Pioneer SA-940 idle current change when supplied with alternate output transistors

hairybiker

hairybiker

New Member
I have a SA-940 in which it differs from the service manual.
The output transistors are 2sc3546 and 2sa1389 pairs.
Also, the values of idle current preset and resistor result in a minimum idle current of 160ma ( 36mv at test points ) with preset fully anti clockwise.
All solder joints are original and the only parts that may have been disturbed are the output transistors.

Now, the service manual for boards fitted with the 2sc2525 and 2sa1075 pair says that the idle current should be set to between 36 and 100ma ( 8 to 21mv on TP ).
The idle preset and its paralleled resistor are different values to the ones mentioned in the SM.

Does anyone know of an addendum to the service manual?

Thanks

Mark
 
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Just a thought, does anyone have a SA940 or equivalent fitted with the updated transistors that could measure their idle current?
 
Hi Mark,

It’s probably going to be the same. And I doubt you will find any updates to reflect component changes.
I'd have to look at the schematic but the way they designed the bias generators ( stv diode stack, R, pot) usually it is for specific devices. Once you change devices the bias generator setting or ranges do allow for the outputstage bias to be set to their claimed spec. In your case it sounds like you need to reduce the bias voltage generator range to be lower. Once again I need to refer to the schematic to offer up options.
 
Rcs16, Yup, that's what I figured.
It uses the Pioneer usual NSA setup.
As I cannot find a datasheet that shows the usual gain graphs I can't just make an educated guess. Although a good guess is probably going to be OK.
Customer was complaining of distortion on a particular bassy track.
The only way I've managed to get distortion from it was driving it to clipping.
All components check out OK, apart from the differing values in the bias circuit to the schematic.
Personally I think it is a high expectation from a basic 70/80's amp.
 
Speakers were checked as OK.
But.. last night I remembered a soon to be important point mentioned.
"I've had to have the balance to one side for a while now".
So I checked the balance control and all appears OK.
I then touched the bass control... the output went all over the place .

A strip down of the bass pot revealed a stack of carbon across the center point of the track and covering the wipers.
The track is a split track with a null point at the center. The crap covering it is obviously causing issues.
I will clean it up and also while I'm at it clean the treble , balance and volume pots.
I also noticed that the loudness button was a bit iffy so I will have to squirt some cleaner in the switches aswell.

I will update here when sorted.
 
All sorted, cleaning of bass and balance smoothed it out.
I have left both idle current pots fully counter clockwise as they arrived.
Just a reassemble and listening test to do.
 
hairybiker said:
I have a SA-940 in which it differs from the service manual.
The output transistors are 2sc3546 and 2sa1389 pairs.
Also, the values of idle current preset and resistor result in a minimum idle current of 160ma ( 36mv at test points ) with preset fully anti clockwise.
All solder joints are original and the only parts that may have been disturbed are the output transistors.

Now, the service manual for boards fitted with the 2sc2525 and 2sa1075 pair says that the idle current should be set to between 36 and 100ma ( 8 to 21mv on TP ).
The idle preset and its paralleled resistor are different values to the ones mentioned in the SM.

Does anyone know of an addendum to the service manual?

Thanks
Someone
Click to expand...
Someone may have tried to substitute different transistors versus The originals. They may or may not work properly. As long as the unit isn't getting too hot and stays a bit warm ( heat sinks) the idle current is probably okay you can put your hand on them and feel both the left and right side.
 
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