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

Discussion in 'Power Amps' started by Armaegis, Mar 30, 2017.

  1. Vishal

    Vishal Rando

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    Lets install new rca connectors and speaker posts

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    Pull out and drill old rca out

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    Bus bar original hole too small has to be drilled bigger

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    looks much better

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    Replacing audio input and out cables with pure silver wire.

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    next is just tidy it all up and do some other needed mods.
     
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  2. Vishal

    Vishal Rando

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    One thing to worry about is shorting against the relays and the AC mains connections.

    Decided to 3D print some covers:

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    didn't have to be an excellent quality print, serves it purpose to stop any shorts and holds things in place.
     
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  3. Melvillian

    Melvillian Friend

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    Bought one of these and I'm a believer, this amp is wonderful. I have them paired with fairly inefficient speakers so I'm probably not getting the most out of the amp, but it still brought my desktop speakers to life.

    @JK47 I know you provided a brief description, but how would you compare the M-25 to the M-22? I will be holding on to the M-22, but was wondering if the M-25 is a better option for my desktop Harbeths.
     
    Last edited: Jun 13, 2020
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  4. Vishal

    Vishal Rando

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    Decided while inside the amp I would add cooling fans, got some 40x40x10mm 12v fans which fit perfectly inside the heatsink under the circuit board covering panel.

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    Temp sensor inplace for testing

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    testing fans and different temp and speed sensors

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    Finally finished, chassis cleaned and polished, anything black and metal was completely resprayed. Came out exceptionally good and looking like new.


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    Last edited: Jun 14, 2020
  5. Vishal

    Vishal Rando

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    Guys what bias have you set yours to? Service manual says 850mv but I feel that's too much, my heatsink temp went to 74 degrees on idle!

    Has anyone here measured there heatsink temp?

    Good news is the fans work well depending on speed of fans I can drop the overall working temp by 4-15 degrees so they serve there purpose.
     
  6. Melvillian

    Melvillian Friend

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    Last edited: Jun 14, 2020
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  7. Vishal

    Vishal Rando

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    I've read through this before and it's not very helpful for actually setting up the bias.

    Has anyone on here actually taken temp readings based on them setting what they think and hear is right, what's everyone running temps on the amp on idle and playing music?

    Best thing to do really is get it on a distortion meter and adjust accordingly.

    Thinking that maybe perhaps because I've resprayed the heatsinks that may cause a thermal transfer of heat.

    Won't know for sure until I do more testing.
     
  8. JK47

    JK47 Guest

    After all, you were warned.

    Yes.
     
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  9. Vishal

    Vishal Rando

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    Well that was helpful. If anyone has a temp gun or the ability to measure temperature could you please measure your hestsinks let me know what you get.

    Thanks
     
  10. skem

    skem Friend

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    I have my bias turned way down. I can neither hear nor measure a distortion difference at lower bias. I have the original Sanken transistors on my output stage. If you have replacement ones, you need to use a distortion analyzer. other people have different experiences. @legarem was very certain he needed his bias hot for it to sound good.

    By the way, for those who have followed @legarem and my search for a good coupling capacitor, I recently switched to a 3.3F AlumenZ, which improved the situation. I briefly liked the 10uF AlumenZ for its better tonal density, but it gives up some speed. Something in between might work.
     
    Last edited: Jul 2, 2020
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  11. Vishal

    Vishal Rando

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    [​IMG]

    One other issue I had come up was a very strange buzzing coming from the transformer, it just felt a little loud and like it was struggling. I thought it was normal until the amp as it got hot started to struggle with loud volumes, the speaker relays would switch on and off intermittently.

    I traced the issue down to the tracks on the bridge rectifiers, I guess over years of use and heat and amps being drawn from this thing it caused issues with the connections.

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    I decided to underpin it and solder wire to complete the track connections, this sorted the issue out, the buzzing stopped and the amp performed so much better! Guys it's worth doing this if your tracks have suffered from heat issues and generally soldering new components in.

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    this next picture shows the components I moved under the board to fit the large chosen 3.3uf coupling cap, this allowed it to fit on the board.

    As for the Bias I tried 850mv but it just wasn't happy there, I have to original output transistors. 750-800mv worked a charm, my temps came down and it runs smoothly and not boiling to the touch!
     
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  12. skem

    skem Friend

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    @Vishal, how did you diagnose the traces as being inadequate. It seems improbable, so I am just wondering how you reached the conclusion.

    there is a protection circuit that throws the relays when the chassis gets hot or when voltage go out of wack.
     
  13. Vishal

    Vishal Rando

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    You are correct about this protection circuit and its not improbable about the issue I had.

    It just a process of elimination and I've come across a similar issue aswell.

    As you said when the voltages go out of whack it the protection circuit comes into play.

    It my situation that area was so discoloured and black due to heat it affected some of tracks linking the rectifiers to the molex plugs, they looked fine but under closed inspection they were weak and damaged.

    This caused the lack of current and voltages to pass through making the buzzing sound and the protection circuit going crazy.

    Once I fixed this I have had no issues at all, I also know of other m22 owners who have done the same.

    Sometimes when there is an issue with a rectifier in a power supply circuit you will hear a buzz from that area or the transformer. It was just logical to me to check that area, of course I took each recitifer and capactitors out and checked them one by one, and they all measured fine. The next thing to do was to sort the circuit traces out and clean the contacts, I even made sure the molex connectors were tighter buy using a special tool so there was a good push fit connection.

    Amps been singing for weeks and not breaking a sweat.
     
  14. Vishal

    Vishal Rando

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    Also look at the picture of the traces, zoom in and if you know what you are looking for you can see where the traces are bad, if you were to scrape away the coating revealing the trace then use a endoscope you would see it, remember how warm it gets in there this wouldve caused expansion and shrinkage, also me physically working on the boards would've made an issue already there weaker.
     
  15. skem

    skem Friend

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    got it.

    I have noticed discoloration in the varnish under the diodes on almost all M22 pics I’ve seen. I replaced my rectifier diodes with some outrageously expensive devices with almost no Vf, and it runs cool.
     
    Last edited: Jul 3, 2020
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  16. Vishal

    Vishal Rando

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    Again correct, once I did this my amp actually ran cooler!
     
  17. PTS

    PTS Friend

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    @Vishal, what transistor type does your amp have? I know some SBAFers have M-22s with the original Sanken, which is awesome.

    Each channel has 2 NPN and 2 PNP power transistors. On mine, someone replaced the original 10MHz Sanken with lesser 2MHz Motorola transistors. Genuine vintage Sanken are becoming very hard to find, sadly.

    The tech I've used said "I recently discovered a very wideband (40MHz) device that should be a drop-in replacement for the Sankens. The hFE is in the same ballpark as the Sankens, which is a bonus. The devices are made by ON Semiconductor for the military and for industry specifically for power amplifiers and high-speed switching circuits. In general a device with a broader bandwidth will have less phase shift and will therefore be more stable into capacitive loads. It probably won’t improve the sonic performance without some adjustments, however, because the overall bandwidth is pulled down by a dominant pole in the front-end. This pole would have to be pushed up in order to extend the bandwidth of the amplifier. As an added benefit, the phase response could be recompensated, and this would reduce the high-frequency distortion. This can all be done, but it would take some time to get right."

    Parts and bench time cost could add up to $500 - do you guys think the sonic differences between transistors is subtle, or is this worth pursuing?
     
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  18. skem

    skem Friend

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  19. PTS

    PTS Friend

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    I spoke to Jeff about this via PM a while back, but interested to get @dBel84's opinion.

    Jeff said that transistors are an area where the technology has moved on since the M-22 was designed, so thinks there's definitely room for improvement. He agreed with my tech that the M-22 is over-compensated. It's not just the dominant pole at the front, but probably also the boat-load of Miller compensation.

    I'm thinking with such a wide band (40MHz) it's possible that the modified ON Semiconductor would be an upgrade over the Sanken? Lower high-frequency distortion, in theory.
     
  20. dBel84

    dBel84 Friend

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    I guess when it comes to restoration, I lean towards the dictum of changing as little as possible.

    In your case, you do not have the original sankens, so the change will impact what you currently have, which in itself is already a change from the original. Does bandwidth itself necessarily equate to better sound. Not in my experience. Will your Amp sound different as a result of the modifications? Most likely , but this is because it would have to be redesigned to accommodate the new transistors. It seems a reasonable experiment to go down this path in the knowledge that this investment will be purely for you and not to add value to your vintage product as no one else is going to see this same value should you decide to sell it. If what you truly want is a wide bandwidth class A amplifier , it might be better to start fresh. There are a few designs out there, or a scaled up Crimson would near exactly define this product.

    I guess what I am trying to say is that the M22 is a vintage design with compromise and charm for what it is and it has stood the test of time. Retrofitting it would lose that charm but is not inherently the wrong thing to do if that is what you prefer.
     
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