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Old 8th December 2005, 03:12 PM   #1
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Default Variac use and old solid state amp.

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.
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Old 8th December 2005, 09:29 PM   #2
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Default Re: Variac use and old solid state amp.

Quote:
Originally posted by portlandlay


I read some where that this is a bad idea with solid state amps in general.

Any help is appreciated.
that thing only some sh*thead can write.
variac is invented for DIY crowd in holly salvaging crusade......

what amp is in question and in what condition it is ...

be more specific and you'll hear more comments
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Old 8th December 2005, 09:40 PM   #3
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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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Old 8th December 2005, 09:53 PM   #4
anatech is offline anatech  Canada
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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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Old 8th December 2005, 10:04 PM   #5
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Quote:
Originally posted by portlandlay
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........

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Old 8th December 2005, 10:15 PM   #6
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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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Old 8th December 2005, 10:31 PM   #7
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Quote:
Originally posted by portlandlay
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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Old 8th December 2005, 11:29 PM   #8
anatech is offline anatech  Canada
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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
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Old 9th December 2005, 01:50 AM   #9
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You guys have been a great help. I feel better about it now. I hope she runs!
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Old 19th November 2009, 01:57 PM   #10
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Well 4 years later this project still sits. Two kids later I am ready to get back to it. Portlandlay was an account I did not remember so this is the new me

I hope to be able to pic up where I left off. Now where was that nut?
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