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Pioneer SX-2500 and that protection circuit

tabpat

tabpat

Member
Hello all,
I have a Pioneer SX-2500 that is giving me (us - if you include a local tech) fits. It plays well up until 1/4 volume, then goes into protection mode. It takes a few seconds and comes back. Now...if I pan only to the right channel, it will rattle the windows in my house. Powerful, deep rich sound as long as your hearing can tolerate it. However, if I pan only to the left...it cuts out again at 1/4 volume. This is a capacity coupled amp. The protection circuit is a strange little thing without relays. The bias has been checked and re-checked. All caps are new as well as many transistors. Everything seems like it should, but it seems like the left channel ends it's day at about 5 watts.

Any ideas?
 
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The protection circuit is basically a timed mute circuit. The input to each power amplifier is muted by Q6 and Q7. That part of the circuit is probably OK. So the input to the protection circuit is where the problem is. One channel's input circuit is R1, R3, D1, C1, and Q1. The other channel is R2, R4, D2, C2, and Q2. After those components it's all common for both channels. Leaky transistor would be my guess. The rest of those components are easily checked. Which components are for which channel I don't know as the schematic really sucks. Is the turn-on delay time the same as the recover from protection delay time, should be.

Craig
 
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Pins 1 and 2 are one channel's input, 3 and 4 are the other channel. Pins 1 and 3 monitor the voltage and therefore the current thru bottom output xsistor emitter resistor of each channel. Pins 2 and 4 monitor the output of each channel. At idle there should be no voltage, AC or DC, on pins 2 and 4, pins 1 and 3 will have a few mv on them. Both Q1 and Q2 should be off as they both need a positive voltage on their bases to turn on. When turned on they short to gnd one side of the multivibrator resetting it starting the time delay which in turn mutes the inputs to the amps with Q6 and Q7. Clear as mud now, right?

Craig
 
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FWIW I'd start with the lower emitter resistor in the suspect channel - One of the functions is to mute when over current is sensed. If that resistor is out of spec, protect will trigger early.
If you can't find any bad components, then, if it isn't too difficult, I would reverse wires coming into protect: 1 to 3, 3 to 1, and 2 to 4, 4 to 2 (all 4 must be done at the same time) then see if problems stays the same (problem with amplifier) or switches too other side (problem with protect board) YMMV
 
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