't have page numbers in the diagrams section, either. just side tabs.

I presume you have a gentle soldering iron, 35 Watts or so with a fine tip. 

Yes I have a 30W fine tip, and also a 6-8W fine tip battery powered one.  While desoldering, I tried the 6-8W and it doesn't wanna stay hot enough so I've been using the 30W.  I don't have a desoldering vacuum tool, so I made a solder sucker.  On the ones where I have access to both sides, I've just been heating one side and blowing the solder thru from the other. been working great so far, but I'll have to suck it out for these caps cuz I'm not taking the boards off again.  I already took the trigger and sweep boards off and cleaned everything up and inspected them today so far.

Before you do anything, you could check TP1558 which is right in there near the C1559 cap and see if you get the 5 V on your DMM and if this changes as the scope warms up. Also look at the box in the troubleshooting flowchart on the right side of the page on page 248, referring to the tantalum decoupling capacitors.....

I may actually have replacements for these on hand. If I do, should I just go ahead and swap them out, or at least the PSU one first? Just in case it is causing a "ripple" or overvoltage, so it doesn't cause any further damage? Or is it unlikely that it would damage anything else running it with the possibly bad cap?
I don't have any page numbers like that, only section tabs and then the section number - page format. What section and whats on the page beside it?

(ETA: It's too bad you don't live next door to me... I just checked my tantalum stash and I have both needed values in there....  But I'm not sure about the voltage rating of my 6.8 uF ones, they are just color coded not marked. The 33uf 10V one is marked though. So best order from DigiKey or hit up your local parts guy. )

Hopefully I have some in my stash as well.  I have quite a large variety of caps of all kinds. For something that inexpensive, I will order from DigiKey since we know they have high quality components, but for future orders or more expensive things, take a look at OnlineComponents.com and let me know what you think.  When I ordered MJL21194's I found they had the best price on em, and they were actually the 21194's and not the G's, and are genuine ON transistors. Let me know what you think when you get around to checking them out.

But for the meantime, everything is back together, the cover is still off.... I'm going to check my stash for replacements and wait for your reply......

[TinselKoala]

Interestingly enough, my manual, which is for my exact model, and in the correct serial number range, says that the cap on the PSU +5V rail is an electrolytic 33uf 10V..
My manual is the original one that came with the scope when the power company purchased it originally, and has the fold-out schematics and PCB layouts.  It doesn't sound like we are both working with the same copies.  Mine doesn't have page numbers in the diagrams section, either. just side tabs.

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"

Yes I have a 30W fine tip, and also a 6-8W fine tip battery powered one.  While desoldering, I tried the 6-8W and it doesn't wanna stay hot enough so I've been using the 30W.  I don't have a desoldering vacuum tool, so I made a solder sucker.  On the ones where I have access to both sides, I've just been heating one side and blowing the solder thru from the other. been working great so far, but I'll have to suck it out for these caps cuz I'm not taking the boards off again.  I already took the trigger and sweep boards off and cleaned everything up and inspected them today so far.

I may actually have replacements for these on hand. If I do, should I just go ahead and swap them out, or at least the PSU one first? Just in case it is causing a "ripple" or overvoltage, so it doesn't cause any further damage? Or is it unlikely that it would damage anything else running it with the possibly bad cap?
I don't have any page numbers like that, only section tabs and then the section number - page format. What section and whats on the page beside it?

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.

Hopefully I have some in my stash as well.  I have quite a large variety of caps of all kinds. For something that inexpensive, I will order from DigiKey since we know they have high quality components, but for future orders or more expensive things, take a look at OnlineComponents.com and let me know what you think.  When I ordered MJL21194's I found they had the best price on em, and they were actually the 21194's and not the G's, and are genuine ON transistors. Let me know what you think when you get around to checking them out.

But for the meantime, everything is back together, the cover is still off.... I'm going to check my stash for replacements and wait for your reply......

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.