QX-949 Power supply issue

[KeithD]

I am in the process of re-capping and replacing bad transistors on this excellent unit. I have been playing it for about a month, but had noticed some static on the rear channels. So decided to get the 2SA726's out of the power amp boards as well as the 2SC1451's. Completed the two power amp boards, but static was still present. Decided to continue on the the power supply board. Recapped that and replaced Q6: 2SA720 with KSA1013YTA. Static issue was still there. Voltage measurements on pin 5 of the power amp boards would not go below about 0.5V and was quite variable, causing the static. Measured the power inputs to the board, and found B1 and B2 were a bit variable.

I was warming it up this afternoon to make more measurements. When I came back to it after about 20 minutes, it was in protection. After several measurements found that pin 6 was reading 3V (should be 13.5V) and pin 8 (+B3) was reading 2.4V (should be 30V!). Both were quite variable. Power on that circuit comes from pins 4 (37.9VAC) and 5 (38VAC).

There are four transistors on that circuit: Q1:2SD313P, Q2:2SC1318, Q3:2SC945, Q4:2SD313P. Emitter of Q4 feeds pin 6. Emitter of Q1 feeds pin 8.

Any thoughts on what might have failed here? What should I test?

 

[petehall347]

noise may be headphone assembly ...
check fuse connections ..also an area where noise can come from .

 

[KeithD]

Yes, checked fuses and they were original and showed signs of corrosion, so I cleaned the holders and replaced all fuses.

Bigger issue is the low voltage on pins 6 and 8 on the power supply...

 

[petehall347]

what voltage d2 d3 cathodes ? then q1 q2 q3 voltages b c and e .

 

[petehall347]

q4 wont work properly until q1 is happy .

 

[KeithD]

Readings on Power Supply board:

D2 cathode: 0.1VDC, 38.7VAC
D3 cathode: 52VDC, ~0.2VAC

Transistors: E, C, B

Q1: 3.5, 53, 4
Q2: 4, 52, 6
Q3: 1.5, 6, 1
Q4: 0, 47, 4

All transistor values very highly variable, voltages were jumping around a lot, so I'm reporting what seemed like the average value.

 

[petehall347]

check d2 .
its strange readings though as d2 d3 cathodes connect together .or they are supposed to .
see if d2 anode is at 52v dc

 

[KeithD]

check d2 .
its strange readings though as d2 d3 cathodes connect together .or they are supposed to .
see if d2 anode is at 52v dc

Sorry, measured anode end of D2 instead of cathode. Cathode end of D2 is also at 52VDC.

 

[larryderouin]

Shotgun the Q's on the P.S. The 2sd313p's are a known weak sister in the power supply. Pioneer under-designed the 747 and 949 P.S.'s like the SX-1010 and they tend to cook everything on the board.

 

[petehall347]

also check d6 .. 13v zener .

 

[KeithD]

Shotgun the Q's on the P.S. The 2sd313p's are a known weak sister in the power supply. Pioneer under-designed the 747 and 949 P.S.'s like the SX-1010 and they tend to cook everything on the board.

OK Larry, that sound like a good idea. Here is what I have for replacements:

QX-949 AWR-038-0 Q1 2SD313P-E or D 512-KSC2073TU
QX-949 AWR-038-0 Q2 2SC1318-Q or R 512-KSC2383Y or 512-KSC2690AY
QX-949 AWR-038-0 Q3 2SC945-R or Q 512-KSC1845FTA
QX-949 AWR-038-0 Q4 2SD313P-E or D 512-KSC2073TU
QX-949 AWR-038-0 Q5 2SB507P-E or D 512-KSA940TU
QX-949 AWR-038-0 Q6 2SA720-Q or R 512-KSA1013YTA

Already replaced Q6, but can do all the rest if you agree on the replacement list. Which do you recommend for Q2?

also check d2 .. 13v zener .

Pete, I don't think D2 is a zener. Service manual has:

QX-949 AWR-038-0 D2 SIB01-02 or 1S1886

Replacement: 1N4004

Correct?

 

[larryderouin]

D7 & D8 are the Zeners. D7 is 14V 1/2W and D8 is 13V 1/2W. You can use a 1n4004 for D1 thru D5 as necessary.

Q list looks ok.

 

[petehall347]

whoops ..good catch ..i meant d6 .
d8 is for the negative side

d6 d8 13v ..d7 14v

just corrected my earlier post

 

[larryderouin]

How the Hell did I miss THAT. Waiting on new glasses after 15 years on the old ones. Cheap bastard that I am. LOL!

KeithD. Add D-6 to the list of Zeners. Same as D-8.

 

[petehall347]

i have one schematic here shows them all as regular diodes ..the other schematic is correct and shows zeners where they are supposed to be zeners ..

 

[KeithD]

Q list looks ok.

I've got two replacements listed for Q2: KSC2383Y or KSC2690AY. Is there a preference?

With respect to diodes, here is my replacement list. I will be adding this to the BOM thread if it looks ok to you guys.

QX-949 AWR-038-0 D1 SIB01-01 or 1S1885 1N4004
QX-949 AWR-038-0 D2 SIB01-02 or 1S1886 1N4004
QX-949 AWR-038-0 D3 SIB01-02 or 1S1886 1N4004
QX-949 AWR-038-0 D4 SIB01-02 or 1S1886 1N4004
QX-949 AWR-038-0 D5 SIB01-02 or 1S1886 1N4004
QX-949 AWR-038-0 D6 WZ-130 (Zener diode) 512-1N5243B
QX-949 AWR-038-0 D7 WZ-140 (Zener diode ) 512-1N5244B
QX-949 AWR-038-0 D8 WZ-130 (Zener diode ) 512-1N5243B

 

[larryderouin]

Ksc2690ays preferred
diode and zener list looks good

 

[KeithD]

OK, thanks Larry. Will be working on power supply transistors tonight.

By the way, there is also a Power Supply "B" board (AWR-039) that has caps and diodes on it. Diodes are SR3AM. Don't know if you've had experience with the B board. I was seeing fluctuation in the B2 voltage to the rear channels and wondered if that was the source of static and voltage variability in the power amp rear outputs? Could a bad diode on the B board cause that? The B2 and B1 are also coupled to the filter caps, but I have no reason to suspect those at this point.

 

[larryderouin]

I'd check the voltages at the output of the AWR-039 board before ripping everything out. It's possible but not probable that one of the diodes in the brige is bad. If they are ok, then just replace the caps and be done with it. I'd bet you have a transistor in one of the rear boards that is breaking down OR a bad cap on one of the boards. The 949 has a lot of 2sa725's and 726's on it. Replace those and any 2sc1312's which are on the TOP TEN TRANSISTORS LIST. KSA-992 and KSC1845's.

If the sr3am are bad, replace with 1n5404.

 

[KeithD]

I'd check the voltages at the output of the AWR-039 board before ripping everything out. It's possible but not probable that one of the diodes in the brige is bad. If they are ok, then just replace the caps and be done with it. I'd bet you have a transistor in one of the rear boards that is breaking down OR a bad cap on one of the boards. The 949 has a lot of 2sa725's and 726's on it. Replace those and any 2sc1312's which are on the TOP TEN TRANSISTORS LIST. KSA-992 and KSC1845's.

If the sr3am are bad, replace with 1n5404.

Thanks, I have replaced all e-caps on both power amp boards as well as 2SA726's and 2SC1451's. If the change in transistors on the power supply A board does not solve the static, I'll move back to the power amp boards and replace:

QX-949 AWH-027-0 Q5 2SC869-C, D 512-KSC2383YTA
QX-949 AWH-027-0 Q6 2SC869-C, D 512-KSC2383YTA

After that its:

QX-949 AWH-027-0 Q9 2SD357-C, D 512-KSC2690AYS
QX-949 AWH-027-0 Q10 2SD358-C, D 512-KSC2690AYS
QX-949 AWH-027-0 Q11 2SB527-C, D 512-KSA1220AYS
QX-949 AWH-027-0 Q12 2SB528-C, D 512-KSA1220AYS