AJD1964
Well-Known Member
This unit was all working 100% until today, during a right channel amp board overhaul. Before this, other than cleaning switches and pots, I overhauled the Power Supply board (054) first. It was working so well after the PS was done, I wanted to get the power amp boards done to prevent the ticking time bombs. I ran this unit for over 2 hours Friday night to test the power supply – all good up until now. Here’s what has been done so far:
Power Supply Assembly (AWR-054-0)
Replaced all electrolytic caps, transistors, and the 3 Zener diodes, D8, D9 & D10.
Main filter caps also replaced; 18000uF 75v. I used the original bleeders.
.. Again – all good after power supply work, and all voltages were in spec at the pins.
Power Amp Assembly (AWH-032-0) replacement parts installed:
C1: .68uF 25v CSSA: 505-MKS20.68/63/5 .68uF 63v
C4: 10uF 16v CEA: 647-UPW1E100MDD 10uF 25v
C8: 330uF 6.3v CEA: 647-UPW1A331MPD1TD 330uF 10v
Q1: 2SA726: 512-KSA992FBU
Q2: 2SA726: 512-KSA992FBU
Q3: 2SA726: 512-KSA992FBU
Q4: 2SB528: 512-KSA1220AYS *optional, did it anyway.
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU
Q8: 2SB528: 512-KSA1220AYS *optional, did it anyway.
Q9: 2SC1448: 512-KSC2073TU *optional, did it anyway.
Q11: 2SC945: 512-KSC1845FTA *optional, did it anyway.
Q12: 2SA733: 512-KSA992FBU *optional, did it anyway.
Q7: 2SD358; & Q10: 2SA740; Still Original in the Board – Do not have replacements at this time.
D1 – D8; Still Original in Board.
VR1: 2.2K ohm multi offset: 652-3296P-1-202LF 2K ohm 25 turn (At approx. 1K Ohms center when installed).
VR2: 470 ohm single bias: 652-3386H-1-501LF 500 ohm single turn (AT Zero Ohms when installed).
Output Transistors: Still original (in this channel at least); Cleaned out old thermal glue from Heatsink, and cleaned micas; Reinstalled with new white thermal compound.
The symptom:
After re-installing the amp board, and output/heatsinks, I went to power up on the DBT (150W clear bulb) to begin the offset check on VR1. Turned on the switch, the bulb dimmed down to normal, but I noticed it was staying in protection, but bulb still dim. Then the magic dragon puffed – by taking out R17 470 ohm resistor. Obviously, I never got far enough to adjust any offset or idle current with the new pots.
Things I’ve checked:
- Transistor orientation – All of them at least 3 times over (assuming silkscreens are correct on these boards).
- Wire connections at top & bottom of the board – all good, actually had to fix a couple during the job.
- Traces on board – I do not see any lifts, or bad solder joints. A LOT nicer shape than the 054 board!
- I see Q5 & Q6’s emitter resistors tie to my smoked R17.
- I see the STV3’s are straight off the collector of Q5. If I fried them, I have a nice parts car I guess.
Where would you guys begin from here?
Power Supply Assembly (AWR-054-0)
Replaced all electrolytic caps, transistors, and the 3 Zener diodes, D8, D9 & D10.
Main filter caps also replaced; 18000uF 75v. I used the original bleeders.
.. Again – all good after power supply work, and all voltages were in spec at the pins.
Power Amp Assembly (AWH-032-0) replacement parts installed:
C1: .68uF 25v CSSA: 505-MKS20.68/63/5 .68uF 63v
C4: 10uF 16v CEA: 647-UPW1E100MDD 10uF 25v
C8: 330uF 6.3v CEA: 647-UPW1A331MPD1TD 330uF 10v
Q1: 2SA726: 512-KSA992FBU
Q2: 2SA726: 512-KSA992FBU
Q3: 2SA726: 512-KSA992FBU
Q4: 2SB528: 512-KSA1220AYS *optional, did it anyway.
Q5: 2SC1451: 512-KSC3503DSTU
Q6: 2SC1451: 512-KSC3503DSTU
Q8: 2SB528: 512-KSA1220AYS *optional, did it anyway.
Q9: 2SC1448: 512-KSC2073TU *optional, did it anyway.
Q11: 2SC945: 512-KSC1845FTA *optional, did it anyway.
Q12: 2SA733: 512-KSA992FBU *optional, did it anyway.
Q7: 2SD358; & Q10: 2SA740; Still Original in the Board – Do not have replacements at this time.
D1 – D8; Still Original in Board.
VR1: 2.2K ohm multi offset: 652-3296P-1-202LF 2K ohm 25 turn (At approx. 1K Ohms center when installed).
VR2: 470 ohm single bias: 652-3386H-1-501LF 500 ohm single turn (AT Zero Ohms when installed).
Output Transistors: Still original (in this channel at least); Cleaned out old thermal glue from Heatsink, and cleaned micas; Reinstalled with new white thermal compound.
The symptom:
After re-installing the amp board, and output/heatsinks, I went to power up on the DBT (150W clear bulb) to begin the offset check on VR1. Turned on the switch, the bulb dimmed down to normal, but I noticed it was staying in protection, but bulb still dim. Then the magic dragon puffed – by taking out R17 470 ohm resistor. Obviously, I never got far enough to adjust any offset or idle current with the new pots.
Things I’ve checked:
- Transistor orientation – All of them at least 3 times over (assuming silkscreens are correct on these boards).
- Wire connections at top & bottom of the board – all good, actually had to fix a couple during the job.
- Traces on board – I do not see any lifts, or bad solder joints. A LOT nicer shape than the 054 board!
- I see Q5 & Q6’s emitter resistors tie to my smoked R17.
- I see the STV3’s are straight off the collector of Q5. If I fried them, I have a nice parts car I guess.
Where would you guys begin from here?