Variac use and old solid state amp.

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I am a solid state newbie. I have this old class A amp that I purchased on ebay that I am putting in new PS caps and a few on the board as well as new output transistors.

Since it is likely that this has not been running in some time I want to use a variac to form the (electrolytic) caps on the board. I read some where that this is a bad idea with solid state amps in general.

Any help is appreciated.
 
Thanks Choky,

This is an Early Sugden A21 integrated. It looks to be in really nice shape. Cosmetics are excellent. The filter caps were physically leaking so I ordered new ps caps and output coupling caps (were 40v 2500uF now 50v 2900uF).

Two other caps looked like the cases were comprimised and one other cap had been replaced. I am replacing the bad ones and the one the one that was replaced (along with the same one in th other channel).

I also have new transistors for the output as one had been replaced and did not math the originals.

So I want to get it all put together and run the voltage up over a days period staring at say 30-50 and watching if any caps get hot etc.

I read that this was a good way to blow out the transistors?
 
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Hi portlandlay,
Choky is right. Only in some designs does a variac cause trouble. In general, I use one every time.

To reform your caps, you can increase the voltage in one day. Just watch the bias current in the output stages and DC offset voltages for signs of trouble while watching the caps. Look at the waveforms on the capacitors with an oscilloscope to determine if they are healthy.

-Chris
 
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portlandlay said:
Thanks Choky,

This is an Early Sugden A21 integrated. It looks to be in really nice shape. Cosmetics are excellent. The filter caps were physically leaking so I ordered new ps caps and output coupling caps (were 40v 2500uF now 50v 2900uF).

Two other caps looked like the cases were comprimised and one other cap had been replaced. I am replacing the bad ones and the one the one that was replaced (along with the same one in th other channel).

I also have new transistors for the output as one had been replaced and did not math the originals.

So I want to get it all put together and run the voltage up over a days period staring at say 30-50 and watching if any caps get hot etc.

I read that this was a good way to blow out the transistors?


If this is what I think, you can go here:

http://www.tcaas.btinternet.co.uk/index-3.htm

look at schm and read that pdf

something will be clearer to you.

with variac and first tests with sand amps,crucial thing is that you DON'T connect load at output.
only when you are sure that everything is OK ,connect dummy load at output and try it again.

my advice is- always replace every electrolyte cap in old amp.

and-you don't need few days on variac with new caps........

:devilr:
 
Good. Thanks for the input. Choky, I will replace all the caps but I want to see if I can get it up and running and then go after all the other electrolytic caps. If I do everything now and it does not work it will be harder for me to figure out what is wrong.

Also, thanks for the link - that is were I am getting my information.

Anatech, I will watch the bias but I do not have a scope to look at them. Like I said if I can get her running I will replace them all.
 
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portlandlay said:
Good. Thanks for the input. Choky, I will replace all the caps but I want to see if I can get it up and running and then go after all the other electrolytic caps. If I do everything now and it does not work it will be harder for me to figure out what is wrong.


'lytics are cheap ,and I can't see any reason why don't change them all at once;
with this approach,you don't risk any leak in signal caps (and that amp have few).....

but- it's your call ;)
 
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Hi portlandlay,
Good point, one I agree with (but I am a technician and know what to look for). Keep an eye out.

If it runs, you can gauge the changes made, good or bad once you change the capacitors. I also fully agree with choky in that all the caps should be changed. I just like to see where things go.

-Chris
 
Look at the waveforms on the capacitors with an oscilloscope to determine if they are healthy.
Someone made a similar recommendation to me in another forum and I wasn't able to get any specific details on the procedure(s). If anyone has some, or a link to a related thread or site, it would help.

IIRC, the recommendation I got involved feeding low frequencies through the caps and watching for distortions, but I got no guidelines on frequency range, levels, or even what to watch for. I almost want to say that they were talking about doing it to caps "in circuit" too, but I'm not sure.

-Thanks!
 
Not a bad idea...

since you don't know the history of the amp. But you should also have an AC ampmeter connected in series. By watching the ampmeter when you power it up, you'll be able to check for abnormal current draw and shut it down before things get out-of-control. This goes for amps with linear power supplies.
But if it uses a switching power supply (like QSC's Powerlite series), at some point the amp will just simply turn on.
 
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