Technics SA-5760 Receiver No Output

At first, the protection relay would click on and off sporadically and I was able to trace the problem down to the driver board having voltage spikes in the signal path of one channel. The board mounts vertically, so I had this bright idea of removing it and putting it in the freezer for a minute, then reinstalling it and warming up the transistors with the tip of my soldering iron one at a time. Silly me, the cold must have killed whatever was working intermittently and now I have positive rail voltage in all sorts of places on the board that it shouldn't be.

-I've tested all of the diodes on the board and they are all good.
-I've replaced all of the electrolytic capacitors on the board because I plan on recapping the entire unit anyhow.
-I've replaced all but one of the transistors on the positive, "push" side, of the channel
-I swapped the remaining positive, "push" side, transistor out with the one from the working channel

I'm still getting about +58V in all sorts of places it shouldn't be. Including on the base and emitter of both the main push and pull output transistors for one channel.

It won't let me upload the service manual I found on HiFiEngine, but I do have a diagram for just one channel that I've been writing voltages on. Everything in blue ink is from the working channel, voltages in pencil are from the non-working channel.

Can anyone give me some ideas for what else I can check? I'm 99% sure that the positive rail voltage is leaking into the pull-down side of the circuit, but I can't tell from where.

 

I almost planted one once, but I'm far more of an audio nut than I gardener, it seems.

That's where I was looking too. I've already replaced TR605 with a new generic and just now swapped out D601, but I'm still getting about 58V on the base / emitters of the main output transistors for this side.

Near as I can tell, R669 is good. My multimeter doesn't read down that low very well, but it looks to be in good shape physically and doesn't show as open.

Frozen circuit boards seemed like a good idea at the time, but I think I'll try some freeze spray next time instead. Definitely one of those "seemed like a good idea at the time" moments.

Here's a little bit better schematic snippet, sans scribbles, but with the added bonus of power transistors!

 

Collector of TR607 is +60.2V
R611 Shows a voltage drop of almost zero (it fluctuates upwards of 10mV, but always goes back down to zero after a second or two). When I power everything off and test the resistance on it, I get 15.58k ohms

 

voltage at the wiper of R601 = 23.91V
voltage at TR601 emitters = -21.20V
Voltage across R613 = 38.4V
voltage at TR601 input base and feedback base - To be honest, I don't know for sure which is which. I would assume that the left-most side would be input and the right-most side, feedback. Regardless, the C601 side has -7.94V and the R617 side has -7.73V.

The blue on the original drawing are measurements that I took from the working channel

 

Hmmm.. I've had that one pulled before and tested resistance. Right now, in circuit, I get 100k and climbing. If I reverse the dmm leads, I get 0.L. I may try swapping it with TR604 first just to confirm. Maybe it's only leaking under load

 

Yep, I still have about +60V on the base and emitter of the main output transistors on one side. *sigh*

 

Yeah, that does make sense. You know what? I went back and double checked the measurement on the emitters of TR601 and I got a positive value. Then I remembered that I switched them from one channel to another, so I checked the other one and it's negative. I'm waiting for the capacitors to discharge so I can check c-e resistance. It didn't solve the problem with the rest of the unit, but I bet one of them is bad. I'll have to go through and double check the other measurements and get you an updated schematic.

I've actually dealt with that supplier before for those pesky, hard to find, Darlington power packs and had really good luck with them.

 

I'm starting to think that TR601 and its counterpart, TR602, are both at different states of failure. Swapping them may have fixed the negative voltage leak, but I finally realized that I cut off the expected voltage listing when I tried printing off a portion of the schematic (woops!). I updated the measurements before I realized that the schematic had the expected voltages listed ABOVE the part numbers at the very top of the page.

Long story short, now both channels have about 60V on the base and emitter of each output transistor and none of the voltages around TR601 / 602 are where they should be.

 

That makes sense. I went ahead and ordered a couple. If that doesn't get everything back where it should be, I'm sure I'll be back. Thanks for your help guys!

 

Just to cross all my eyes and cross all my tee's, the replacement A798's did fix the voltage issue. The relay clicks after about 3-5 seconds and the bases and emmitters of the output transistors are back down where they should be. Sine wave looks good on the scope too!

Thanks again for your help guys!