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Pioneer SX 1010 overheating

M

mu34tant

New Member
No signal no load draws about 5 amps. Both amplifier heat sinks get scalding hot. The +56 v power supply regulator transistor gets hot. +56 is .5v lower than -56. Differential amp transistors have been replaced. Worked for a two years since then. Maybe one has failed? both channels get hot along with regulator heat sink.
No dc offset and idle current is normal. Any suggestions are appreciated.
 
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+56 and -56v difference is within spec. Normal Bias(Idle Current) in the SX-939 and SX-1010 is not what you think. Pioneer Engineer's skipped a step in their math and got it wrong. Do NOTE that the 50mv value for BIAS by PIONEER is INCORRECT!!! Correct value is 25mv per side. Re-adjust as needed and re-evaluate. I doubt it will change things much tho. How cooked is the Power supply (Picture helps). They can get pretty crispy (location (bottom of unit), orientation(upside down), lack of airflow (no room around it!). Normal operating conditions the 1010 should be at most, lukewarm to touch.

No dc offset, meaning it's not adjustable, or it's non existant? Open Pot? What is ACTUAL Idle current. (Before and After Adjustments!)

Voltage readings of the POWER SUPPLY PINS, (vertical format, one pin reading per line), then Voltage readings of the power amp pins.
 
l r ________ p/s
8 52.3 52 ________ 1 .1
9 52.3 52.3 _______ 2 .0
10 .6 .6 _______ 3 .0
11 -52.2 -52.2 _______ 4 .0
12 -52.2 -52.2 _______ 5 .0
13 -.6 -.6 _______ 6 6.7
14 0.0 0.0 _________ 7 0.0
15 0.0 0.0 __________ 8 12.8
16 0.0 0.0 __________ 9 -13.5
17 0.0 0.0 _________ 10 -58.2
18 0.0 0.0 ________ 11 -23.3
19 0.0 0.0 _________ 12 0
20 0.0 0.0 __________ 13 33.0
______________________ 14 56
_____________________ 15 26.1
i can do ac reading if needed
dc offset is adjustable set to .007
i set idle to 30mv two years ago adjusted today to 25
 
With the bias set at 25mV, are the heat sinks getting hot? If so there is no way the current could read that low. I suspect your meter or connections. Decrease the bias setting as low as it will go. Do the heat sinks cool off? With the power off, set your meter to OHMs and read the resistance between the bias testpoints. Should be 1 Ohm.
 
Yes that is the correct bias setting. 1 ohm between idle current test points on both channels. This thing plays music fine. Just overheats. The positive 56v regulator transistor gets hot first then the output heatsinks get hot.power supply.jpg
 
Hmmm, something has to be drawing too much current. Does it get hot with no speakers attached, no input, volume at zero?

Pass transistors on those amps always run very warm, along with the voltage dropping resistors, but with nothing connected to the outputs at 25mV bias the heatsinks should pretty much stay room temp.
 
Actual temps might help. Do you have a laser pointer temp sensor? They are inexpensive. I would start taking temps of the power board and amp board components. It might yield a good clue as to what your problem is.
 
Do you have an oscilloscope? You should look at the output of both channels with no signal, check for oscillation. Also, turn the bias all the way down and see if it gets hot! Sanity check.
 
I replaced the relay driver transistor and the heating stopped. I have replaced all protection circuit transistors. The protection circuit is driven by the bias circuit from the amplifiers, both channels are fed into it. The only problem I have now is the left channel(the channel close to the transformer) will not adjust dc offset. It will move when I turn the pot, c1, but then go back to -.3v. I have replaced all the bias transistors. Any suggestions?
 
well I got the one channel fixed and both are cool now. Except the left channel wont adjust dc offset. I can turn the pot and for a second it will adjust then go right back to where it was. Any suggestions?
 
Seems like the trimmer is working, but then the circuit balances right back to where it wants to be. I would diode check any diodes in the circuit and compare the results to the good channel's diodes, perhaps you have a faulty one. After that I would do an ohm check on all resistors in the circuit and compare again to the good channel. I would not bother to lift any leads as I would be just looking for something that does not match between the two sides.
 
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