Odd issues with Pioneer SX-737 rebuild

[EastPoint]

Hi folks. I have an SX-737 that I am rebuilding. When I got it, it was filthy and was missing one of the output transistors. I replaced that with a new MJ21194G. I rebuilt the entire amp board--all new transistors and new electrolytics. I was pretty careful with polarity. I have replaced all the blue Sanyo caps except in the tuner board so far, and I replaced all the transistors in the preamp board. I redid the power supply entirely--all new electrolytics, transistors, and diodes. I replaced a few transistors and caps in the protection board. I have not added the diode mod or replaced the relay yet.

I cleaned and lubed the controls with Deoxit and Faderlube.

So, it plays fine and sounds good, BUT I have some issues.

1. I cannot get the bias down below 50mv on the left channel.
2. I cannot get the DC offset below 0.25v on both channels.
3. I get a decent amount of FM playing on the right channel on the Aux input, when I turn up the volume.
4. FM radio will not completely go silent when I turn the volume all the way down.

I took it off the dim-bulb and found that power supply voltages increased, but it still has the issues. I am using the updated bias/DC offset instructions found here. I know the service manual is incorrect on this.

Power supply voltages are:
1. zero
2. zero
3. fluctuates around -.6
4. fluctuates around -.6
5. zero
6. fluctuates around -.025
7. -38
8. zero
9. 35.3
10 .-35.64
11. fluctuates around -.03
12. -11.5
13. -11.5
14. 35.16
15. 26.9
16. 27.05
17. 4.5
18. zero
19. 13
20. 13
21. 13

 

[Watthour]

Regarding the offset, did you replace the Q1/3 and Q2/4 pairs? Did you happen to check the gain one those pairs and match them?

 

[EastPoint]

Yes, I replaced those with KSA992s. They're from the same batch and should be relatively matched.

 

[john stumpf]

you've no doubt noticed that some of your power supply voltages are low to quite low.did you check the power supply board before you moved on to the next?

 

[EastPoint]

Not yet. I’m curious what’s wrong with the ps board—I redid everything on there.

 

[john stumpf]

maybe nothing. if it were i, i would go back and check the pinout and polarity of EVERY part i installed and when your sure go back and do it again.i installed a transistor wrong last night but caught it before i soldered it in and i'll bet everyone here has done it as its sooo easy to do. use a magnifying glass if necessary. i do,. look for good solder connections and "gear adrift" like clipped leads becoming an un intentional part of the circuit. did you just change the one missing o/p transistor?

 

[zebulon1]

PS board AWR-057-0:
R10, 1K resistor needs some checking. Your missing the +24v for the EQ.
Pins 1 through 6, 11 and 8 are AC. The PS looks good except for Pin 17.

FM issue: Make sure the harness is routed correctly and all the shields are installed.

The amp board had an issue before you received it (Some failed repair?). Everything needs checking, except the ceramics. Lift one of the two legs from the components and check for condition. Take notes or mark the good components so not to recheck them. Compare their twin brothers to the other board for a cross check.
I would separate the pre amp from the main by desoldering the signal input pins from the board wire-wrap and all from the amp. Pins 7 and 15. Then work the amp looking for self inflected troubleshooting mistakes. Poor connections and such. Tape off the loose pins until the amp is sorted.

No tricks to this repair. You have need too dive in and check things out. A piecemeal approach will waste time and add frustration.

 

[EastPoint]

OK, I checked R10 and it's fine. One of its solder joints was loose, so I resoldered that. I have like 6v on pin 17 on the PS now. I checked all my soldering and transistor pinouts and it all looks fine to me.

The FM bleeding on Aux is a known SX-737 issue apparently and is not an issue. It doesn't happen when there's an Aux source hooked up.

I desoldered and pulled pins 7 and 15. I checked all the resistors on the amp board and they're all fine. I tried swapping back the original driver transistors, and no change in the problem. I noticed that the socket for the MJE transistor wasn't quite seated in the heat-sink, and fixed that, but no change.

What voltages should I have on the various output transistor pins? I noticed that I have voltage on amp board pins 21 and 31, but not on pins 24 and 27. Is this supposed to be the case?

 

[zebulon1]

Double check the voltage from pin 17. I think you should have 24 volts. The 34 volt is working so it has too be something around the resistor R10.

 

[EastPoint]

I have like 26 volts going into resistor R10 and then 4.5 volts coming out of it. Pin 17 is 4.5v now.

 

[larryderouin]

Change R-10 with a 1K 1/2W resistor. Also Work back from pin 15 (Should be 31.5vdc with 117VAC on the plug) to find the low voltage source on pin 15.

 

[Watthour]

R9, R10, and C19 are suspicious. There may also be some inordinate load at the tone amp, mic amp, or EQ amp.

 

[EastPoint]

I replaced R9, R10, and C19 with new parts that test good. No changes.
270 ohm resistor has 35v into it and 26.6v out
1k resistor has 26.5v into it and 5v out

Pin 15 is at 26.6v
Pin 17 is at 5v, fluctuating a little from about 5-5.1v

 

[zebulon1]

Remove pin 6 from the EQ board. Pin, wire wrap and all, like the amp board pins you did earlier.
See if the voltage in pin 17 of the PS responds with the correct voltage.

 

[EastPoint]

Well that's silly. I pulled the EQ board pin 6.
Now I have 32v on PS pin 15, 32v on PS pin 17, and the amp board measurements appear to be fine as well. So presumably I have something shorted in the EQ board?

 

[zebulon1]

Correct.
I would rebuild the EQ board. It has the 2SA725 problematic transistors and the 2SC1313's which can be updated to the better signal Fairchilds.
2SA725 = KSA992
2SC1313 = KSC1845
Both Fairchilds are ECB pin out.

 

[EastPoint]

I fully rebuilt the phono eq board. One of the orange electrolytics measured 54 ohms and way under for uf. Now with it hooked up I’m getting 22.76v on pin 17 and 30.34 on pin 15.

 

[zebulon1]

You don't waste any time.
Good job.

 

[EastPoint]

OK, so I shotgunned the rest of the mic board and the audio amplifier board (with the pots attached). I reattached the two amp board pins as well. Currently the only things not fully redone in this are the tuner board (replaced most of the sky-blue caps, one left I think), main caps, relay, and some of the protection board. Everything else is fully recapped and re-transistored.

Power supply voltages appear to be fine.
Amp board has no trouble with bias and DC offset--I set those easily.

However, I am still getting music with the volume at zero. Both channels, on all inputs, play noticeably when the volume is all the way down. Could it be a bad volume control?

 

[Watthour]

Could be the wipers on the volume pot have moved, that there is conductive sludge in the pot tracks and/or across the connections, or the spindle/stops have been distorted.