MAC 4275 speaker relay??

Discussion in 'McIntosh Audio' started by genewoodall, Sep 21, 2014.

  1. genewoodall

    genewoodall Active Member

    Messages:
    117
    Location:
    Austin, TX
    Hello:

    I just purchased a MAC 4275. I would not have considered this model, but It is near mint cosmetically and I got it very cheap knowing that it has an intermittent problem which is crackling and channel dropout in the left channel. It does this on either speaker 1 or 2 taps.

    The dropout/noise shows up randomly and giving the volume control several quick turns gets rid of it, usually for a listening session. I have cleaned the volume control.

    It works perfectly as a preamp tuner hooked up to another amp via preamp out and sounds surprisingly good. No dropout whatsoever.

    When listening t the headphones I get the dropout, but it goes away when I disengage the speaker tap button.

    I think the speaker relay is the next suspect. I have cleaned the controls but I can’t find the speaker relay although I can hear it. I am afraid it is hidden between two boards and could be a really tough one to get to, dealing will all the ribbon connectors.

    Anyone ever dealt with this problem or have an idea as to where the relay is located?

    Thanks
    Gene

    MC 7200, MC 2120, MR 78, C 30, MAC 4275
     
    Last edited: Sep 22, 2014

     

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  2. Classic_CJ

    Classic_CJ Well-Known Member

    Messages:
    585
    I think this was a unit that they had designed, but had a vendor build the boards and such for it. Could be wrong, but both the schematic in the manual and the board design look very Japanese to me. But whatever, I don't think I've ever had one of these in the shop, so perhaps they were pretty reliable!

    There should be one big power supply and audio board in the unit, and the relay should be roughly in the middle of that board. The manual lists the Mac part number as 310-182 with a vendor number of MR72A24U.

    I did a few quick searches on the vendor number with Octopart, Mouser, and Newark and got no returns on any search, but NTE crosses it to an R25-11D10-24, a fairly common relay format IIRC.
     
  3. 62caddy

    62caddy Trust but verify Subscriber

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    Having owned a MAC 4280 with the same symptoms, I can almost guarantee the problem is with the jacks at the MAIN AMP inputs.

    They are designed so that the receiver does not require jumpers from the preamp to the amp but when an RCA IC is plugged into the MAIN AMP inputs, the internal feed is disconnected and the amplifier section will then be fed from the external input.

    Unfortunately, connecting a pair of IC cables between the PRE OUT & MAIN IN does usually will not solve the problem. There is some relay or contacts inside that have to be repaired.

    This was a very common issue with the MAC 4275/4280.

    ClassicCJ is correct - the internals of these two models were Japanese and assembled in Binghamton.
     
  4. genewoodall

    genewoodall Active Member

    Messages:
    117
    Location:
    Austin, TX
    No problems using it as a preamp/tuner

    I am not sure what you mean about the pre out and main in, but the unit does not show any problems when I use it as a preamp, connecting cables between preamp out and an external amp. Also, when using headphones and the unit connected directly to speaker, no standalone amp, the headphones will exhibit the noise and dropout unless I turn the speakers off via the speaker selector switch (s). Then the noise disappears immediately.

    The boards were definitely sourced from Japan. I believe the transformer, case, switch gear, knobs etc., were made by McIntosh. The darn thing sounds really good, with a good tuner and phono sections. I have owned other McIntosh receivers and Japanese receivers of similar or more power. This is noticeably better than the Japanese receivers, and nearly equal of a 4200 sound wise, although the build quality and omission of features is obvious. It makes a very good preamp tuner
     
    Last edited: Sep 28, 2014
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  5. genewoodall

    genewoodall Active Member

    Messages:
    117
    Location:
    Austin, TX
    It was the speaker relay. NEC MR72A-24U is the original part. McIntosh has replaced this part. Part# 310182 has been replaced by part# 087043, the same as used in the later MAC 4300V.
     
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  6. strymeow

    strymeow New Member

    Messages:
    9
    Sorry to revive an old thread, but since discussion of these units online is pretty limited, I thought I'd put this update in for posterity.

    I have the 4280, a recent acquisition that is near identical to the 4275 except the 80 has a remote, and it is exhibiting this exact same behavior, down to the headphones. I have sent the unit off to Audio Classics for service. In my talks with Ryan at AC, he suggested the relay may be the issue, as genewoodall confirmed was the case for his. I will provide another update when the service is complete.

    In the meantime, I wanted to post a small correction to 62caddy's statement about the preamp out function on the 4275/80, also for posterity. The use of the preamp out on the 4280 does *not* deactivate the internal amp; rather both the speaker 1 and 2 connections can be used simultaneously as an external amp, per the Mcintosh manual. This is not to call 62caddy out, especially considering how much other great info he has posted about his former 4280 (thanks for that!) but instead is so that someone like me who comes along looking for info about the 4280 in the future will have a clearer understanding of its capabilities.

    Finally I'll echo genewoodall's comments about the sound the 4280 puts out, which really is superb--particularly the tuner and phono section. I know 62caddy moved on, and maybe some day I will too, but it's a fantastic machine. Thanks again to all who have posted about the 4280.
     

     

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  7. 62caddy

    62caddy Trust but verify Subscriber

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    I think you may have misunderstood.

    If you reread my post, I stated the problem lay in the MAIN AMP input relay which disconnects the internal connections between the preamplifier & power amplifier sections of the MAC4275/4280. When a pair of male RCA IC are connected at the MAIN AMP inputs, the internal preamplifier connections are broken.

    The preamplifier outputs otherwise continue to function normally and are ordinarily not the cause for the problem as described by the OP.

    Thanks and welcome to AK .[​IMG]
     
  8. strymeow

    strymeow New Member

    Messages:
    9
    Indeed, I did misread. Thanks for the correction to my correction. ;) And thanks again for your post history on your old 4280.
     
  9. JPJazDog

    JPJazDog New Member

    Messages:
    2
    Location:
    Palm City, FL
    I finally could jump into the McIntosh water with the purchase of a 4275. I appreciate all of the expertise shared here and look forward to continuing discussions about this receiver. JPaul
     
  10. Species8472

    Species8472 AK Subscriber Subscriber

    Messages:
    74
    Location:
    Milwaukee
    I'm confused. It been said that "the problem lay in the MAIN AMP input relay" and "There is some relay or contacts inside that have to be repaired". The BOM lists only one relay and that is connected through to the speaker terminals but It also lists a pair of "light dependent resistors". Could these be the culprits?
     
  11. 62caddy

    62caddy Trust but verify Subscriber

    Messages:
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    "Relay" is probably not the correct terminology. The only reason I used it is because that is how Audio Classics referred to it when I had the issue with my own MAC4280. (I think the term was also used on the repair order). Of course there are also other things that can cause intermittent channel losses too.

    What is the receiver you have and what is it doing?
     

     

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  12. c_dk

    c_dk Addicted Member

    Messages:
    5,992
    Location:
    West Michigan
    Instead of the "horseshoe" jumpers that Mac used in previous generation units to couple amp and preamp sections for example, they went to make/ break RCa jacks.

    The internal contacts of these jacks often need cleaning.
     
  13. Species8472

    Species8472 AK Subscriber Subscriber

    Messages:
    74
    Location:
    Milwaukee
    It's 4275. From day one and for years I used the pre-amp and amp separately. It was when I used it as a single unit it would have intermittent issues with the left channel. When the channel gets funny it is apparent on the output meters. Is the effect on the meters a clue as to whether it's on the amp or pre-amp side of things?

    To c_dk
    Now that you say that I just noticed the dotted lines on the schema. (doh!) Is the main in pre-out detection just mechanical?
     
  14. 62caddy

    62caddy Trust but verify Subscriber

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    An interruption in the preamp signal would definitely be shown on the output power indicators. Since the preamplifier functions normally on its own and only when the receiver is used with both sections are problems showing up, it's highly likely the microswitch is faulty at the jacks. Someone here had said he was able to clean the contacts using a toothpick(?) (or something similar) and Deoxit on a MAC4300 when it was exhibiting the same behaviour.
     
  15. bebens

    bebens New Member

    Messages:
    2
    I had this exact same problem and had the Speaker Relay replaced. Its available from McIntosh p/n 087 043 and it replaces the original p/n 310 182. See p.10 of the Service Manual which you can find on line. With this update the crackling and drop outs have ceased.
     

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