SX-950 Power amp board transistors

[supergilson]

I'm replacing all the power amp board transistors due to corrosion. This unit sat somewhere nasty for a while!

I can't find a current list of every transistor for this board. I'm having trouble with Q5&6, Q7&8.

 

[tsd71]

Q5-KSA1381
Q6-KSA1381
Q7-KSC3503
Q8-KSC3503

Q1,2,3,4- KSA992 gain matched
Q9- KSC2073 or MJE15032G
Q10- KSC-2073 or MJE15032G
Q11- KSA-940 or MJE15033G
Q12- KSA-940 or MJE15033G

Also I change the trimmers two of each
652-3266P-1-103LF VR1 and VR2
652-3386H-1-101LF VR3 and VR4
and the big diodes 1S1885 with UF4004

 

[supergilson]

Thanks!

I see I got the wrong trimmer. The manual says 1k on the list, but 10k on the drawing. Indeed it was 10k on the board!

 

[cferry1]

Hello To Members ,

pioneer sx-950


Presently having issues with Idle Current Adjustment ( a little bit of a problem ).

CURIOUS, Would the HFE rating or dash letter of Replacement Output Final Power Transistors create problems ?
Example replacing the 2SB557-R [ 40 - 80 hfe ] vs using a 2SB557-O [ 70 - 140 hfe rating ].

How about the Amplifier Drive 1st stage . . . example replacing the 2SA818-Y with a 2SA818-O or replacing thru
the 2SC1628-Y with 2SC1628- different subscript letter ?//?//?

any help / suggestions appreciated.
cferry1

 

[SaturationPt]

Problem being drift or inability to adjust?

 

[cferry1]

Longer Story ...
1) Originally Could Not Zero DC Balance.
This turned out to be a circuit board trace internal short at the 1st Stage Differential Amp - Q2 base. Corrected by Lifting the net and rewiring.
2) While Performing Idle Adjustment -
Corrected a Out of Tolerance VR4 ( the Idle Current Adj. Pot with replacement) .
Replaced the Varistor two times, just because.
3) Idle Current . My last success - with only 2 of 4 Final Output Power Amplifier Transistors installed I was able to get a stable 20 Mv. Yes, All appeared well awaiting another set of Transistor's to be delivered.

4) New Mishap -
This item # 4 occured with 2 of 4 Output Final Transisitors installed. After Zeroing DC Balance OK and Carefully Adjusting Idle Current Voltage to 20 mV per procedure.
Powered On Unit with FM input / Volume Level knob at 2. Pop !!! XZ@% - The two Output Final Power Transistors blew ( shorted themselves - after inspection ) and Idle Current Potentiometer VR4 started smoking before I rapidly Powered Off Unit.
I am suspecting that the VR4 circuit took a over current load or the Varistor saw a Power On Noise Spike that it couldn't handle.

5) After 10 minute Warmup per procedure. With Either 2 or 4 Final Transistor's installed. Right Channel Adjusting Zero DC Balance OK. The Right Channel Idle Current Voltage measures 0.00 volts with the Adjustment at initial setting of Full CCW. The Left Channel ( Good Channel ) reads some voltage with potentiometer at initial Full CCW position. It appears to take twice as much Potentiometer Resistance to get to the 20 mV adjustment per procedure than the Good ( Left Channel ).

I am suspecting that Q6 and Q8 collector current is much higher ( at times ) than what is expected. Guess.

Suggestions:

 

[tsd71]

Always use a dim bulb tester before powering up to house power, all those components would still be good if you did.