Capaciters/diagnosing and replacing

Steven Welborn · Post by **Steven Welborn** » 20 Feb 2002 7:16 pm

It seems that one of the likely or common culprits in ailing older amps, and in my particular case an older Evans fet500 HV, are caps or filter caps breaking down with age and use. It occured to me that maybe a nontech type on a tight budget who's handy with solder guns,meters, and knows a few electronic basics could diagnose and replace for his/her self. Could it be a simple thing, like unplug amp, apply voltage to one side of cap, check other end for proper reading or something along those lines Or am I revealing too much electronic flathead ignorance here ?

. Thanks for your help and patience.

Glenn Austin · Post by **Glenn Austin** » 20 Feb 2002 10:05 pm

If the filter caps are shot on your amp, you'll hear hum at 120Hz, because obviously they are not filtering. You can also visually check the caps for signs of age like bubbles or leaks. You should be VERY CAREFUL about poking around in the power supply, because filter caps store a lot of voltage, even if the amp has been off a week. The shock can be LETHAL.

Everett Cox · Post by **Everett Cox** » 21 Feb 2002 1:06 am

Steve-- Yes, be careful as the larger caps can store and hold a pretty good jolt. The 'rule of thumb' we used to follow, though' was "If it's as big as your fist, it can kill." But I really don't know about today's components.

Anyway, one thing you can do -- after at least one lead of a cap is disconnected, put an ohm meter accross it. Use a high range on the meter. As the cap charges, you'll see the needle swing from low to high resistance. That will give a CLUE that the cap MAY function but won't tell how well it might work at operating voltage.

Elecrolytics are polarity sensitive so you'd try connecting the meter both ways. One way should give a good needle swing while, equally important, the other way should not.

Hope this is of some small help. --Everett

Bill Crook · Post by **Bill Crook** » 21 Feb 2002 5:53 am

Hey folks......

While not wishing to rain on anyone's parade, IF you haven't got basic ELECTRONICS 101 down pate,

STAY OUT OF THE AMP !!!!

You can and will do more damage to it and yourself than you ever thought of.

A good portion of my workload is is of this nature. I've actually had amps brought to me in a box and a paper sack,with the owner saying " I think I messed it up."

Save yourself time,money and problems by takeing it to a "Teckie" type person to begin with...

Just my 2cents

[This message was edited by Bill Crook on 21 February 2002 at 05:54 AM.]

patrick donovan · Post by **patrick donovan** » 21 Feb 2002 6:56 am

If you hear a 60 cycle hum in the amp, the filter capacitors are probably shot. Replace them all, they are not that expensive. Unplug the amp. Wait 10 minutes or so to allow the bleeder resistor to discharge the caps. Then short across the leads of all caps with a screwdriver to make sure they are discharged. Replace them paying attention to the polarity. If you hook up an electrolyic capacitor with reverse polarity...it will explode..(been there, done that). If you don't feel comfortable doing it, have a tech do it, it shouldn't cost much.

Regards, Patrick

Bill Terry · Post by **Bill Terry** » 21 Feb 2002 7:24 am

Caps are cheap. I don't try to verify whether they're good or not. If it's in doubt I replace it, particularly old eletrolytics.

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Tommy Detamore · Post by **Tommy Detamore** » 21 Feb 2002 11:03 am

What is all this I've been reading about "forming" the caps after you put them in? The way I understand it, it regards bringing the amp slowly up to full power using a Variac....[This message was edited by Tommy Detamore on 21 February 2002 at 11:04 AM.]

Jim Phelps · Post by **Jim Phelps** » 21 Feb 2002 2:29 pm

Found this at: http://aga.rru.com/FAQs/general.html

Do I need to "form" my new caps on a variac?
You never need to form new caps. The only caps that need forming are older electrolytics in amps that haven't been played for some time, and NOS electrolytic caps that have been on the shelf a long time. These caps should be brought up slowly on a variac. If you have just bought an old amp you suspect hasn't been played in a while, take it to a tech and have this done. If you expect to need to do this more than a couple of times in the next few years, do some research and get a variac, and learn how to do this. It's not hard, but there are things to consider (such as filament voltage).
[This message was edited by Jim Phelps on 21 February 2002 at 02:30 PM.]

Donny Hinson · Post by **Donny Hinson** » 21 Feb 2002 3:32 pm

Jim is right on the "forming" thing...most of the time it isn't necessary unless you're talking about many years of "non-use". Anyone who is good with a soldering iron can change caps, but I don't recommend inexperienced people playing with solid-state amps. The fragile circuit boards and the static-sensitive devices combine to offer significant problems for those who have little experience.

Best leave those to a pro.