MC2505 dead channel

Discussion in 'McIntosh Audio' started by showett, Jan 19, 2018.

  1. showett

    showett AK Subscriber Subscriber

    Messages:
    88
    Location:
    NW burbs IL
    I just bought 2 MC2505's from eBay for what I thought was a good deal knowing they both had problems. Both amps have the same problem with one completely dead channel, no hum no noise just dead silence. Both meters do show signal at the same level. I have only cleaned the controls so far and have read the service manual and saw the meter circuit comes just before the autoformer and I am wondering is it possible to have a bad autoformer or could both speaker switches be bad. Thanks in advance.
     
  2. dewickt

    dewickt Will fix about Anything

    Messages:
    3,383
    Location:
    Knoxville, TN
    Dirty speaker switch, try jumping the switch as the first step in trouble shooting. Switch can be cleaned with DeOxit, unfold one tab and pry open just far enough to give it a good dose of D5 cleaner, work the switch and it should start working like new, Note - this part is NLA, only have two options, clean it or jump it out of circuit.
     
    John James likes this.
  3. showett

    showett AK Subscriber Subscriber

    Messages:
    88
    Location:
    NW burbs IL
    Awesome, thanks.
     
  4. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,863
    Location:
    France
    Hello showett

    I am wondering is it possible to have a bad autoformer or could both speaker switches be bad.
    Bad autoformer still possible but I can't answer without precise informations
    You have to know that speaker switches are going bad with the time on MC2505 and MC2105 . They are a lot failing ; you can replace them but don't forget that they are double (L+R)
    You can easely test your transformers by switching the speaker position to off and watch the signal with a scope. If you don't do that, and one channel is dead with DC offset , switching the power on with speaker switch in ON position will connect the DC offset to ground and damage resistors (smoke) and output transistors .
    If you want to refix it yourself , give a picture of the power boards with part numbers (ex 043916)
     
  5. showett

    showett AK Subscriber Subscriber

    Messages:
    88
    Location:
    NW burbs IL
    Dirty switches were it, jumped them out and both amps are working perfectly. Now I will try to clean the switches. Next question is LED's or not, I want to re lamp my C28 also.
     
  6. can-man

    can-man Active Member

    Messages:
    331
    Ok dummy hear but what do you mean by jumping the switch?
     
  7. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

    Messages:
    12,242
    Location:
    Duvall, Washington
    Using test leads to connect the wires at the switch.
    You clamp on the ends of the wires where they connect to the switch and then the wire is a connection that "jumps" over/around the switch.

    If that works, the switch is bad so you either unhook the wires and splice them together or solder short wire across the connection making a "jumper".

    There are two channels and each channel has 2 connections on the switch.
    So 4 connections, so 2 jumpers.
    The IN get jumped to OUT on each channel.
     
  8. BilboBaggins

    BilboBaggins AK Subscriber Subscriber

    Messages:
    2,736
    Location:
    Ottawa, Canada
    I had one I was looking at for a friend and it had a dirty speaker switch. I thought it was the autoformer as well, Deoxit, and it was fine for a couple of years. Problem came back and he asked to try to fix it. Deoxit wouldn't fix it this time. I bypassed the switch with a plan to wire in a replacement switch when he gets one.

    Glad you found the problem!
     
  9. showett

    showett AK Subscriber Subscriber

    Messages:
    88
    Location:
    NW burbs IL
    I was amazed that the switch was dead silent no matter how many times I tried it especially considering it's carrying speaker level power.
     
  10. twiiii

    twiiii Super Member

    Messages:
    4,953
    Location:
    west Texas
    fortunately for me the problem has always been the Speaker on and off switch needing to be cleaned.
     

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