Is this a normal DC servo?

[Forman313]

Trying to fix a Perreaux E160, and it seems to be working.
The main problem was very loud mains hum. One of the legs on the rectifier bridge made contact with the GND layer. QC really isnt their strong suit.

Also a couple of 2w resistors needed replacing. Should have been 5w parts, but I guess they were out.


This whole circuit looks strange to me. Whats VR1 doing there?


Anyone familiar with the design?
The outputs are Hitachi SK2221/SJ352 MOSFETs.


I think most of the components are there.

 

[rayma]

Whats VR1 doing there?

The servo seems normal. The pot may be for hum or noise balancing.

 

[Ian Finch]

You have the balanced output of the LTP, Q2,Q3 collectors, both connected directly to the negative rail. Consequently, so are the bases of the balanced VAS transistors, Q5,Q6. Are there not a couple of resistors needed there?

 

[PRR]

The servo is straightforward, once you separate the essential NFB parts from the added servo parts.

Bottom of your diagram, V- and Gnd seem to be connected and is likely drawn wrong.

 

[Forman313]

The servo is straightforward, once you separate the essential NFB parts from the added servo parts.

Bottom of your diagram, V- and Gnd seem to be connected and is likely drawn wrong.

Ahh... thanks 🙂

You are quite right. I skipped some components that diddnt relate to the servo or pot-circuit.. but ended up drawing most of it. So I might as well finish it.


Replacing a bypass cap with a trace might not be the smartest thing to try

 

[srh]

VR1 is adjusted for minimum distortion at 20kHz.