Hitachi HMA-7500 MkII project

Discussion in 'Solid State' started by Bert 1100, Feb 12, 2018.

  1. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,150
    Location:
    Planquiri, Switzerland
    I bought this power amp about a month ago, from a local hoarder. It wasn't cheap (300$) but the fact that I could see it in person and test it beforehand ....and it has great big meters and MosFET, and twin power supply....OK, OK, I couldn't help myself :biggrin::rolleyes:.

    Anyway, It's all working reasonably well. The only few things are burnt-out bulbs, intermittent leds (4 Ohm - 8 Ohm - BTL) and a non-adjusting idle current for the left channel (oops, that had me worried !)

    That idle not adjusting turned out to be just a very dirty trimpot, the wiper just made zero contact onto the track...after desoldering to test and a thourough clean with deoxit it's adjusting just fine.

    This model is older than I first thought, from 1980, soon 40 years old.....

    The goal is to restore it fully : replace all electrolytic caps, all fusible resistors, most ceramic caps (more likely not needed but as long as I'm in there, and it's cheap enough...).
    I'll also redo the lighting to leds.
    And the main and secondary rectifiers will be changed to soft recovery ones.

    Right now it's pretty much all disassembled (a bit frightening :) ), and I have all of the parts. For the main caps (8 of them...) I chose Nichicon LS series, upping the capacitance from 5600uF to 6800uf.

    Next is a good wash for everything apart from the transformers, relays and meters.

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  2. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,150
    Location:
    Planquiri, Switzerland
    Now a quick question : does anybody know of drop-in replacement relays ?
    I couldn't see any with the proper pin placement, the contact legs are offset.
    the originals are DEC type AP2U 24VDC...
    See the photo below, the relay was unsoldered (photo from my Marantz PM-750 DC, same relay)

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  3. tarior

    tarior Dirty pool, old man? Subscriber

    Messages:
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    Location:
    Kelso, Washington
    Throw your old bias trimmers in the garbage and buy new ones.
    That pic isn't particularly helpful.
     
  4. rottalpha

    rottalpha Yamaholic Subscriber

    Messages:
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    Location:
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    Really nice amp.

    Yes get rid of those trimpots.

    :lurk:
     
  5. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,150
    Location:
    Planquiri, Switzerland

    I got new Bourns trimmers, 6 identical ones in there, 500 ohm. They are very easy to adjust, so I went with single turn ones.
    About the relay photo, I'll unsolder one and give actual dimensions.
     
  6. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,150
    Location:
    Planquiri, Switzerland
    Ok, these are the relays I'd like to find a drop-in replacement for...anybody ?
    They are working well, but in a rebuild like this having new ones would be good. If not I have some Schrack relays that I can glue upside down on the board next to the original placement and run wires to them....but if there are drop-in replacements, that would be much nicer :)

    dimensions are in millimeters.
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  7. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,150
    Location:
    Planquiri, Switzerland
    The 2 boards and the heatsinks are now cleaned up ( dishwater soap, hot water, then alcohol to remove the old flux, then soap and water again, then compressed air)

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    I've now removed the glue, was quite a pain, it took a good soaking with acetone.

    I wonder why my leds were intermittent :rflmao::rolleyes:

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    Q801 and Q802 were getting quite hot in their flameproof sleeves...

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    rottalpha and awillia6 like this.
  8. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,150
    Location:
    Planquiri, Switzerland
    I've about finished with the 2 boards, but as usual :no: :rant: I've forgotten a few parts in the order. This has been happening pretty much with all my latest projects :rolleyes:

    I'm missing 2x 220uf 6.3V caps, I think I used them up to finish the Yam CR-620 I was doing last :whip::biggrin:
    Also I failed to see 2 of the 10uf 16V were bipolar, so I don't have those either...

    Anybody to help with the relays ? (see post N°6)
     
  9. rottalpha

    rottalpha Yamaholic Subscriber

    Messages:
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    Location:
    Middle of Crook County
    And I thought that only Yamaha is really good with provisioning these kinds of solder joints
     

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