Please help: McIntosh MA6100 (recently refurbished by Terry DeWick) volume control problems

Discussion in 'McIntosh Audio' started by drz, Feb 4, 2019.

  1. drz

    drz New Member

    Messages:
    11
    This is my first McIntosh post, I am a relatively new to vintage audio, and know nothing about repairs. I am trying to be a safe beginner.
    My dumpster rescued MA6100 was DOA, so I sent it to Terry Dewick. He rebuilt the left channel with new output transistors/ audio path upgrade, etc. The unit quickly developed volume control problems. Fader lube fixed that for awhile. I know it is not a good match, but I paired it with newly acquired AR9s. The second time I turned on, it came on at full volume!
    I bought a volume control from an ebay vendor, installed w/o issue, and it works, however, when turning it on, the left channel comes on first (no sound from the right) until I turn the volume up, then the right comes on but is not as loud as the left, and stays that way. Moving the balance to 2:15 fixes the imbalance.
    I described the problem to the ebay seller. He replied as follows:

    "HI Tony,
    there could be several reasons.
    1. When Terry serviced the
    unit , he might have adjusted the signal strength for each speaker to even
    out the original volume/balance control imbalances.
    2. It also could be
    caused by a new pot!

    Here is few solutions.
    1. You can even out the
    balance by installing a resistor to lower the signal strength to the
    strongest deck of the pot. Let's say the right speaker plays louder.
    Install a resistor to the deck of the pot that is responsible for the right
    speaker. (The value of the resistor will depend on the strength/imbalance
    of the signal)
    2. Send the control back for evaluation, if it is faulty, I
    will replace it with another one."

    I can't figure out how to adjust the signal strength, and there is not enough space to place a resistor in the small space around the volume control. I hope I can fix the problem without removing the new volume control.
    I greatly appreciate any help!
    Sorry about the long question. I hope I am posting in the right forum.
    Thank you,
    Tony
     

     

    Please register to disable this ad.

  2. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,345
    Location:
    NC
    Hi Tony and welcome to AK. May we know who you purchased the Volume control from the auction site?

    Have you contacted Mac parts to see if they have a replacement? Terry does great work from what I have heard. Lots of good help here so please respond.......
     
    drz likes this.
  3. drz

    drz New Member

    Messages:
    11
    Hi AITinkster, Thanks for your welcome and reply! The new pot was purchased form captainfantastic07.

    I originally purchased a new volume control from McIntosh, but that control was very tight, it took a lot of torque to turn the switch from off to on, it was much tighter to turn and made grinding sounds when rotated. I contacted McIntosh, was told that the new controls are not as good as originals, but that was the only control available. I decided to return that control.

    Captain Fantastic has good reviews, so for attempt #2 I am trying his volume control.

    Terry is still doing great work! Family issues are keeping him very busy, and I wish him all the best. From what I have read, these volume controls can malfunction at any time. Most certainly not Terry's fault!!

    I have read a few posts about the MA6100. It seems there are no bias adjustments and the only pots I saw are for the tone controls and they are best left alone.

    I am going to have another look to see if I missed something, but I truly appreciate any advice!
     
  4. c_dk

    c_dk Addicted Member

    Messages:
    6,508
    Location:
    West Michigan
    Sit down with the schematic and follow the path to ground via the shield drains for the various boards and controls.

    Find them in the unit and determine if they have been repaired.

    Reflowing these preamp connections can be very time consuming and therefore expensive so if it checked out OK when the amp section was blown he might have judged it good to go.

    If any of these ground connections go wonky the unit will behave badly in strange ways.

    I believe units of this vintage should not only be repaired but also get the extra treatment of restoration to head off these known issues.

    I believe the Canadian sourced control from Mac parts is the best option but does need to be customized to work best as it is a generic replacement for 5 previous part numbers.

    The auction site should be labeled buyerbewarebay.
     
    drz and KeyserSozee like this.
  5. drz

    drz New Member

    Messages:
    11
    I will attempt to do as you suggest, later in the week. I disassembled the original volume control, cleaned and reassembled, and it tested ok with a DMM. There were no visible shorted/burned areas. I did it as an educational exercise, not sure if I should trust the control. If necessary I will try another McIntosh Volume control.

    Terry repaired the power supply, protection circuit, audio path update, replaced the left channel output transistors, replaced two 132-032 transistors, one MPSA04 NPN transistor, and one visibly damaged .33 ohm 5 watt resistor, all on the left driver board. It was not a full restoration.

    I wonder if the MA6100 was damaged when it powered on at full volume? I was trying to gently use it with newly acquired AR9 speakers. I turned off as quickly as possible. Maybe that unrestored right channel was damaged? Long term, I probably need something else for the AR9s.

    In fairness to the seller, he has all positive feedback, there have been several positive reviews of this same control, we are communicating, and he offered to replace the control if it is defective. But I am always wary.

    Thanks c_dk for your advice! I'll post my findings. Meanwhile here are pictures of Terry's work. IMG_1975.JPG IMG_1976.JPG IMG_1977.JPG IMG_1979.JPG IMG_1980.JPG IMG_1981.JPG IMG_1985.JPG This is my first attempt at pictures, hope they can be viewed.
     
    KeyserSozee likes this.
  6. drz

    drz New Member

    Messages:
    11
    Big mistake! The pictures I posted are the "before" Terry's work pics.
    Here are the Terry's work pics. IMG_2269.JPG IMG_2270.JPG IMG_2271.JPG IMG_2272.JPG IMG_2273.JPG IMG_2275.JPG Sorry Terry!!
     
    KeyserSozee and John James like this.

     

    Please register to disable this ad.

  7. c_dk

    c_dk Addicted Member

    Messages:
    6,508
    Location:
    West Michigan
    Well I doubt that he found a secret stash of the Alps controls Mac contracted for back in the 90s so most likely they are something from Alpha.

    Others have posted that there are no identification markings on the control itself so who knows where they are sourced from.

    The part Mac has is from a well know manufacturer of MILSPEC controls........will end users pay for a true milspec unit, most likely not but I have found them to be good, once creatively modified.
     
    drz likes this.
  8. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,345
    Location:
    NC
    Thanks, let us know how that control from Capt Fantastic goes, interested to hear. :)
     
    drz likes this.
  9. drz

    drz New Member

    Messages:
    11
    To c_dk: Thanks for the explanation about manufacturers of good quality controls. That is good to know. You are correct: here are no markings on the control that I installed. I took a chance. We shall see. Just wondering what type of creative modifications are needed by MILSPEC controls? I had to modify the terminals (shorten, bend, twist, taper, etc.) on the unknown pot to get it to fit into the MA6100. It appeared that the McIntosh sourced control would need the same physical modifications. It really wasn't that hard...

    To AITinkster92: I'll be sure to let you know.

    I noticed that Tom Bavis recommended the following resource for Volume controls: Mark Oppat at: http://oldradioparts.net/controls.html.
    Does anyone have any experience with Mark?

    Thanks, Tony
     
  10. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,345
    Location:
    NC
    Tony, I've had a couple Fisher Volume controls rebuilt by Mark, he's ok however he has to rebuild these from already used parts he has in stock. Al
     
    drz likes this.
  11. c_dk

    c_dk Addicted Member

    Messages:
    6,508
    Location:
    West Michigan
    The U.S. military certified manufactures to build products to their specs, PEC, the Canadian company met that milspec certification.

    I preload the control with added on resistance to shift the center tap value and twek the channel difference. This will adjust the taper to be more of a audio taper than linear when put in circuit.

    You can get out your slide rule to do the math for a voltage divider but I choose to try to match one of the original's taper......
     
    drz likes this.

     

    Please register to disable this ad.

  12. drz

    drz New Member

    Messages:
    11
    I finally found time to look into the problem pot. Per c_dk's advice, I re soldered all grounds and anything that looked less than 100%, and cleaned the board contacts.

    The function of the EBay volume pot was still off as described in the first post. I tested the speaker output and found discrepancies. I removed the ebay pot and tested it for resistance and found a 15 to 19% difference in the two ganged pots.

    I installed the original McIntosh pot that I earlier dissembled, cleaned,, lubed, reassembled because it tested almost spot on. The good news is the MA6100 is operating nicely at least for now. Perhaps some piece of solder or something got loose and ended up in the pot or someplace else? We may never know. I am delighted to have a Terry DeWick repaired McIntosh Integrated amp to enjoy again. Eventually I may restore the rest of the unit but for now, time to enjoy. BTW, Terry's test results (on the invoice) for the amp were "Test <.1 % THD at 70 W, 1 % THD at 76 W L & R into 8 ohm load."

    I read about linear, analogue, taper, etc. It brought back memories of my slide rule and the formulas written on it for physics 45 years ago. We called that class "Wires and fires" as I remember. A pleasant memory, thanks c_dk! Lots left to learn!

    Al: I will contact captainfantastic07 to return the pot for testing. I'll post the outcome.

    Thank You Gentlemen! Tony
     
    KentTeffeteller and John James like this.
  13. drz

    drz New Member

    Messages:
    11
    Al, I heard back from the seller. He will not accept a return on this out of balance volume control because I altered the terminals. I can accept that.
     
    Last edited: Feb 12, 2019 at 7:32 AM
  14. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

    Messages:
    3,345
    Location:
    NC
    drz, sorry you had problems but in fairness you can't alter a part and expect a seller to accept it back, next time check the control with a ohm meter before installing, if it's faulty then you'll know beforehand. A couple resisters could of been installed to even out the output on each ganged side and possibly you would of been good to go.....best of luck Al
     
    drz likes this.
  15. drz

    drz New Member

    Messages:
    11
    Al, Thanks for your advice. I edited my post accordingly. I considered resistors but there was no room around the volume control and as a novice I couldn't decide where else to install. In the future I will be sure to test before installing. Unfortunately the pot I received was perhaps not as high quality, plug and play as advertised.

    The silver lining is that it was fairly easy to disassemble, clean and reassemble the original pot, and it is working perfectly! Again, Thanks for everything... Tony
     

Share This Page