Pioneer M-22 Rebuild

darkhawk22

darkhawk22

New Member
Hey Guys,

I've just purchased an M-22 and it's in great condition for the age. It works well but cuts out on the right channel at low volume. I'm going to do a recap of the boards and replace the switching relays. I've ordered most of the caps but i need some help crossing the ASR-025 and ASR-026 Relays? Any help appreciated. There is a guy on eBay that does rebuilds on these so i sent him an inquiry to see what relays he uses but have not heard anything back yet.

Thanks!
 
Do not assume that a component is faulty unless you measure or test signals through it. I do not recommend Crap-shoot troubleshooting to determine which comp to change.
darkhawk22 said:
It works well but cuts out on the right channel at low volume.
Click to expand...
This does not sound like a relay or an ecap problem. Try cleaning the pot with deoxit and follow up with fader lube to see if that solves the vol control problem.
 
Recapping is NOT a repair operation. It is done after a unit is fully functional.

Speaker protection relay contacts could need some work, or there might be a fractured solder joint.

Enjoy,
Rich P
 
I've just done a lot of work getting my M22 running again - and Jeromach (another member here and on diyaudio) has completely rebuilt an M22.

I'd agree look at the relays first for those symptoms. Next on the list would be to replace the 2SA798 input pairs as these were causing lots of chaos in my M22. Replace these with a hfe-matched pair of 2SA992s per side.
 
Ok so boards are recapped, the relays were cleaned, still had intermittent sound on one of the channels. I opened it up and went to test voltage across relay, Pop, and smoke from the right side. I narrowed it down to one of the 2SA744's and noticed that I did not have one of the capacitor leads soldered while I was in there. I have some replacement 2SA744's on order and tested the unit with only the leads to the right side main transistors pulled and all seems stable. Also, in the mean-time I have ordered the caps and diodes to replace the ones that get crazy hot. I'm hoping that the only thing I popped was the 2SA744. Anything in the path of that I should check?

Also, I did get in some replacement OMRON relays but the casing and overall dimensions are too large. They could probably be used on their sides however and the internal contact plating is silver. Maybe I'll try that once I get it back to a fully functional state :)
 
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Also, prior to installation I wanted to verify. I ordered for the 30D2 diodes a replacement type NTE5802 - 3A @ 200V.
 
OK - so when I got my M22 (I just sold it today coincidentally) it had replacement outputs - MJXXX24 and 25. 2SA744 are one type of your outputs.

Have you pulled and tested all of your outputs?

Where did you order the replacement 2SA744s from? If these are random ebay finds I wouldn't recommend them - instead I'd suggest replacing with the same types that were in my M22 - new and from a reputable vendor. Same goes for your NTE diodes - don't take the risk with this amp - NTE parts are generally bought in batches by NTE and resold to approximate specs under their own codes - you don't really know what you are getting. Find the voltage and current specs of the 30D2 (these are in the power supply right?) and get more good parts to the same specs - ie: onsemi or Fairchild.

You really need to build a DBT for this - install a larger bulb, say 150W

You don't want to take any risks with this class A circuit, there is much more current going around in this circuit and far more chances of a cascade of things going wrong very quickly.
 
Thanks for the input, yes I tested all the main output transistors and only one was showing a short.

The replacements are from a reputable vendor, haven't gotten those yet. I ordered some replacement pots from Digikey to install prior to installing the replacement output transistor. Too bad you sold yours I need someone with a functional unit to get some readings on some pins to verify I have the proper voltage etc... going to the main transistors prior to reattaching the cables. Specifically Pins 13, 14, 15 and 16.
 
Some good news, the 2SA744 replacement came so I checked pins to left side to measure voltages and had the same approx value on the right side. I installed the 744 and powered it up and all went well. I set the idle current to 850 right on after 10 Mins. Set the outputs to approx 0mV, one was 15 approx. and the other 20mV approx. Re-installed the unit on my system and now I'm back to where I was originally! The left side is intermittent and either crackly at high volume level or inaudible. It's not the speaker cable and not the input to the amp as I have checked both of those by swapping. Any ideas on what i'm missing? Maybe a bad board transistor on the left side?
 
Replace the relay - sorry I dont know a replacement. Good to see that looks like your problem.

You should also replace the input pair 2SA798 on each side - these eventually fail and are probably exposed to more heat in this class A circuit.
 
M-22 is working excellent! Cleaned the relay again and it has been working great ever since. Sounds great! Recently found an M-25 as well, will post pics of that one.
 
Hallo, I have one little question.

A friend of mine bought an M22 a long time ago and found now out during an inspection that Motorola MJ15024 / MJ15025 is mounted instead of the 2SC1402 / 2SA744 output transistors...maybe the whole circuit has been revised to apply the Motorola ... I don't know, I have couldn't check.

Was it a bad move by the previous owners? How much could sound be different? Could it be worse or different enough with Motorola?
Why were the MJXXX24 / 25 and not other equivalents or the originals used?
Many thanks for the answers.
 
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rcs16 said:
Ask the person who did it?
Click to expand...

Hi, sorry read the beginning well: "A friend of mine bought an M22 a long time ago and found now....."


So it is impossible to ask the old owner who bought it from another owner who bought it from another owner ... so endlessly. Lol
 
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best-groove said:
Motorola MJ15024 / MJ15025 is mounted instead of the 2SC1402 / 2SA744 output transistors
Click to expand...

I wouldnt consider this an issue.

In an M22 I bought years ago - these outputs were installed. They are modern and thoroughly field-tested. These 24/25 are made to run at high current.

No other changes had been made in my circuit downstream from the outputs. The DC offset and bias were all fine - and the amp was working fine - so chances are your friends' probably is too. Ie: at the time the original outputs were replaced, hopefully the person doing this re-calibrated the amplifier
 
slimecity said:
these outputs were installed.
Click to expand...

Very well, this is an encouraging news to tell my friend that it is no longer alone in the world to own an M22 with these power transistors! :D
I also hope that the repairer has performed all the fundamental calibrations; the amp that my friend owns has never had a problem since I bought it and the heat

Thank you very much.
 
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