Pioneer SX-939: power supply keeps blowing.

Is this a multi-voltage unit?

How many watts is the DBT?

Check the grounds carefully, look for interrupted foils.
 
What's the voltage at the wall. You want the setting higher than the wall voltage. Less strain on the transformer (helps with keeping it cooler). So if your voltage is over 120 but lower than 130, set the selector for 130.
 
If you're having regulator problems, this may help. A lower voltage supplied to the regulator circuitry means the regulator doesn't have to work as hard.
 
EchoWars said:
If you're having regulator problems, this may help. A lower voltage supplied to the regulator circuitry means the regulator doesn't have to work as hard.
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Changed it back to 130V.

I'm betting this is a bad resistor. This power supply has taken a fair bit of heat.
 
There's not a large number of them on the supply board anyway. Just do a sweep and check all of the resistors and see if they all fall within tolerance.
Also, if you haven't already done so, as one suggested earlier check the ceramic cap connected in that circuit.
 
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Replaced the cap and few resistors (12k, 11k, 1.5K, 33ohm) and the ceramic cap. Q5 starts to get hot, hotter than the other parts, using the DBT. Looks like that didn't fix anything. Load off that part of the circuit is still disconnected.
 
Raccoon1400 said:
Replaced the cap and few resistors (12k, 11k, 1.5K, 33ohm) and the ceramic cap. Q5 starts to get hot, hotter than the other parts, using the DBT. Looks like that didn't fix anything. Load off that part of the circuit is still disconnected.
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Did you check the others as well? There are a couple of others that could cause problems in that area as well.
 
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Also to check: is the power symmetrical (is the ground wire of the transformer winding for this part of the supply connected?).
 
gslikker said:
Also to check: is the power symmetrical (is the ground wire of the transformer winding for this part of the supply connected?).
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He said he checked Q4 for shorts. But I'm still a bit suspicious as to whether the device is on the leaky side.
 
glen65 said:
Did you check the others as well? There are a couple of others that could cause problems in that area as well.
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I checked a few others, I'll check some more. Which ones were you thinking of?

glen65 said:
He said he checked Q4 for shorts. But I'm still a bit suspicious as to whether the device is on the leaky side.
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Q4 is new. MJE15033. Would take more than 8A to damage it.
 
I've replaced most of the resistors in that part of the circuit. 11k, 12k, 1.5k. The 5.1k ones, and the 2.7k one have been checked and are ok (at least as far as my DMM can tell).
So everything looks okay, and the load is currently removed. Q5 still gets searing hot within a few seconds on the DBT.
 
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Plenty of cooks in this kitchen but I'd consider looking for what I call non-standard faults. The unit came this way- the previous owner did who knows what. Is q4 or 5 mounted to heat sinks using mica insulators? Could one be breaking down(cracked) under full load? Next thing I'd do (seems crude) but I'd connect one lead to ground and go thru that circuit looking for abnormal low resistance to ground. If I saw something I'd logically begin trying to isolate it by pulling components. It's a crude approach but it has worked for me a couple of times.
 
Lux Man said:
Plenty of cooks in this kitchen but I'd consider looking for what I call non-standard faults. The unit came this way- the previous owner did who knows what. Is q4 or 5 mounted to heat sinks using mica insulators? Could one be breaking down(cracked) under full load? Next thing I'd do (seems crude) but I'd connect one lead to ground and go thru that circuit looking for abnormal low resistance to ground. If I saw something I'd logically begin trying to isolate it by pulling components. It's a crude approach but it has worked for me a couple of times.
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Q4 is mounted to a heatsink, but I replaced it and the insulator.
I can't find any abnormally low resistances to ground.

But wait! Q4 and Q1 are switched! Q4 should be PNP! They got reversed! And it wasn't me! (Though I should have seen it earlier) The last guy must have switched them, as it came to me with Q5 and the zener blown. I put in my replacements according to what parts were on the board, I didn't cross check with the schematic.
 
Wonderful! I'd check the rest too, even though you want to think they are correct. As I mentioned the previous owner did who knows what, my sx-1280 had the same issues- had to go thru every transistor where it had previous hands touching it. Check polarity of caps too.
 
BTDT! Time to start wearing those BIFOCAL Glasses! :D

Best to compare the schematic/board drawing with the actual board BEFORE changing parts out.
 
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