Mr74 stereo bulb blows...

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

  1. macman007

    macman007 Mcintosh..Made In USA

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    My newly aquired MR74 has a burnt out Stereo indicator bulb. Like a good lil Ak'r, I immediatly set to replace it . I put in a #44 and it blew immediately. I Think its supposed to have a #47 but I'm not sure. I took an 1866 out of it. Both the 44 and 47 are 6.3 volts with .15a and .25a respectivly. Wouldnt the 44 work also but be dimmer? Or is it the other way around? Do I have excessive voltage perhaps? Going to measure socket voltage now.

    35.4 Vdc....in stereo mode no bulb in socket terminal to terminal.


    Wish me Luck

    Artie
     
    Last edited: Jun 9, 2005
  2. skippy_ps

    skippy_ps AK Subscriber Subscriber

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    Yuk yuk. MPX lamp for a 73 and 77 is #1828, 37.5 volts. I'm guessing that the 74 takes the same bulb.

    It would seem that a 6v #44/47 bulb would be pretty unhappy in that socket. :D

    Murray
     
  3. macman007

    macman007 Mcintosh..Made In USA

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    You are correct sir. Just goes to show whats in there isn't necessarily whats right. Garbage in garbage out. Well off to the 'shack I go. Thanks dude.

    Artie
     
  4. skippy_ps

    skippy_ps AK Subscriber Subscriber

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    I would have done the same thing. I'm just glad it was you instead of me. :D

    Murray
     
  5. macman007

    macman007 Mcintosh..Made In USA

    Messages:
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    Location:
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    Now I have the correct bulb but no illumination when in stereo. I have 37.8vdc at the bulb caseing with the bulb installed and the same on the yellow wire on the socket lug.I am reading 5.7vdc between the socket case and the chassis. Now , what am I missing here?

    I dont mind learning as I go but this is going from a simple bulb replacement to who knows what. However, I wont give up. I know with help I can lick this. I'm looking at the schematic and i cant find where the wire goes after it leaves the socket. Comes in red and goes out yellow. Is the yellow the ground? Or does a transistor have to switch to provide ground. Or am I barking up the wrong tree. Between the busted 2205 and this I think I'm all Mc'd out ....lol. Well after a good nights rest it may make sense. Or maybe not



    Artie
     
  6. skippy_ps

    skippy_ps AK Subscriber Subscriber

    Messages:
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    Just taking a quick look at this on my MR-77, you should show open between the bulb socket and chassis. It looks like the socket is hot from the pwr supply board and the center pin gets ground from the mpx board when stereo is detected. That's what it looks like to me anyway.

    Murray
     
  7. macman007

    macman007 Mcintosh..Made In USA

    Messages:
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    the 2 transistors that are in the mpx board after the resistor have voltage on all three legs on them between the emmitter of one and the collector of another.According to the schematic its supposed to be 0v at both these points. Maybe I "opened up" a transistor when the bulb blew...or perhaps the meter probe arced at the socket and shorted it.They are NPN's so a quick trip to the shack will provide me with replacements. I will swap them in following the schematic and recheck voltages afterwards. Maybe I'll get lucky. It WAS working...killing 6.3v builbs...now nothing
     
  8. dewickt

    dewickt Will fix about Anything

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    Found out long ago that if a #47 is put in place of the #1828 it will zap the driving transistor, most any NPN capable of .6W will work.
     
  9. macman007

    macman007 Mcintosh..Made In USA

    Messages:
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    Location:
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    NPN BC 238's sound right for 2? Might as well do the pair.


    Artie
     
  10. macman007

    macman007 Mcintosh..Made In USA

    Messages:
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    Location:
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    Success......it seems I now have a stereo light. It was the transistor. I must have shorted it out inadvertantly while testing, or when the 6.3v bulb was inserted. Lesson...just because it was in there dont just replace it with the same. Always go back to basics and consult the schematic. I could not have done this without the help of Terry and my fellow AK'ers. For a total of 1.67 cents I now have the stereo light functioning and all seems well with the MR74. What a sweet unit. The strength of the tuning section in this unit is amazing. It picks up stations my other radios can only pass by, both fm and am. These vintage units have a much better capture than any modern tuner I have encountered. The blue multipath light is glowing wide and blue...it's really a trip and the multiplex ligte is bright and red when the unit is in stereo. I am nothing but impresse to say the least....... :D

    Now if i can get the Mc2205 back in one piece....

    Thanks To One and All,

    Artie
     
    Last edited: Jun 12, 2005
  11. Paul R

    Paul R Member

    Messages:
    61
    Where can I obtain... cm 1828 8.3mm (T2) bulb ....for my mx113 stereo indicator..the filament in the one I have is broken.Thanks in advance
     
  12. dgwojo

    dgwojo AK Subscriber Subscriber

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    I have 20 left if you need a couple, Dave. :yes:
     
  13. Paul R

    Paul R Member

    Messages:
    61
    Dave thanks ...2 would do just fine ...cost and shippng charges please and I will take care of it asap
     
  14. mike175gr

    mike175gr Super Member

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    This thread is another example of what a great site this is!
     
  15. macman007

    macman007 Mcintosh..Made In USA

    Messages:
    1,077
    Location:
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    Be glad to send you one. Hell, I got 9 left. I bought a box whole box for 4.59$
     
  16. Paul R

    Paul R Member

    Messages:
    61
    re 1828 bulb

    Thanks MACMAN007 DGWOJO has already obliged me..I also need the dial indicatoe bulb for my MX113..if you can help pls let me know...cheers
     
  17. dewickt

    dewickt Will fix about Anything

    Messages:
    3,378
    Location:
    Knoxville, TN
    Dial indicator bulbs - 95% are good - red filter is burned and no light can get thru, small needle nose, grab filter and pull out about 1/4" to get to new material, will be red again.
     
  18. Paul R

    Paul R Member

    Messages:
    61
    Terry ...I will try your suggestion and post...thanks
     
  19. Paul R

    Paul R Member

    Messages:
    61
    Terry...Thanks a million...the white plastic "prefilter/heat reducer?" was burnt black ...the red "gel ' colored one was ok.Initially after pulling out the red one I did not see a light through the front glass even though there was a light evidenced by looking down the assembly.I realized there had to be something else blocking light from coming through.. it was then I saw the white part which I pulled out and inverted back into place...thanks again...now if only you could tell me why no AM at all!!! P.S. My sons eyes are just gleaming with every fix ..after all it will be for him...cheers ..P.S.... MX113
     
    Last edited: Jun 14, 2005
  20. dj_AmTraX

    dj_AmTraX Super Member

    Messages:
    1,833
    Location:
    San Francisco
    My MR73 "red" dial indicator is half lit. Does this work for it? I will post pictures whe I get home.

    The "Stereo" bulb doesn't light also. I replaced the bulb already, but no luck. Can someone please tell me how to trouble shoot without all the tech terms.

    Thanks!
     

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