Fixing UA La-610 Compressor (need schematics with voltage points)

Discussion in 'Tech Talk - Electronic Repair Modifications DIY' started by pcrecord, Jun 20, 2016.

  1. Boswell

    Boswell Moderator Distinguished Member

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    Great work, Marco!

    Did you measure the resistor you took out, or was it obviously open-circuit?
     
  2. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    Yes, the circuit was open.
     
  3. rmburrow

    rmburrow Active Member

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    Glad to hear you got the LA610 units working again. Also glad to hear the problem was an inexpensive resistor. DIY repair work saves $$...the investment is time and perhaps some money spent on a digital multimeter, audio generator, and a scope.
     
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  4. DonnyThompson

    DonnyThompson Distinguished Member

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    Great thread!
     
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  5. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    I couldn't have done it myself with new gear that are based electronic chips. A circuit like the LA-610 may seem old fashion to many but it has the advantage of having nearly all its part exposed and easy to reach. :)

    I had far less chances with my focusrite Saffire 56 which now serves as dual headphone amp...
     
  6. djmalo

    djmalo Active Member

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    This has been a very helpful thread. In fact, having the same exact problem with my LA610 currently. Wondering if you remember the number of the resistor you replaced that fixed it? Saw some discoloration on R20 on this board, but didn’t wanted to reach out first.
     
  7. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    My resistor blowed because the circuit became out of specs with time. If your unit has the same problem, I can't give you an exact value at the risk of causing more damage because in my situation I had to test 5 value starting with the original before I got acceptable behaviours...

    This was the answer I sent in private :


    Hi DJMalo,
    Not being an expert in electronics, there is one thing I know ; it's rare that the lightning hits twice at the same place :) Unless it's a design flaw.

    I had the chance (or not) to have a blown out resistor in my unit and I could see the damaged part being carbonised.
    Since it was visually evident, I proceeded in trying the same value, then lower but it wasn't helping.. so I went higher value until the compression had a normal behaviour. One thing I did was to remove the black cover of the T4 to test if it was defective. By pointing a light at it, it made the compression react. So I knew that part of the circuit wasn't blown out.

    My LA610s are old and this unit I bought from ebay might have been runing non stop 24/7 in a studio for what I know.
    So the specs changes.

    Now be carefull if you don't have experience working in such complex circuits. To make the tubes work it uses high voltage that can kill someone...
    Good luck
     
  8. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    Again a copy of the conversation with DJMalo, for the sake of having this thread with the complete informations :
    ----
    In any case here is what I said in the thread :

    Intriguing enough, I first thought a smaller value would send more current to the light in the compressor but no it was the other way around. I guess it uses some kind of signal cancellation and the current needs to be lower at one end to let it hit the Opto cell. So after a few tests, I ended up with a 4.99k resistor instead of the original 1.2k. Man these old preamps circuits are alive and changing overtime and that's why we love them !!! As for now, the action of the compressor knob gets better precision that it ever did since I got it. (used) For exemple ; to get the same degree of compression I needed to put it at 7 when I got it and now at 4 (very similar to my other LA-610)
    I've used it everyday all week without a glitch, so I think I'm good ! :love:


    Now the danger is that with adjusting this value it may impact the the longevity of other components.. So if you do so, you do at your own risk.
    Start at 1.2K or around and go up with different resistor value. . .
    Don't you ever short the chassis and any of the surrounding circuit near the tube and capacitors... Those capacitors hold a lot of voltage even when not plugged to AC current
     

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