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-   -   An oldie. Maybe a goodie? (http://www.diyaudio.com/forums/solid-state/137182-oldie-maybe-goodie.html)

ungie 23rd January 2009 08:40 PM

An oldie. Maybe a goodie?
 
2 Attachment(s)
Looking for comments on the attached schematic.
I thought it might be fun to not reveal the manufacturer yet!
Currently ~220mV offset on both channels. :xeye:
Any thoughts where to begin?

SQLGuy 23rd January 2009 09:09 PM

DH-500?

mlloyd1 23rd January 2009 09:37 PM

looks to make me think of the old big perreaux power amp.
what was it, the 5150 or something like that?

is the 220mv dc only (not low freq or high freq oscillations as verified on a scope)?

i might check if the electrolytic cap in the feedback leg is leaky.

mlloyd1

jacco vermeulen 23rd January 2009 10:05 PM

PMF5550

ungie 24th January 2009 01:47 PM

Quote:

Originally posted by mlloyd1
looks to make me think of the old big perreaux power amp.
what was it, the 5150 or something like that?

is the 220mv dc only (not low freq or high freq oscillations as verified on a scope)?

i might check if the electrolytic cap in the feedback leg is leaky.

mlloyd1


Quote:

Originally posted by jacco vermeulen
PMF5550
Very good guess mlloyd1 and jacco nailed it!
Sure enough this is the beastly Perreaux PMF5550.
Another boat anchor pulled from the "trash".
Now, before I get too deep into yet another big amp repair (the Rowland Model 7s I am repairing are still in progress), is this thing even worth fixing from a modern day performance perspective?
mlloyd1, I have only put a meter on the outputs so far and the AC voltage is very low in both channels (2mV).
I have only had a chance to clean it up (years of dust and cobwebs inside), check for any toasted parts, check the power supplies, etc.
Any ideas how to measure bias on this design with no source resistors?
Any idea what the correct bias current is for this amp?
Thanks!

jacco vermeulen 24th January 2009 02:55 PM

The PMF models were biased in Class A to 10% of the nominal output level : 50W for the 5550, aka 2.5A, is 200mA per output device.
The current is measured at the rails.

SQLGuy 24th January 2009 04:59 PM

I searched some pictures of this amp. Very impressive!

What are those devices that attach to the insides of the heat sinks?

jacco vermeulen 24th January 2009 05:06 PM

Real biggy output coils. (the transformer in that amp is a 6KVA specimen)

Michael Chua 24th January 2009 06:24 PM

Quote:

Originally posted by ungie
Currently ~220mV offset on both channels. Any thoughts where to begin?
You should be able to adjust DC offset with RV2 pot.

Mike

ostripper 25th January 2009 12:50 AM

Quote:

You should be able to adjust DC offset with RV2 pot.
That is strange.., AC coupled offset adjust???
Does that use the ESR of the cap to affect the offset?
OS


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