MA5100 reconstruction

Discussion in 'McIntosh Audio' started by manito, May 15, 2018.

  1. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    upload_2018-5-15_10-3-15.png

    I found this amplifier in poor condition, however it came with its cabinet (covered with a black plastic and adhered with glue) and their respective panlocs. without the lower metal cover and without the upper cover of the transistors heat sinks, I also found a lot of accumulated dust, rusty chassis and when inspecting in detail some transistors were missing, one of the two triacs was missing and also the fuses were missing. I had the impression that he had a problem at some point, perhaps in the right channel because all the power transistors of this channel were missing and the diodes, resistors and ceramic capacitors were disconnected and someone tried to repair it but when not getting the spare parts of this amplifier was abandoned for many years and perhaps trough the time it began to deteriorate further as the right channel power stage circuit board was partially broken.

    upload_2018-5-15_10-5-12.png

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    I thought it was a good project to rebuild and I gave myself the task of making a list of the missing pieces as well as the pieces suspected of being bad, I checked some capacitors and found that the vast majority had values very different from those that appear in The service manual so I decided to get them all except the big cans.

    When I started looking for the missing pieces I found that most of them are not currently sold but that there are modern substitutes with very similar values, researching the forum found most of these substitutes as well as some replacement guides that can be found in pdf files in the Web.

    I got most of the pieces on the internet and others in the electronics stores in the city where I live.

    While the pieces that I ordered online arrived I started to disarm the front and put the glass plate safely in a safe place in a box as well as the knobs and other pieces removed, take advantage of the front was dismantled to clean and adjust the black vertical rocker switches, the post was very helpful:

    http://audiokarma.org/forums/index....can-these-be-tightened-or-retensioned.769625/

    upload_2018-5-15_10-7-3.png

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    Continuing with the cleaning, I used a brush to remove the dust and I used epoxy glue to rebuild the broken circuit board, I placed a piece of copper wire to restore the continuity of the broken traces.

    Once the spare parts arrived, I started disassembling and emptying the cans of capacitors C304, C305 and C306. Then I installed the individual capacitors according to the original values, soldering them all together by their negative pole and distributing them in such a way that they could be placed on the same way the body of the can to preserve the original aesthetic.

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    Last edited: May 15, 2018
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  2. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    On the left-side power circuit board, I installed the missing transistors Q207 and Q209 using the MPSA06 and MPSA55 substitute transistors, and I also changed the electrolytic capacitors of MALORY C203, C204, C207 and C208 on both power circuit boards, left and right channels by nichicon TVX 100μF 16V and nichicon TVX 220μF 10V.

    Left channel Power Circuit Board :

    upload_2018-5-15_10-12-30.png

    Right channel Power Circuit Board :

    upload_2018-5-15_10-13-10.png

    On preamp circuit board change the MALORY C9, C10, C19 and C20 electrolytic capacitors by nichicon TVX 100μF 16V.

    Preamp Circuit Board:

    upload_2018-5-15_10-16-37.png

    On output monitor circuit board I removed the triac along with its retainer and instead installed the modern substitute replacements NTE 5623, installed the missing 3A fuses and changed the electrolytic capacitors of MALORY C223, C224 by bipolar generic capacitors from 10μF 50V, all this with the valuable help of AK members post:

    http://audiokarma.org/forums/index.php?threads/triac-for-a-mcintosh-ma5100.817194/#post-11526801
     
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  3. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    Output monitor circuit board:

    upload_2018-5-15_10-20-9.png

    In the power output section I installed the modern substitutes with the TO-220 package for the transistors Q211, Q212 (TIP50) Q213, Q214 (MJE15028G), Q215, Q216 (MJE15029G), and with the TO-66 package for the transistors Q217, Q218, Q219 and Q220 (2N3772). All the transistors I apply insulating film and silicone grease. I also installed the thermal cut out No. part. 153-007 without silicone grease, does anyone know if I should apply this silicone grease on it?


    upload_2018-5-15_10-21-15.png




    After all these changes and thinking that everything is in place I would like to know how to start testing the amplifier without causing damage, before I can connect it to the outlet and turn it on. in the service bulletin recommend the use of a VARIAC but I do not have one of those, I only have the lamp bulb tester, so besides that if you look something wrong installed I ask all of you to please tell me and also tell me how I can begin to do the tests and in this way get back to life this much loved device.
     
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  4. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    Both Kev and I have had issues with using the MJ15028/29 as driver subs.......I pulled them and followed Mac's recommended subs Kev added a variable bias circuit. I think we both found them to want to go into oscillation.

    If I was going to experiment again I would use a older "slower" lower gain to220 type.
     
  5. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    I tried to get the substitutes suggested by McIntosh with the encapsulated TO-66 (thinking that it would be as original as possible) but I only got a pair of 2N3767 so I decided to leave all with the TO-220 package, searching the forum I found that suggested the MJ15028/29, I appreciate your experience with these drivers and I would like to experiment with other transistors, will you have any suggestions in mind for these drivers?
     
  6. c_dk

    c_dk Addicted Member

    Messages:
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    Location:
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    As I have stated before, only one company has enough marketing muscle to have a small batch semiconuctor manufacturer build to spec obsolete transistors and then move them through their inventory system to keep them available. That is NTE. That Mac parts has been known to buy and ship NTE devices that don't match the NTE cross reference to sub for obsolete parts shows that they do actually test their recommended subs and the possible deficiencies of the NTE cross reference system.
     

     

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  7. Hipocrates

    Hipocrates Anti-Muppet

    Messages:
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    That usually do the job.
    I ordered some NTE 124, 218 and 384 (TO-66) but Is going to be a while to experiment with those, (several projects first). But maybe you want to try them. They are ''slower" that the on-semis.
     
  8. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,938
    Location:
    France
    In my point of view , this is not normal

    XX1.png


    one should be with orange dot
     
  9. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    ok, I'll wait to get the drivers, I'll do a second check that everything is in place and I'll try with the bulb tester.


    good suggestion, that opens more possibilities, I will try to get those subs, thank you very much.
     
  10. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    good observation, when these devices are found one does not know how many hands have passed through them and what things they have done, I will review the numbers that have been recorded and if necessary I will remove to check them with a transistor tester. Thank you.
     
  11. Hipocrates

    Hipocrates Anti-Muppet

    Messages:
    1,493
    Location:
    Costa Rica
    No problem
    This is what I have to test

    132-028.... 2N3738 (TO-66, mouser, littlediode -expensive-)... NTE124 (TO-66).....TIP50 (TO-220)
    132-023.... 2N3767 (TO-66, mouser, littlediode -expensive-)... NTE384 (TO-66).... MJE15032(TO-220)
    132-024.... 2N3741 (TO-66, mouser, littlediode -expensive-)... NTE218 (TO-66).... MJE15033(TO-220)

    Notes.
    I usually don't use NTE but sometimes a man got to to what a man got to do.
    TO-220 can be ''to fast" but some people had good results
    You can find some TO-66 on ebay, but I had some bad results with some from India and China, they tested bad under working conditions.

    I'm not saying that NTE and Onsemi will work for sure, but they look like inexpensive widely available options.
     

     

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  12. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
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    The NTE subs you have listed do not pass Mac engineering's scrutiny.......I know that is the NTE sub listing but not what Mac parts ships.
     
  13. Hipocrates

    Hipocrates Anti-Muppet

    Messages:
    1,493
    Location:
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    I don't like them neither. Like I said, have to test them first.
    They goal here is an alternative for those 2NXXXX.
    Are you telling me that McIntosh part send those parts on this day and age? That's cool... Not to my ''authorized'' dealer that's for sure, they just gave me the commercial part number to go and fetch.
     
  14. c_dk

    c_dk Addicted Member

    Messages:
    5,924
    Location:
    West Michigan
    From what I have gathered from a few conversations with my last contact within engineering and service, McIntosh has had to turn in some cases to NTE devices to find a semi reliable source for obscure parts. They do not use NTEs cross but do after testing supply selected devices. I do not know every part they do this for, nor do I know that they test every device that they use or send out like they used to decades ago.......see the paint on old vintage devices as evidence of that testing.

    You can not use the NTE cross reference as a crutch for marginal repair efforts....it might be a guide but way to often it is not reliable and can not substitute for proper repair procedures.

    Again your final bench testing with the correct testing equipment is paramount in any proper repair.....
     
    Last edited: May 19, 2018
  15. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    You were right, both transistors had the number 132-021, the one on the right was the wrong one should have the number 132-032, so I decided to place the same subs that I used in the Left channel Power Circuit Board; MPSA06 and MPSA55, thanks.

    upload_2018-5-19_15-45-42.png
     
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  16. manito

    manito New Member

    Messages:
    43
    Location:
    South of the Border
    Finally I changed the drivers for the MJE15032/33 transistors, I connected the speakers, my ipod for the auxiliary input and the dim bulb tester and I turned on the amplifier and the bulb remained lit, the music was heard by the speakers with little volume and a little dirty, but the bulb never stopped shining! , so there is some short circuit out there. Someone could help me telling me where I can start looking for that short (and how to do it), thank you.
     

     

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