Completely dead SX-1250

So I set both VR2 pots to around the middle and took it off the dim-bulb. With just straight 120v, no bulb, each VR2 pot ranges from 0-10 mv or so. Maxes out at 10. I have the positive probe on pin 7 and the negative on pin 19. Both boards are 100% plugged in. I am just using the regular preamp jumpers, not the 5.1k resistor that the manual calls for. I am adjusting the pot that's right by the top, which appears to be VR2 according to the manual. It does adjust the voltage, just not very much. With both VR2 pots set in the middle or so, this plays music fine through headphones. Any idea what I'm doing wrong?
 
The amp boards need the regulated +- 65 and the unregulated from the big filters to run.
The regulated +- 65 is good.
What about the big filter caps? Read the voltages at the screw terminals. If you never checked these filters before; you ground the black probe to the chassis and use the red probe to read DC from the screw terminals.
They should be +-70v's DC.
 
Alright, so previously the outputs were not bolted down--I had removed the screws to test them and hadn't screwed them back in. I screwed all the outputs in, and then brought it up on the dim-bulb and got a range of 0-50 mv or so. I tried it without the dim-bulb and I can easily bias both channels to 100 mv. Not sure what the story was, but we seem to be fine.

On a related note, I saw a restoration thread online that suggested biasing the SX-1250 at 75 mv rather than 100, for cooler running and safer operation. Thoughts?

Also, the rails drop to like 55 and -55v when all the boards are hooked up. Is that normal? They're a nice solid 65/-65 without the output boards attached.
 
Alright, so previously the outputs were not bolted down-
:)
The bias depends on how hot you want to run the amp at idle. the higher the bias, the lower the thd, to a point. 30-100 mA bias/pair are the usual ranges for a EF design.
The PS to the o/p stage is unregulated, as you draw current the average voltage will fall, the sch shows nominal voltages, with a spec'd biased ops and with full load, 8 ohm condition.
 
We here in the forum set the Idles at 65m's or so for the 1250's. You won't notice any issues and the set runs cooler.
The outside of the outputs is the collector connection of the transistor. The screws hold down the outputs to the heatsink but are insulated from any metal chassis bits. Such as the heatsinks themselves. The sockets are wired to the amp board. On the 1250 its the molex plugs on top. Without the collector there is no voltage to run the outputs. Hence no current flow through the amp. Nothing to bias.
Can't say I ever saw that before but I've only been doing this repair thing for ten years.
Funny.
Sounds like your back in business. The wonky thermal fuse with its loose connection was all it was(?).
Did you repaste the outputs?
Think about rebuilding the Power supply. Its a good for the reliability.
Sounds like you have the chops for this work.
 
Love you guys. Didn't repaste the outputs, but the stuff on there seemed fine. I can add more if I need to. I greatly appreciate all the assistance.
 
So, obviously it died again. Worked great for a week and then just wouldn't start up one day. All fuses are fine except the thermal fuse is blown. A friend suggested that one of the main caps might be bad, but I tested those and they all tested right around 16-18k uf with minimal ESR. The caps on one channel had leakage of around 1.3% and the ones on the other had it around 2%, but I don't think that's likely to be an issue. Any thoughts? I can go through the whole procedure again, but I'm assuming the outputs are fine. Should I just replace the thermal fuse, dim-bulb it, and start it up?
 
Recapping the stabilizer isn't the best approach. Completely rebuilding it is.
That could be a likely issue but not high on the probability list.
There's a short in the set or the mechanical components are not operating correctly, ie, the soft start relay. 653-LY1F-DC48

Monitor the unreg supply that powers the soft-start relay after you do your repairs.

Replace the thermal fuse:
Disconnect the Stabilizer board
Disconnect the unregulated DC from the large filter caps and monitor the DC voltage from the rectifier on the AWR-107 fuse board pin 13, looking for 48V and or any AC voltage. To measure AC ripple on a DC supply, put a 0.1uf capacitor (100v or more) in series with the red multimeter lead.
Watch the soft start relay when operating the power switch. AC and DC voltage.
 
Hi, I’m back. I disconnected the stabilizer board and the main caps. I am getting around 48.6 volts DC on pin 13. When I check for AC I am seeing around 200-400 mv, using the .1 uf cap like you said.
 
Hi, I’m back. I disconnected the stabilizer board and the main caps. I am getting around 48.6 volts DC on pin 13. When I check for AC I am seeing around 200-400 mv, using the .1 uf cap like you said.

Check both the positive and negative voltages.
 
When I try it that way I get around 275mv one way and -275mv the other way (red lead to ground, black lead to .1 uf cap to pin 13.

It does change though—the voltage went all the way up to about -1v and then abruptly dropped to -250mv again.

Then after that it slowly rose to -375mv and then dropped right to to -250mv.
 
Connect the caps back up (Not sure what I was thinking) and rebuild the stabilizer using the list for the diodes, transistors and replace the soft start relay. Call it a day.
 
OK, I already fully recapped the stabilizer board before. So I just need to do the diodes, transistors, and relay then?
 
Redid everything but three diodes and the relay. It comes out of protect fine and seems to work. I'll get those other four from Mouser.
 
I put in the last three diodes and replaced the soft-start relay. It seems fine now. Voltages on the stabilizer board dropped to around 62v after the rebuild, so I adjusted those. I tried powering it off and on a dozen times and it is playing fine. We'll give it some time and see if it pops again.
 
I checked the amp board settings and it's still dead-on at where I set it last time--0v offset and 65mv bias, both channels.

However, after running it for about an hour at a very normal listening volume (like, the sound of someone talking), both heat sinks are moderately warm. Is this normal for the 1250?
 
Can you quantify "moderately warm" in numbers? Mine gets warm to touch, or about 120°F after a half hour or so of running even at no volume. I set my idle currents at 35 mA (lower than the original 65 mA) for the new output transistors, and don't notice any degradation in sound quality.
 
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