Pioneer SX-1250 Loud POP, Protection Mode, and Burning Sme

[Ben Rothman]

Hey guys, my SX-1250 is down for the count. This unit is all original, I’ve only cleaned the pots on it. I have a restored 1010 that’s my daily driver, and the thought was to restore the 1250 when something happened. Well, something finally happened. Last night I was listening to a record, and I tried to hit the loudness switch. In the middle of pressing it a loud bang and pop came through the speakers, and the 1250 immediately went into protect mode. It smelled horrible, something was burning inside. I unplugged the unit and brought it down to the bench. When I turn the unit on, the first relay clicks, but the second relay will not click on. I’m not sure what happened, but I think it’s time for a full rebuild. On a side note, my speakers didn’t get damaged! The protection circuit did its job.

 

[jd3rdcoast]

Start with the Stabilizer board rebuild. That is the weakest link and typically needs full rebuild. I just did one. Oddly enough it would come out of protection but the +65V would not adjust. Had multiple shorted and open transistors, diodes and couple heat damaged resistors. Replaced all transistors, regulators, caps, diodes and out of spec resistors. All good now. That might at least get you back to a functional state so you can then proceed with a more in-depth rebuild vs. shot gunning and getting into deeper problems. Easy to pull those boards to work on.

 

[Ben Rothman]

Start with the Stabilizer board rebuild. That is the weakest link and typically needs full rebuild. I just did one. Oddly enough it would come out of protection but the +65V would not adjust. Had multiple shorted and open transistors, diodes and couple heat damaged resistors. Replaced all transistors, regulators, caps, diodes and out of spec resistors. All good now. That might at least get you back to a functional state so you can then proceed with a more in-depth rebuild vs. shot gunning and getting into deeper problems. Easy to pull those boards to work on.

I thinks that’s a smart idea. I’ve opened up the unit and there is no obvious sign of physical damage which is a good thing. The main 12amp fuse did blow.

 

[zebulon1]

Oh man,
I guess I was tired of waiting for a refresh.
It could be a component on the boards for the pre-amp?
EQ under the cover? +-23v supplies?
You would have to remove the faceplate and access the boards.
There are several things you could do to isolate it but I think a good visual check is in order.
If your going to rebuild it anyway.
Best to find and correct the problems before you restore. Unless you've done at least ten of them. (Piano tuner story of mine.)

 

[ivandezande]

I’ve bought and installed quite a few of the new stabilizer board off ebay made by an AK member and it’s so, so much better than any rebuild of the original is (which I have rebuilt a ton of).

 

[petehall347]

use your nose to find the area of damage ,

 

[jd3rdcoast]

I thinks that’s a smart idea. I’ve opened up the unit and there is no obvious sign of physical damage which is a good thing. The main 12amp fuse did blow.

Exact same symptom of the one brought to me. 12A fuse blown, guessing when Stabilizer board just couldn't do it anymore.
Here are my notes on the failure:

Replace main fuse w/10Amp AGC 250V. DBT stays lit (high current draw) with both amplifiers disconnected. Problems diagnosed on Power Stabilizer Board (AWR-106). Several open transistors , capacitors, Zener Diodes, burnt resistors. VR1 will not adjust for -65V. Q1(Short B-E), Q2(Short B-C), Q3 Short B-C), Q4 Shorted (B/C/E), DZ1(Open), R5 Burnt, C15(Open). Rebuilt Stabilizer Board=Transistors, Capacitors, Diodes couple resistors. Now can adjust +/-65V & 25V. Calibrate Amplifiers DC Offset and Bias.​

When fired back up could not set the -65 so went back and finished those transistors as well then all was well. Might as we do the other few transistors and be done with it. And did full recap as well.

 

[Ben Rothman]

Oh man,
I guess I was tired of waiting for a refresh.
It could be a component on the boards for the pre-amp?
EQ under the cover? +-23v supplies?
You would have to remove the faceplate and access the boards.
There are several things you could do to isolate it but I think a good visual check is in order.
If your going to rebuild it anyway.
Best to find and correct the problems before you restore. Unless you've done at least ten of them. (Piano tuner story of mine.)

Hey sorry for the late reply! School is in full swing until my thanksgiving break next week. I think it could be on the EQ board. I do wonder if the output transistors blew during the “big bang”.

 

[Ben Rothman]

I’ve bought and installed quite a few of the new stabilizer board off ebay made by an AK member and it’s so, so much better than any rebuild of the original is (which I have rebuilt a ton of).

I know exactly who you’re talking about. I recently purchased their SX-1010 power supply, and the build quality is fantastic.

 

[Ben Rothman]

Exact same symptom of the one brought to me. 12A fuse blown, guessing when Stabilizer board just couldn't do it anymore.
Here are my notes on the failure:

Replace main fuse w/10Amp AGC 250V. DBT stays lit (high current draw) with both amplifiers disconnected. Problems diagnosed on Power Stabilizer Board (AWR-106). Several open transistors , capacitors, Zener Diodes, burnt resistors. VR1 will not adjust for -65V. Q1(Short B-E), Q2(Short B-C), Q3 Short B-C), Q4 Shorted (B/C/E), DZ1(Open), R5 Burnt, C15(Open). Rebuilt Stabilizer Board=Transistors, Capacitors, Diodes couple resistors. Now can adjust +/-65V & 25V. Calibrate Amplifiers DC Offset and Bias.​

When fired back up could not set the -65 so went back and finished those transistors as well then all was well. Might as we do the other few transistors and be done with it. And did full recap as well.

These are good notes to have. My plan is to have the issue diagnosed, then repair it. After that I will proceed with a full restoration to make sure the 1250 can play for another 50 years.

 

[Ben Rothman]

use your nose to find the area of damage ,

Once I opened up the 1250, the smell had gone away.