Mr74 stereo bulb blows...

Discussion in 'McIntosh Audio' started by macman007, Jun 9, 2005.

  1. dewickt

    dewickt Will fix about Anything

    Messages:
    3,387
    Location:
    Knoxville, TN
    Watch out for the tuners that use the 37V bulb for the stereo lamp, not all of them do, if the demultiplex has a 270 ohm 1 watt resistor it will use a #1828 37V lamp, if there is a jumper in place of the resistor it will use a #1835 55V lamp, check first before replacing the lamp...MR73, MR74, MX113 etc...
    MR73 that has no AM probably has a bad FET (dual gate MOS) Q5, located in a socket next to the tining capacitor, I use a 3SK45 as a replacement.
     

     

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  2. dj_AmTraX

    dj_AmTraX Super Member

    Messages:
    1,833
    Location:
    San Francisco
    It has a #1828 bulb, but it doesn't work. I don't know if that is the oringinal bulb. I have a multimeter, but don't know where to probe. Can someone please post some pictures of where the demultiples or the resister is located?
     
  3. EastPoint

    EastPoint AK Subscriber Subscriber

    Messages:
    4,359
    Location:
    Milwaukee, WI
    Hi folks. I put the wrong bulb in the stereo indicator on my MR-77 and now it won't light, just like the OP. I'm assuming I blew the transistor as well. Is the board at the back left the MPX board? The two transistors with orange dots on them are marked BC 238--are those the ones I should replace? Here's a pic.

    IMG_7569.JPG
     
  4. dewickt

    dewickt Will fix about Anything

    Messages:
    3,387
    Location:
    Knoxville, TN
    The transistor with the small aluminum heat sink is the lamp driver, try the correct bulb as the driver is fairly robust and may not be blown.
     
  5. EastPoint

    EastPoint AK Subscriber Subscriber

    Messages:
    4,359
    Location:
    Milwaukee, WI
    I tried the correct bulb and it definitely won't light. The bulb is good--I tested it with a battery and it lit dimly.

    What I actually tested in the circuit was a drop-in LED replacement for a 47 bulb, which flickered briefly but wouldn't stay lit.
     
  6. EastPoint

    EastPoint AK Subscriber Subscriber

    Messages:
    4,359
    Location:
    Milwaukee, WI
    Well whaddaya know, the new bulb works! Thank you so much.
     
  7. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

    Messages:
    3,292
    Location:
    NE Ohio
    Fixing the dial light required two plastic strips; a milky white diffuser strip next to the lamp and the clear red strip next to the hole. If you only use the clear red strip, the filament for the lamp will show through. I used a double thickness of red because the color wasn't deep enough. I got the plastic sheets from JoAnn's. Push the two strips down into the tube and then cut them flush at the top when you're happy.
     

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