Cap on Oscilloscope

[large_ghostman]

Hi, my mystic powers tell me I have a capacitor about to blow in my Oscilloscope. How do I know? Well I havnt had the back off yet but plugged in a little smoke with that unmistakable smell of frying electrolytic capacitor floats up!

My problem is I have turned it on twice and both times it starts up fine and works, as its a Gould scope and well known for blown caps taking the entire scope out when they go I really loathed to plug it in again. I have a LCR meter but I dont know if the cap will show low ESR because some of the Gould caps I have replaced in the past have dried out gone OC or nearly OC and had low ESR.
I could take apart and plug in and see if I can spot smoke quickly, but thats a bit risky as Goulds have alot of Tant caps and if its a big power cap they tend to take out all the smd Tants when they go.

So am I likely to be looking for low ESR or as its likely to be a dry cap will the cap value be way off? I dont normally get to them before they go bang so I am unsure, obviously I might take the back off and its obvious but just case I wanted to ask

 

[jpanhalt]

Hi, my mystic powers tell me I have a capacitor about to blow in my Oscilloscope. How do I know? Well I havnt had the back off yet but plugged in a little smoke with that unmistakable smell of frying electrolytic capacitor floats up!

My problem is I have turned it on twice and both times it starts up fine and works, as its a Gould scope and well known for blown caps taking the entire scope out when they go I really loathed to plug it in again. I have a LCR meter but I dont know if the cap will show low ESR because some of the Gould caps I have replaced in the past have dried out gone OC or nearly OC and had low ESR.
I could take apart and plug in and see if I can spot smoke quickly, but thats a bit risky as Goulds have alot of Tant caps and if its a big power cap they tend to take out all the smd Tants when they go.

So am I likely to be looking for low ESR or as its likely to be a dry cap will the cap value be way off? I dont normally get to them before they go bang so I am unsure, obviously I might take the back off and its obvious but just case I wanted to ask

How can something go open circuit and have a low esr? Isn't OC very high resistance?

If you are getting smoke from a capacitor, but not the fumes for a blown electrolytic, it will probably be hot and appear burned. If it a puffed electrolytic it will look puffed. I don't have a lot of experience with puffed/blown electrolytics, but in that experience once they blow, it is obvious. Just puffed is not so obvious.

John

 

[granddad]

Replace the lot ?

 

[Les Jones]

Hi LG,
If you can get hold of an ESR meter that will do in circuit testing it is a Quick way of identifying faults. I built a slightly modified version of one that was in the Elector magazine in September 2005. I found it quickly identified faulty capacitors in the line output stage of CRT type TVs and monitors. I also fixed a TX camera module just by testing capacitors in circuit. Although the transistor tester discussed recently on the forum tests ESR I don't think it will work very well with capacitors still in circuit. I think John is having a go at you for saying low ESR when you mean high ESR.

Les.

 

[large_ghostman]

Sorry John my confusing things, I was trying to say I didnt know if it would fail OC or short, so I dont know if the ESR will be affected. I had a quick look and nothing shows, so I will test the caps but there is a shed load of them lol. I have a LCR meter that does ESR etc etc so testing isnt a problem, its white smoke but still starts up. I will prod around the PSu first see if any are bad there, its my back up scope but really nice and small so its handy to have around.

What I was trying to say (and failed miserably) was normally when they smoke I get them go open circuit, but I have had a couple go short C. So last time I wasnt able to tell much with the ESR meter. I am surprised its still working ok?? So I assume its one of the larger caps on the PSU. DEEP JOY!! Its a PITA scope to take apart! I have to strip it right down to get the board out for testing thats my weekend taken care of.

 

[Nigel Goodwin]

So am I likely to be looking for low ESR or as its likely to be a dry cap will the cap value be way off? I dont normally get to them before they go bang so I am unsure, obviously I might take the back off and its obvious but just case I wanted to ask

No - exactly the opposite - you're looking for high ESR.

The actual capacitance rarely chances much, and may still be within tolerance.

First thing is to take the top off and do a visual inspection, you might well notice capacitors with domed tops, or 'leakage' around them - generally the problem is down to sub-standard capacitors been fitted in the first place.

I'm presuming your scope has a switch-mode PSU?, as almost all cases are in switch-mode supplies.

 

[large_ghostman]

No - exactly the opposite - you're looking for high ESR.

The actual capacitance rarely chances much, and may still be within tolerance.

First thing is to take the top off and do a visual inspection, you might well notice capacitors with domed tops, or 'leakage' around them - generally the problem is down to sub-standard capacitors been fitted in the first place.

I'm presuming your scope has a switch-mode PSU?, as almost all cases are in switch-mode supplies.

Thanks Nigel, had the top off and all looks fine! utterly amazing considering all the smoke. I will strip it down at the weekend and test the caps, I wasnt sure which way the ESR went once they got to the smoking stage. The ones I have changed in the past have been >95% within the 20% stated value range for capacitance so your right its a rubbish way to search for them, lucky enough my LCR meter does ESR.

Yes the PSU is smps, the gould scopes were not great quality apparently, but packed with things way beyond there time. Having said that, most Goulds that survive today cant of been that bad to get past 20-30 years. I keep toying with the idea of getting a cheap RIGOL, but to be honest for a bit less money I can get a decent Lecroy boat anker for slightly less money and I like old test gear.