6800uf (M) 100v caps trouble

[EGM]

Hello, I'm new to this site but have been in the field for years. This is my first posted thread.
I've been doing some repair work on numerous amps, receivers lately. Replacing main filter caps on two different amps I've come across a problem. Running through and replacing 20 year old caps in two amps I have had the same problem two times and can't seem to figure out what's up. On the latest, I've replaced two 6800uf 100v caps on a Fisher CA-800. When powering up the amp the power meters pegged out. I looked the amp over and hadn't done anything wrong. No reversed caps and no debris left on the board. I use compressed air to clean up everything prior to turning anything on. The amps original caps were 6800uf at 68wv. Obviously the replacements will work. I tested the caps prior to installing and all seemed fine. They actually tested great. Near 6800 and .01 esr. I had replaced all the caps at one time and didn't know if I stupidly got one reversed but I checked and all was fine. After a couple hours scratching my head I decided to reinstall the original filters. Powered it up and it was fine. I checked the new caps again and they seemed ok so, I reinstalled them again and whammo! Meters pegged out again. If replacing 68v with 100v isn't the problem, what could it be? Any help would be favorable.

[Per Hansson]

Re: 6800uf (M) 100v caps trouble

There are several ranges of capacitors.
In some cases the equipment will be tuned for a particular capacitor type.
And veering too far off from that will produce problems.

So saying that it's a 6800uF cap really tells nothing...

[EGM]

Re: 6800uf (M) 100v caps trouble

I did test them and their capacitance was right around 6780uF actually both tested right near 6800. These should be good replacements for the originals at 6800uF @ 68wv

[kc8adu]

Re: 6800uf (M) 100v caps trouble

could be leaky.
or like some sprague snap ins i got have the jackets on backwards.
try to reform them.

[EGM]

Re: 6800uf (M) 100v caps trouble

I'd like to build a reformer but, I don't get many bad ones. Just the past 4 months put a few on my door step. I still want to see if the cans were installed backward. Not enough time in the day!

[montemcguire]

Re: 6800uf (M) 100v caps trouble

Maybe the problem with the amp is not these caps? The old ones test OK, the new ones test OK, the amp works well with the old caps and the new caps, and then fails with the old caps and then the new caps. Seems that amp failure has little relation to the 6800uF cap. Maybe the power meter itself has a flaky solder joint that causes the needle to peg randomly as the amp is handled during the refitting and testing of this 6800uF cap?

Time to look further at this amp and see what else is going on! For starters, look at the ripple across the 6800uF cap. If it's low, and the DC value is good, that cap is fine. Test all of the DC voltages on the easy to test parts and compare it to what you'd expect according to the schematic. Best of luck isolating this problem - sometimes tracking a problem down can be annoying, but you've made a start and have some information that should point you away from the 6800uF cap.

[redwire]

Re: 6800uf (M) 100v caps trouble

I'd say the problem is elsewhere or poor connections/duds to the new 6,800uF filter caps.

With vintage gear like this I shotgun it (=replace all) little electrolytic caps, they are also important.


The STK's are fragile and if you banged them up I could see one acting up , but not both channels.
Also, compressed air/vacuuming makes a lot of static and ESD. You have to be careful.

Note the protection relay does not detect DC or amp troubles; it's just a power on time-delay relay. Don't connect speakers!

I'd start with measuring the amp's rail voltages +/-20V and +/-56V and speaker output voltages.

For both "meters pegged out" it's either oscillating or DC problem.

Fisher CA-800 Service Manual at hifiengine 14.3MB but here is schematic only.