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Pioneer SX 626 distorted channel eliminated but...

ondaride's picture

Hi folks,
I picked up this unit and started it with variac step by step.
After hooking up headphones, I noticed distortion in L channel. The sound as if the output transistors had been overdriven.

I gave a good spark of de-oxit around volume control, balance, selector switches and on/off rotary switch.

After dissassembling the unit and examining, I removed the output transistors, reapplied head-sink compound on each and re-soldered them each.

The distortion is GONE, but...

when I touched each and every transistor, the 'outer' transistor of the L channel was substantially hotter than the rest. Than I measured collector voltages and found that it's too high on 'outer' transistor. Supposed to be 29-sh Volts, but was near 45V! I tried to adjust it with VR1 pot, but was not successfull. I was only able to drop it by 4V down to max. The other channel I only moved pot a small notch to be exactly half of 'inner' transistor's collector.

I checked R15, R17 and R21 for proper values and these three are all fine and give good reading with small tolerance...

So far no culprit found, but have no desire to put together unit being not properly operating. I've also read about major overhaul which I'm not desire to do. Appreciate your help in advance!

Re: PIONEER SX 626 distorted channel eliminated but...

RV1 is one of the bias controls. I looked at the service manual and there doesn't appear to be a real bias adjustment procedure. I recommend you adjust these controls (one for each channel) for 1.1 volts DC on the Bases of transistors Q3 and Q4. If bias can't be adjusted properly then you probably have one or more bad components in the circuit.
It should be noted that this receiver uses two NPN transistors in the output stages; there should be 58 volts on one Collector and 29 volts on the other in each stage.

ondaride's picture

Re: PIONEER SX 626 distorted channel eliminated but...

Well. After reading Q3 Vb = 7.7V!
Q9 Vc= 55V -- a OK
Q11 Vc= 41.6!
Any attempt to decrease would end up with overheat, but I've always stayed alert to disconnect and reset the pot settings back to the original mark. WIERD.
Then, when I tried measuring Vc of Q1 = 17V which is 3V over. The left channel humming dissapears as I touch my test lead of Q1's collector terminal and sound was heard similar to tube amp! That was kinda good news, but still would never believe that this amp will survive 3 hours of listening. It will blow the slo-blo fuse, but output transistor can do that faster than fuse sometimes :(
Than I tested 2 caps C1 and C3 and their readings are perfect. I tested L-channel large filter cap also reads perfect. Not sure if they're still good, but input and driving transistors definitely being replaced. I tested output ones and they test good, but not sure how long they wer running that hot...
Since I could easily touch the right channell all of transistors, I doubt that I will have to overhaul it, but still looks like full overhaul of the L channel to me.