Capacitor Discharge Rate Testing!

[shagster]

Hi, I have 2 identical capacitors from an old scope I'm trying to get running.

Both caps measure ok in ESR and Capacitance values imo.
Data sheet says tolerance spec is -10 +50%. And these are way under that.
Both of there original values are marked as (430uf 200vdc).
After ~40 years, they now measure 485uf and 465uf. Well under the +50% tol.

Anyway I charged them up to 24 volts dc and discharged them slowly down to 10 volts using a Digital Multi Meter. Funny thing was one of them just took over 4 minutes to discharge down to 10 volts. And the other took over an hour to discharge down to 10 volts. I'm not sure about either of them now

What do you guys think?

[stj]

Re: Capacitor Discharge Rate Testing!

they are done
one probably has high leakage but both probably have much lower ripple(charge) current than originally.

ripple current is an important parameter, but unfortunatly there are only a few bits of test gear that check it - all big expensive desktop units

[redwire]

Re: Capacitor Discharge Rate Testing!

If they've been in storage/not used I would reform them, put a few hundred volts on them for an hour and then recheck the leakage.

[CapLeaker]

Re: Capacitor Discharge Rate Testing!

I’ve seen this before, that one cap discharges faster than the other in old equipment. I always ended up replacing them. One is for sure done for.

The ESR meter on high volts, larger caps means Jack sh*t. You need a capacitor leakage tester to test these kind of caps properly.

[eccerr0r]

Re: Capacitor Discharge Rate Testing!

tbh 4 min, 1 hour, whatever, it's not bad using just a DMM's input impedance to discharge, seems fine to me, What is the function of these capacitors in the device? SMPS bulk filter with bleeders on them?

[shagster]

Re: Capacitor Discharge Rate Testing!

Yea, dunno. Maybe I'm probably better off changing them out.
Might give the reform a try first just to see if it makes a difference though.
Thanks for all the interesting advice anyway guys, much appreciated

Bought this Tektronix 485 scope on ebay, it looks like it was out of action for a long time. Needed a good clean. The units mains glass fuse was vaporized. So something is bad. Checked for shorts on the inverter pcb, but nothing really looks wrong. One thing that did look dodgy was the board connectors, some of them were pretty green. Going to just remove them and clean them in an ultrasonic cleaner. I'm guessing that caused the blown fuse.
I can pick up similar caps for around 30 euros a pair, I'm into it for about 200 euros atm. So if I can save a few bob here and there, that would be cool.

These caps are the line storage caps on the inverter board. C1822 and C1823.

[eccerr0r]

Re: Capacitor Discharge Rate Testing!

I vaporized a fuse on my tek scope's CRT PSU as I was trying to figure out what was wrong, turned out to be a broken CRT.

[shagster]

Re: Capacitor Discharge Rate Testing!

Haven't got that far yet. Hope that's not the problem. Visually it looks fine though. It was well packed, so survived the shipping intact. Looks like it originally came over from America to the UK, and now Ireland. Going by the original receipt that came with it.
It was last calibrated in 92 the sticker on the outside says

[stj]

Re: Capacitor Discharge Rate Testing!

check the scope for RIFA film caps - i know they almost always had some
if you find any translucent yellow rifa caps then replace them with panasonic or kemet equivelents

[shagster]

Re: Capacitor Discharge Rate Testing!

Haven't seen the dreaded Rifa caps in this unit yet. It is dated from 1980 though, going by an etched IC on the PCB, so it is possible I suppose. I need to get at the crt transformer board buried in the middle of the scope. But I doubt I'll find any on that board.

[eccerr0r]

Re: Capacitor Discharge Rate Testing!

I know I have rifas on my tek. Just haven't gotten a chance to replace them yet
Need to clean the input attenuator relays first...and also replace a bunch of the aluminum electrolytics too

[shagster]

Re: Capacitor Discharge Rate Testing!

The suspect caps on these 485 scopes are the under voltage rated tantalum caps supposedly.

[momaka]

Re: Capacitor Discharge Rate Testing!

I would say those caps are probably OK, despite their age. Old caps were made on a "larger technology node"... in other words, with much thicker foil for the plates (and that's why older electrolytic caps are bigger.) So they tend to last a while, provided the electrolyte hasn't dried up, which probably hasn't since their capacitance still reads OK.

You can check their leakage current in a very simple way with a series resistor.

First reform them for 30-60 minutes with a 1-KOhm series resistor and at voltage close to their rated (i.e. 200V)... or at least somewhat close (160V DC from the 200V caps on a cheap non-APFC PSU should be OK.)
After reforming, change the 1-KOhm series resistor with a 100k. It's best if you can do this while the cap is still charged to the full voltage output of your supply. One possible way to do this is to wire a switch in series with the 1k resistor and also have the 100k resistor running in parallel to the 1k-switch network. Once the cap is charged fully, use the switch to disconnect the 1k resistor. Then leave the circuit to settle for another 20-30 minutes. After that, measure the voltage across the 100k resistor. Since you know the resistance of the resistor and the voltage across it, you can use Ohm's law to calculate the current going through the 100k resistor. This will be the cap's leakage current. Should get something in the uA range.
... or to avoid doing any calculations, just put your multimeter in series with the 100k resistor and measure the DC current.

As for the blow fuse...
Since this unit came from the US, is there a line voltage switch to choose between 110/115/120V AC and 220/230/240V AC? If yes, was it set to use the correct line voltage?
USA & Canada --> 120V AC
UK and Europe --> 240V AC
If if was set incorrectly, that could well be why the fuse blew.

[shagster]

Re: Capacitor Discharge Rate Testing!

Thanks Momaka for that excellent (how to) on reforming, in my case 43 year old caps. You could well be right! I'm sure it will go on to help others out
I thought the same about the mains voltage switch, but it was on the right 230 volts. Unless it was switched over after, but I doubt it.

But I was impatient and had a look around online for replacements and they seem to be cheap enough to just not take the chance of doing further damage. And I've seen online at least 1 other person had a problem with them here http://rfscientific.eu/sites/default...485_repair.pdf

Here's the ones I went for instead. Looks like they haven't got the same screw footprint. But I thought I could get around that by mounting them 90 degrees to the original layout, like this +, that keeps them central.

https://www.mouser.ie/ProductDetail/80-ALS80A431DA400

[momaka]

Re: Capacitor Discharge Rate Testing!

Thanks for sharing that PDF, shagster! It was a very interesting read. Looks like the mains caps failing may not be an isolated issue just to you, so probably a good thing you're replacing them. If the old caps are failing with high leakage, no amount of time reforming them would fix them.

As for the replacement caps... you could have gone with 470 uF, which is a more standard value and is usually cheaper (though I will be honest, I didn't check prices of said value with screw terminal option... and my cheapass would probably have gone with regular through-hole / snap-in type caps, as they are about 2x cheaper, or more... but for nice test equipment like this, it probably doesn't make much sense to be a cheapskate. )

Anyways, let us know how it went and if you got the scope fixed. Would be nice to have some closure, in case anyone else stumbles on this in their search.