Pioneer M-22 Stuck in Protection, No Offset at Zobel Networks

Basspig

Basspig

Active Member
Dealing with a Pioneer M-22 that won't energize relay. Checked the obvious first, looked for DC offsets. Found both channels under 20mV and working fine.
Then started pulling and testing the transistors in the protection circuit, all produced good curves on a Tek 576.
Looking with a thermal camera, I can see R63 and R73 standing out like beacons, running much warmer than the surrounding devices. Since I had no voltage on the base of Q25, I replaced the 330uF cap, thinking it likely shorted. Nope. Just for good measure, I replaced the rest of the electrolytic caps in this circuit. Still no go.
That leaves resistors and diodes. I've been able to measure about 60% of the resistors in circuit. The rest are in parallel with other devices. Started lifting one side of a few to check, all checked within 1% or marked value, but didn't get far with this due to the fact that leads are bent over and hard to desolder and pull without damaging the PCB.
This is turning into an extended troubleshooting process. Any time-saving tips would be appreciated. Thanks.

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Check for the -13v.
You should be reading something at the base of Q25. Better check for the 36v too, (even though the amps are working).
What's the base readings on the over current detectors, Q19, Q20?
 
Last edited:
zebulon1 said:
Check for the -13v.
You should be reading something at the base of Q25. Better check for the 36v too, (even though the amps are working).
What's the base readings on the over current detectors, Q19, Q20?
Click to expand...

Where is the -13V rail? I can only see on the schematic, +/-45V, 28V & 36V call outs.
36V rail = 36.15V
Base of Q25 = .007V
Base of Q19 = .79V
Base of Q20 = .196V

I'm at a loss about the 13V rail.
 
In the power supply there is a 13v zener D17. It might be shorted causing the missing -13v and the hot R63???
 
I had to replace the relay driver transistor 2sC1384 with a KSC2690AYS in a few SX pioneers. no idea if you need the same replacement.
 
If I recall correctly, there was a bi-polar cap in the M-22's that often went bad. It would cause a (physical) chattering or buzzing kind of sound upon turn on and wouldn't "fire" up. I discovered that there were a bunch of these amps (from the late 70's) sitting in Pioneer's Long Beach warehouse around 1987. So, when I sold a couple dozen of them to my dealers, I'd recommended starting them up very slowly on a variac before initial use. I wonder if this same capacitor issue is causing your problem? I also recall that the capacitor was hard to source(?). Wish I could recall which cap it was.
 
I have -13V at the zener.

I have tested the relay driver and all other transistors on that part of the PCB on a curve tracer. They all produce good curves and I gave each one 2 full minutes to heat up, in case there is a latent failure.
 
I think I found the problem. When I touched the 4.7K resistor R46 with my probe, I heard the relay pull in.
I had already reflowed the entire protection circuit. I just now reflowed the power supply and replaced the capacitors.
Going to let it burn in for a while. See if it holds.
 
After completely reassembling the amplifier, I tested it once more and... after about a minute, the relay dropped out... again. Didn't drop out while the amplifier was apart and the power module and PCB were dangling from the wires. So either there's a cracked trace somewhere, or maybe a metal film resistor is intermittent. Gonna tear it all apart again tomorrow.
 
Disassembled the amplifier and changed out the 4,.7K resistor. Reassembled power stage and powered it up. Relay kicked in, but about 5 seconds later, it dropped out. That's what it'd been doing initially from a cold start. Having poked that resistor with the probe and having the relay engage, I thought it was either bad solder joint, or, perhaps the resistor itself. But it's neither. I still think problem is a bad connection somewhere in that area.
 
Have it hanging out like this and the relay engaged for a while, then dropped out. I still have -13.V at the zener. But nothing at all on the transistor base of the relay driver.
There are numerous diodes in the protection circuit, but I am having trouble cross referencing the part numbers to available diodes. Was thinking of just replacing all of them at this point.


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I would monitor pins 27 and 28 in your first post.

This is the output of each channel. L = 27, R = 28.

If they stay constantly less than 40mV, I'd probably replace Q25, the relay driver. if they kind of jump around, the protection relay might be working as designed.

did you recap the protection board? I've done less than 10 of similar pioneer protection boards last year so not an expert.

I haven't had problems with the diodes but have replaced the old electrolytic caps and some transistors. sometimes, it is nice to start with fresh components like diodes and take them off the naughty list.

while I haven't replaced diodes, I'd might replace the 1s2473 with 1n4148 and the 10e2 with 1n4004. feel free to post your replacements. I didn't spend much time looking for replacements and would use what I have in my spare parts. I don't think it is too critical (and could be wrong).
 
The first thing I did was signal tracing. Amplifier offsets less than 20mV at the time., adjusted to 1mV. Then tracked the voltages from the relay driver backwards. None of them were correct. Pulled and tested every transistor in protect ckt with a curve tracer for 2 minutes each until warmed up. All good curves, no failures.
Recapped the entire protect circuit.
Replaced the resistor that feeds the -13V zener, as the relay came back on when I touched a probe there. Right now, it only works for a few seconds and then cuts out.
I need a can of freeze spray, as I suspect a component temperature issue.
 
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