Pioneer M-22 Power Amp - Epic Japanese Gear From the Past

On the M-25 there 4 variable resistors on each board.
According to the service manual VR2 adjusts idle current and you can measure it between two terminals inside the amp with a volt meter.

VR1 is center potential adjustment. And you measure it from the speaker terminals with a volt meter. (I think this is the DC offset)

VR3 and VR4 (these are the ones further away from the edge of the board) is for current control circuit adjustment. You need a 1khz signal to the input, connect the terminals with a 4ohm 200w or bigger dummy load, attenuate the signal to to 25V and adjust VR3,4 until you get slightly clipped sine waves on an oscillator. Then conform that the waves are not clipped on a 24V signal.

So if you don’t have the equipment, you shouldn’t replace VR3 and VR4.
In my case the amp sounded normal even without adjusting it, but the right side got too hot to touch after 5-10 minutes while the left side remained only lukewarm. After adjusting it both sides are evenly hot, but much slower and not that hot.

 

It seems to be a great deal easier to adjust bias on the M-22, as the service manual only says "set it to 850 mV between the testpoints".

Apparently the guy repairing my amp is having trouble sourcing a variable resistor with enough current handling capacity to not burn out after a few minutes that also fits on the board.

 

I couldn’t figure out the variable resistors from the service manual, but @JK47 told me he used Bourns 100ohm 1/2W resistors, so I bought those (there are two 470ohm resistors too on the M-25) and they are working fine.

 

@dBel84 spill the beans!

 

Not much to spill. Had intentions of going back to pick up some radial ribbon tweeters but life got busy. The amps are stable and sound great, have not put them into the main system as I wanted to recap and change the twin flex to appropriate iec as well as put decent binding posts on the back. Got most of the parts in, waiting for the Christmas break for a few days off to tinker.

..dB

 

I have not looked at the schematic but I find this a little odd. Most of the bias adjust resistors only use small currents to adjust the current across small transistors which then drive the bigger output devices. 1/2 watt would more than likely be more then enough to last another lifetime.

..dB

 

@JK47 - do you have the dimensions for the 33,000 uF capacitors?

 

Thanks. Seems like its a 75 mm x 100 mm size. I think some capacitors on this page would work if no one can find orignals.

http://www.ftcap.de/en/products/electrolytic-capacitors/screw-terminals/

 

Sorry for the bad angle

Joined the M-22 club over the weekend. It's mostly @sphinxvc's setup, which we used to compare my M-22 to his First Watt F3. Going from the F3 to the M-22, the most immediate difference is with the cleanliness of the treble, overall resolution, and the improvements in imaging. The M-22 was able to get louder and had more control over the bass due to its higher power output as well. Overall it's an extremely solid SS amp if you can find one in good condition or price, and if you have the appropriate speakers (90db+ at 8 ohms).

A note for those with 100v M-22's or M-25's: don't cheap out on the step down transformer. Initially I used a 300w Acupwr because the Nissyo 1100w transformer that was shipped with my unit broke (it was modded with a Shunyata cable and the soldering came apart in shipment). I got the Nissyo fixed over the weekend and it was a clear step up from the Acupwr. I'm not sure if it was because the Acupwr was lacking in power, or if it was because the improper voltage. My Acupwr measured at 105.5-106v whereas the Nissyo measured at 99-100v. @sphinxvc, @Dr. Higgs, and I cleary heard the difference when we swapped from the Nissyo to the Acupwr. The M-22 lost a lot of bass control. It was muddy and indistinct in comparison.

 

Larger transformer --> lower impedance.
Unlike power cords, this matter is serious, and objectively explainable.

 

I ordered the Nissyo @thegunner100 recommended this morning from Amazon Japan for approximately $80 shipped, much cheaper than the Acupwr I recommended earlier in this thread. I'm glad a better, cheaper option was found. Now the Acupwr will be put into storage along side a 100V Stax SRM-1 mkII PRO, that may or not not be sold to someone that put a deposit down on it 6 months ago... Anyway, thanks again for finding the Nissyo and comparing head to head.

https://www.amazon.co.jp/gp/product/B0017H4QC8/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

How did the M-22 stack up to the Almarro?

 

Too bad you guys aren't closer. Would love to throw my XA30.5 in the mix.

 

For what it's worth, I'm also using (in Europe) a NISSYO SK-550EX (rated at 550W). Seems to work well, bass control is good, although I guess it doesn't really matter with (very sensitive) headphones.

 

It seems so. I had no idea Pioneer did stuff like this.

 

So, my M22 has a problem with bias - according to my tech, the bias is normal for about 10 minutes after a new bias pot is installed (somewhere around 800 mV), then it drops to something like 50 mV after that. Any ideas on what might cause this?

 

Sounds normal, the bias will change until the temperature has stabilized. It's mentioned in the manual to coarse adjust the bias initially, then fine adjust once 100% warmed up.

 

It didn't drop by 50 mV, it dropped _to_ 50 mV, and can't be readjusted back to service manual values after that.

 

Might be a bad solder joint. As the unit heats up a connection breaks due to thermal expansion.