early eighties Marantz PM-750 DC

Discussion in 'Marantz Audio' started by Bert 1100, Sep 7, 2017.

  1. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    When this amp showed up on our local auction site for a BIN of 29$ including shipping, I did not hesistate much, even if it was advertised as "dead"
    1982-1984, 2x80 wpc
    Cosmetically it is in a nice shape, except for the (corrosion ? mould?) spots on the buttons.

    And is it dead ? not quite, the one green led comes on, but there is no relay click and zero sound.
    Luckily it looks like nobody was inside before...

    I will start with reading voltages on the the transformer output, and further down to see what's up...
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  2. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    I did some poking around today...
    The good :) :
    I have good AC out of the transformer, proper voltages.
    I have the good voltages on the main supply caps and on the secondary supply caps.

    The not-so-good :naughty: :
    After the secondary supply caps, there is the regulator board that feeds the preamp and phono parts. And the voltages coming out of the regulator board are just much much too low.
    And at the emitter resistors of the output transistors, around 3 VDC, so the relay does its job of not engaging...
    The DC at the output could be from bad voltages to the preamp and the main amp ICs

    So I will start by pulling out the regulated supply's semiconductors (Q8xx) to test them.
    Oh, and R806 reads 10kOhm when it should be 100 Ohm :rolleyes:

    I like a small challenge like this, it's very rewarding when things start working again :D
     
  3. wlhd1610

    wlhd1610 Penny and her new friend Subscriber

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    Looking good!
    Hopefully no SMDs in there or nla chips!

    Bob
     
  4. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    definitely no SMD !
    As for nla chips.....we'll see o_O when the time comes...
     
  5. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    Maybe I should start by disconnecting the loads off the secondary supplies and see what happens to the voltages ?
     
  6. airtime

    airtime Super Member

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    Central Arizona
    Ever try fixing a NAD with SMD - it's a no go!
     

     

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  7. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
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    I disconnected all "users" of the secondary +/- 15VDC and +/- 24VDC and +6VDC...and now I see good voltages coming out of the regulator board :)
    And the relay now clicks in :) :)
    I then feared I had a short somewhere further down in the pre, bringing the voltages down, but "ohming" out the "users" I see nothing frightening...

    I had measured R806 (feeding the -15VDC regulator) in circuit at 10kOhm, the schematic says 100Ohm, so do the color bands on the resistor itself....
    (Edit : I just saw in the manual that R806 is a fusible resistor...)
    So, in go a couple of paralleled 180Ohm resistors (couldn't find a 100Ohm) just to try things out...

    I connect the supplies back, and we have music ! :rockon: :D :banana:

    I still need to connect the phono pre supply back, I hope there no problem there either.
     
    Last edited: Sep 16, 2017
  8. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    The phono pre power supply is connected back, and all seems to be working well :)
    The idle on one channel was way down to 1mV, but both channels adjusted well to the 8mV spec.
    And the DC offset ? I guess there is some servo at work here, I'm measuring 1-2 mV on both channels :)

    Since there are not so many caps and all is very easily accessible, I'll go ahead with a recap and cleanup.
    But that will be later, other project are in line before, I just couldn't help myself and had to find out if I could get this one working :D .
     
  9. wlhd1610

    wlhd1610 Penny and her new friend Subscriber

    Messages:
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    Good deal!!

    Bob
     
  10. slow_jazz

    slow_jazz Lunatic Member

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    Congrats!!!!!!!!
     
  11. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    Update, the amp is now restored and looking good !
    Full wash in the sink with a hose and hot water + kind of windex + alcohol to remove old flux
    All new electrolytics : Nichicon LKS main caps, Nichicon UPW for some PS caps, lots of Elna Silmic II and a few Wima MKS2 films.
    New main diodes and secondary PS diodes.
    New IEC mains connector to replace the obsolete whatever was there.
    Deoxit treatment front to back.

    So it all looks good, but best of all the nice surprise is that is sounds REALLY nice :music:. A bit like the Yamaha CR-820 I restored, a lively dynamic sound, very pleasant, good bass as well !
    The only small problem is that it still has a smell strong enough to make it unworthy :( of living-room duty. I hope with time it will get better.

    Before :

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  12. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    And after :

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  13. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    More pics :D

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    It's pushing 24.5 Volts on 8 Ohm at 1kHz, both channels working, giving 75 WPC before clipping.
    The manual gives it at 80 WPC, my guess is that this is with one channel at a time ?
    Also, I've set the mains voltage for 240VAC, when my line voltage is 230 - 235 VAC...

    So, what's not to like about this amp ? Construction is not bad, still all metal, the looks I like, the sound I like very much, and the price was a bargain !
     
  14. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    Oh, the relay was giving me some trouble with bad contacts. It is a sealed model, so I cut out a bit from the top to access the contacts and polished them.
    But I'd like to get a new relay for it. It's an unusual contact pattern....DEC type AP2U 24VDC...
    See the photo below, the relay was unsoldered.
    Does anyone have a part number for a replacement ? Thanks !

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  15. airtime

    airtime Super Member

    Messages:
    2,107
    Location:
    Central Arizona
    This is an off question, but does it happen to smell like poop? I had that problem with several pieces of that era. I have a feeling it was the PCB or some other component rotting????

    I had one Marantz piece of that era as well. They didn't sound all that bad. Not their best but not any worse than the other offerings of that era. I like to think of it as the dark ages of audio.
     
  16. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    No, it's just the usual mix of warm electronics / old attic / leftovers of windex sort of smell.
    I had an SR-8100DC of the same era that had a VERY strong smell that I thought was of medicine somehow :dunno:

    I have to say this one sounds REALLY nice to me after recap. And that's comparing it to pieces (restored as well) like a Yamaha CR-820, a Marantz 1060, a 2220 and my modern Marantz PM8005 :)
     

     

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  17. robertbet

    robertbet New Member

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    excuse me Bert I'm Roberto from Rome I'm an Md and I don't undestand tecnical if you don't mind please can you write all the capacitors you changed
    best regard roberto
     
  18. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    Hi Roberto, sorry I'm late, I don't come much to AK now.
    I ordered my stuff from Mouser, I'll see tonight when I'm home if I can give you the full list.
    Greetings from Svizzera!
     
  19. Bert 1100

    Bert 1100 Super Member

    Messages:
    1,159
    Location:
    Planquiri, Switzerland
    Here is my list, my order was from Mouser, total was 75 chf which is about 75 usd or 65 eur.
     

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  20. robertbet

    robertbet New Member

    Messages:
    2
    grazie MILLE Bert 1100 !!! compro subito arrivederci roberto
     

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