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McIntosh - Hints and Kinks

Discussion in 'McIntosh Audio' started by dewickt, Jul 1, 2004.

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  1. tonyrome01

    tonyrome01 New Member

    Messages:
    7
    I have a problem with my Mac 1900! The sound from the unit is just fine except for the 1&2 phonos, the sound is low with one channel having no bass at all. However, the sound & volume from the tuner, tape and even from the CD player when connected to one the tape inputs (being there's no aux) comes out just fine. The problem is not the selector pot, it is not with the amp or from any of the parts that governs sound/volume from the two R & L Chs. The problem is also not with the two turntables I've used, they worked just fine on other hi end receivers. Therefore I'm thinking the problem must be just with in the phono inputs circuits going to the amp est. If I know just what & where I might be able to fix it myself. I am not an expert but I have tinkered with these things with some success once I know what the problem is and what bad part/parts is causing it. If someone can put me in the right direction on what to do I would deeply appreciate it, tonyrome01
     

     

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  2. smartin53

    smartin53 Well-Known Member

    Messages:
    538
    Location:
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    Well...it MAY be simpler than anyone thought. I was independently researching MAC1700 mods and came across this thread...

    http://audiokarma.org/forums/showthread.php?t=91542

    It goes into some details on converting the tape output into a preamp out. Ultimately it SEEMS he simply clipped and isolated the tape output, and then ran a shielded wire to the outputs from the preamp boards. The following post indicates where the signal came from.

    http://audiokarma.org/forums/showpost.php?p=972885&postcount=31

    From the discussion, it seems it works well. I'm curious if this makes sense and if anyone had any further thoughs. It is obviously doable.

    I've been juggling the option of switching from my MA6100 to the MAC1700 and then running the audio into my Dynaco ST-70 tube amp. I'd of course need this mod to do that.
     
    Last edited: Jan 13, 2013
  3. dewickt

    dewickt Will fix about Anything

    Messages:
    3,398
    Location:
    Knoxville, TN
    I recently converted an MA230 the same way using the tape head jacks as the amp in and the phono 2 as the preamp out. On a 230 with the 4 gang volume control you can end up with a gain control for the amp in and also a gain control for the preamp out.
     
  4. mrz80

    mrz80 I guess it's in my blood

    Messages:
    646
    Location:
    Gainesville, FL
    Curse you AudioKarma, I've got enough projects on my "this 'll never get done but it's on the to-do list anyway" list! :) I'd pretty much managed to forget all about my long ago notions of modifying my 1700 for pre-out/amp-in; now you guys've got me thinking again about it. I'd reached the conclusion after some brainstorming that it'd be a major undertaking involving ditching the courtesy AC outlets (does anyone actually *use* those in the modern age of outlet strips?) to gain some back panel real estate. I figured on making up a "filler" panel, moving the speaker outs over to where the AC outlets were, and putting pre-out/amp-in RCAs where the speaker outs were. I'd considered but rejected repurposing the Tape Head and Phono2 jacks for pre-out/amp-in, since I was also considering redoing the Tape Head input (and maybe Phono2 as well) as another line-level Aux input. Maybe I ought to go ahead and put this project back on the list, since the unit needs major surgery now anyhow (I have to recap the unit, chase down the high distortion in the phono preamp, and repair the dead FM tuner). Food for thought, food for thought... :)
     
  5. jlovda

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

    Messages:
    3,478
    Location:
    NE Ohio
    MC2100 Power Transistor Insulating Washers

    Does anyone have a diagram showing or describing which power transistors (by physical location) have insulating washers. I'm replacing all 12 in a unit I'm working on. I have replaced 5 of 6 on one side and only one had a washer. I'm confused because it doesn't seem logical. I would assume that all the transistors near the positive rail or the negative rail would need to be isolated.

    Thanks
     
  6. dewickt

    dewickt Will fix about Anything

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    Location:
    Knoxville, TN
    The heat sinks are isolated from the chassis, if 2 of the outputs have the same on their collectors only the third needs to be isolated, only Q25 and Q26 need to be isolated. Two of the heat sinks can have +35 on them and the other two can have output signal.
     

     

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  7. jlovda

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

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    Thanks Terry: One more question. What is the technique for replacing the power resistors on the heatsinks? They are buried deep and I can't seem to move the heatsinks out enough to cleanly get a soldering iron near them.

    John
     
  8. brichter

    brichter Well-Known Member

    Messages:
    701
    Location:
    SF Bay Area
    Hey, McIntosh gurus! (Hi, Terrry & Ron :D)

    I have a technical question regarding an MC275 MkV:

    I bought this used and it no longer has the McIntosh tubes (it's full of JJs now) and whoever rolled the tubes must be a little bit... hmm... challenged, shall we say? The Output tubes are fine, but where he screwed up was the signal tubes, it should have 4x 12AT7 and 3x 12AX7, he put in 4x of the AXs and 3 of the ATs.

    I haven't been able to find a schematic of the MkV anywhere, but have read some stuff that V1 (supposed to be an AX) is the tube that handles the conversion from unbalanced to balanced, and I'm using balanced inputs.

    I have several AX7s I've been rolling in and out of my C2300, so I do have a replacement for the odd man out, but wanted to verify with folks who actually know what they're doing, that V1 would be the best place to roll in an unmatched tube.

    The AXs I have available are Gold Lion Genalex, PSVANE, and stock McIntosh (Telefunken or Sovtek, if I believe what I read on the web), so obviously nothing that I would think is an exact match for the JJs.

    <edit>
    Anyone know where I can find the service manual for this? :scratch2:
     
    Last edited: Feb 3, 2013
  9. instatone

    instatone New Member

    Messages:
    20
    thanks much! I will give it a shot. I just got a 5100 and am excited to demo it. Any other tips about it please let me know.
     
  10. MX117.MC7150

    MX117.MC7150 Listener Subscriber

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    brichter - this has been discussed in one or two threads on the MC275 either here or over the Audio Afici_nado McIntosh forum. Do a search on both. Also, have you looked in the Digital Docs forum here to see if the service manual is there?
     
  11. brichter

    brichter Well-Known Member

    Messages:
    701
    Location:
    SF Bay Area
    Yup, checked our DD forum, we don't have the service manual for the MkV, neither does Berners...

    I found a thread (not sure now if it was here or on AA) that discussed the tube complement and purpose of individual tubes. Unfortunately, it wasn't specifically for the MkV, either, so I was looking for validation the amps have enough in common that the tube functions are the same.
     

     

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  12. MX117.MC7150

    MX117.MC7150 Listener Subscriber

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    Small signal tubes/power tube functions are the same in all MC275s thru the MkV. Not sure about the new, expanded chassis model, but will guess they have not moved or changed their function.
     
  13. brichter

    brichter Well-Known Member

    Messages:
    701
    Location:
    SF Bay Area
    Thanks, I'll go with the PSVANE in V1, then try the Gold Lion Genalex.:banana:
     
  14. vyik

    vyik New Member

    Messages:
    3
    Hi everyone. Im currently in the process of buying all the coupling capacitors to replace my bumble bees caps out of my mc60s. The one capacitor im having a hard time finding is (C3). Its a 100uf 12v cap. Is there a different cap i can use maybe w/ a higher voltage?

    Thank you in advance and i apologize for the noob question as this is my first time.
     
  15. dewickt

    dewickt Will fix about Anything

    Messages:
    3,398
    Location:
    Knoxville, TN
    A higher voltage is OK, I use Nichicon Muse KZ 100uF @ 25V. Going higher in voltage has no effect on sound quality, just size.
     
  16. dmoro

    dmoro New Member

    Messages:
    8
    I have a mac 1900 and a mx117. the right channel of the 1900 gives off a low level popping sound in all modes,and the 117's left channel has a low level static noise with occational low level popping.
     

     

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  17. MX117.MC7150

    MX117.MC7150 Listener Subscriber

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    dmoro - Welcome to AK. Unless you are skilled in the art of cleaning and Deoxiting, a trip to Dr. DeWick's McIntosh Repair Emporium is in order. For safe shipment order you proper double boxes from Audio Classics. Sames boxes for both pieces. Suggest a recap and alignment of both pieces for another 40+ years of satisfaction.
     
    Last edited: Feb 10, 2013
  18. vyik

    vyik New Member

    Messages:
    3
    Thank you very much terry! waiting for new caps to arrive!
     
  19. dmoro

    dmoro New Member

    Messages:
    8
    I do have experience in cleaning and deoxiting switches. I'v also replaced the volume/ on off control on my 1900 twice ( quite a job). I will probably send these units to Terry as I
    don't trust the local shops here in Portland or. to work on mac stuff even though there is an authorized mac sevice shop here.
     
  20. c_dk

    c_dk Addicted Member

    Messages:
    6,313
    Location:
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    Sounds like a failing transistor in the MAC1900, I would suspect a failing summation opamp in the MX117.
     
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