G-9000 Bias Voltage at zero and can't be set after replacing pre driver transistors

HankM

New Member
I have just finished a complete recap of a Sansui G-9000 that came into my shop barely playing. I rebuild lots of recievers. I have rebuilt the PA boards and replaced transistors 5-8 ,the the 2sc1628(NPN) and 2sa818(PNP) transistors with KSC2690AS(NPN) and KSA1220AY(PNP) from Mouser. With the old transistors still in place, I was able to adjust the bias to 16mv with no problem. Both sides started at 3mv like they had never been adjusted with old xors. With the new transistors bias is at zero and can't be adjusted at all. Needless to say at minimum volume there is distortion. Can resistors be changed in the bias circuit like R25+26 390ohm to give me some bias voltage?
 
Well if all you have changed is the transistors, then something is wrong.

Check check and check your work, orientation of transistors, are the NPN/PNP in the right place, have you inadvertently cracked a trace on the board?

Have you checked/replaced R17 and R20?

Have you not accidentally mounted the driver board a pin out on the Molex?

Check and Check and check, if it worked before it should work now.
 
Thanks, all! Yes, I knew the pin outs were different, Sansui made the board with E holes on both sides, easy to install new transistors. Problem is, the new ones were not equal to the originals. So I reinstalled the originals, and bias setting was made easily, eliminating the low volume distortion. Testing for one more day, then shipping this monster to the client!
 
Thanks, all! Yes, I knew the pin outs were different, Sansui made the board with E holes on both sides, easy to install new transistors. Problem is, the new ones were not equal to the originals. So I reinstalled the originals, and bias setting was made easily, eliminating the low volume distortion. Testing for one more day, then shipping this monster to the client!
What do you mean "not equal"? I use those very substitutes and have never had an issue, the KSC has a little more gain, and a higher ft, but the KSC is a good substitute for those transistors.
You must have made an error.
I would get rid of those original transistors personally, they are not the most reliable on the planet.
I am about to do this exact thing to mine, a quick overhaul of the driver boards, then when, I have more time I will do the rest of unit..
 
@HankM just for reference, I have just done a quick freshen up of my G9000 driver boards, I replaced all those 2SC1628/2SA818 transistors with the KSC2690 and KSA1220, replaced all trimmers, all E-Caps. All worked perfectly when I fired up. No problem at all, dialled bias and DC offset in all good, I can only suspect you orientated something wrong in the one you were working on.
I just wanted to come back to this thread and clarify in case someone else was looking for substitutes for those transistors.
 
@HankM just for reference, I have just done a quick freshen up of my G9000 driver boards, I replaced all those 2SC1628/2SA818 transistors with the KSC2690 and KSA1220, replaced all trimmers, all E-Caps. All worked perfectly when I fired up. No problem at all, dialled bias and DC offset in all good, I can only suspect you orientated something wrong in the one you were working on.
I just wanted to come back to this thread and clarify in case someone else was looking for substitutes for those transistors.
On my driver boards tr06,07 are c2071 transistors. And tr05,08 are a939 transistors. Both drivers are the same....
 
@HankM just for reference, I have just done a quick freshen up of my G9000 driver boards, I replaced all those 2SC1628/2SA818 transistors with the KSC2690 and KSA1220, replaced all trimmers, all E-Caps. All worked perfectly when I fired up. No problem at all, dialled bias and DC offset in all good, I can only suspect you orientated something wrong in the one you were working on.
I just wanted to come back to this thread and clarify in case someone else was looking for substitutes for those transistors.
I'm looking for a good parts list, caps transistors and diodes, for my G901DB... I've been looking around but all I can find are bits and pieces...
 
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