Voltages high on my MC2100

Discussion in 'McIntosh Audio' started by MikeRam, Apr 8, 2018.

  1. MikeRam

    MikeRam Active Member

    Messages:
    177
    Location:
    Portland Oregon
    I recently acquired a pretty clean MC2100 and have mostly rebuilt it. I checked the voltages after I was all done and noticed that they are a little high compared to the schematic and the last 2100 I had. The power supply voltage is about 10 volts above normal and the voltage off of the large capacitors are about 1.5 volts higher at 36.5v. I posted a schematic with my voltages in red. AC voltage from the wall was 120v when I measured them. Unfortunately, I didn't take any voltage readings before the rebuild. I have a couple of questions.

    Is this anything to worry about, or is it worth figuring out how to lower them to specs?

    Can someone with an open 2100 measure the AC voltage after the transformer and let me know if they are under the 172v and 55v that I'm showing? What are your voltages right after the rectifiers?

    Since a thread is worthless without some pictures, here are a few:
    DSC_0660_02.JPG

    DSC_0642.JPG
    Original transistors

    LeftRebuilt2.JPG

    DSC_0634.JPG
    Stuffed power supply caps

    Power Supply Schematic MC2100.JPG
    My voltages in red
     
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  2. whoaru99

    whoaru99 Epic Member

    Messages:
    40,951
    Location:
    LoTL
    Despite what the drawing shows, the service manual I just looked at said all voltages on the schematic are with AC input at 117V (and all voltages shown are +/-10% with respect to chassis ground).
     
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  3. MikeRam

    MikeRam Active Member

    Messages:
    177
    Location:
    Portland Oregon
    Then I'm in the ballpark and won't worry about it. I know all my capacitors have voltage ratings well above what they are receiving, but I was just wondering how the higher voltages would affect distortion or heat. I changed the bias resistor to a 22 Ohm 1% one so bias is spot on and heat has not been an issue. I'll button her up and let her sing!

    Thanks,

    Mike
     
  4. BillWojo

    BillWojo AK Subscriber Subscriber

    Messages:
    2,607
    Location:
    Burlington, NJ
    That's a really clean unit. I always like the way they look. Would make a nice companion to my MC40's for bi-amping. Those blue meters just don't do it for me.

    BillWojo
     
  5. MikeRam

    MikeRam Active Member

    Messages:
    177
    Location:
    Portland Oregon
    BillWojo,

    Yes it really was well taken care of. It was a one owner unit and he babied it. He bought it new in the 70's while he was in the army and has carted it around and used it ever since. He gave me the original paper work, sales receipt, and the McIntosh Clinic printout. I think it put out 137 clean watts at the time.

    Mike
     
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  6. techguy0192

    techguy0192 Well-Known Member

    Messages:
    772
    Nice looking 2100.
     

     

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  7. damacman

    damacman Blown and Injected Subscriber

    Messages:
    8,072
    Location:
    Gilbert, AZ
    Man, I’d be tempted to run that with the cage OFF! I bet it sings now!
     
  8. c_dk

    c_dk Addicted Member

    Messages:
    5,997
    Location:
    West Michigan
    40 volts DC on the heatsinks across your heart might not be a good idea with the cage off........SOP back in the 80s was to replace those RCA predrivers with transistors with greater SOA......when they cut loose they take out the outputs and usually burn the boards when the resistors act as fuses.
     
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  9. MikeRam

    MikeRam Active Member

    Messages:
    177
    Location:
    Portland Oregon
    damacman,
    I've only heard them on my test speakers. Can't wait to hook it up to my modified La Scalas :) As nice as it looks without the cage (that's why I took a picture of it that way), I'm going to play it safe like c_dk suggested and keep it on. I'd hate to have my wife get shocked while she's dusting it off or something.

    c_dk,

    Thanks for the reminder about the pre-driver transistors. I know you have mentioned that the upgrade was mandatory for the MC2300's but only suggested for the 2100 and 2105. I was hoping to be able to skip that upgrade but I'll go ahead and order the parts and go for it.

    So I’m assuming that these are still the correct substitutions?

    2N5320 https://www.mouser.com/ProductDetail/Central-Semiconductor/2N5320?qs=Ypxpq5eNvNVHzjidnRyF9Q==

    2N5322 https://www.mouser.com/ProductDetail/Central-Semiconductor/2N5322?qs=Ypxpq5eNvNVVwI%2bWi6SNXw==

    Would this work for the heat sink:

    322400B00000G https://www.mouser.com/ProductDetail/Aavid-Thermalloy/322400B00000G?qs=sGAEpiMZZMttgyDkZ5Wiuty1kLnbuLRyHVt2i0Y43Qw=


    For those who have not seen the instructions for replacing the transistors, I thought I'd include them here so you won't have to search around for them:



    INSTRUCTIONS FOR REPLACING 132-038 AND 132-039 TRANSISTORS WITH 132-153 AND 132-154 TRANSISTORS IN mc2100, mc2105, AND mc2300 POWER AMPLIFIERS.


    DESCRIPTION

    The above listed replacement transistors have similar ratings to the original parts, but have increased heat dissipation capacity when used with the 080-007 heat dissipator.


    INSTRUCTIONS FOR REPLACEMENT

    It is recommended that both the 132-038 and 132-039 be replaced at the same time. Follow the procedure indicated below:


    1. Remove the driver plug-in circuit boards from the power amplifier. (Be sure the amplifier is unplugged from the AC power line.)

    2. Remove the 132-038 and 132-039 transistors.

    3. Replace the 132-038 with the 132-153. Place the bias diode 070-046 under the 132-153.

    4. Jumper together the two circuit board lands that formerly connected to the 132-038 heat sink using a 1 ½” length of wire and a 1” length of spaghetti.

    5. Replace the 132-039 with the 132-154.

    6. Jumper together the two circuit board lands that formerly connected to the 132-039 heat sink using a 1 ½” length of wire and a 1” length of spaghetti.

    7. Fasten the 080-007 heat sinks to the 132-153 and 132-154 transistors.

    8. In addition, the following changes are to be made on the P.C. board.

    a. MC2100: 043-899 p.c. BOARD

    i. Delete the C13-14.

    ii. Delete R17-18 (22 ohms, 1/2W) and replace R17-18 with 18 ohm ½ W.

    iii. Delete C9-15-10-16 (0.0012mfd, 10%) and replace C9-15-10-16 with 680pf disk. Cap.

    b. MC2105: 043-899 p.c. BOARD

    i. Delete C123-124.

    ii. Delete R119-120 (22 ohms, 1/2W) and replace R119-120 with 18 ohm ½ W

    iii. Delete c115-117-116-118 (0.0012mfd, 10%) and replace C115-117-116-118 with 680pf disk. Cap.

    c. MC2300: 044-243 p.c. BOARD

    i. Delete C123-124.

    ii. Delete R119-120 (27 ohms, 1/2W) and replace R119-120 with 18 ohm ½ W

    9. Check to be sure that:

    a. The 070-046 diode must be under the 132-153 transistor.

    b. There must be jumpers as indicated in steps 4 and 6.

    c. The heat sinks must be mounted.

    10. Reinstall the drive plug-in circuit boards into the power amplifier. Be sure to secure the circuit board properly.

    11. The amplifier is now ready for use.


    Some pictures I found that somebody took of the replacement kit and the assembling of it:

    DSCN0560.jpg

    DSCN0565.jpg

    DSCN0568.jpg

    DSC04434-680pF-recto.jpg

    DSC04435-680pF-verso.jpg
     
    Last edited: Apr 10, 2018
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  10. Wayniac

    Wayniac AK Subscriber Subscriber

    Messages:
    300
    Location:
    Dunnellon, Florida
    Do it. I’m running a bi-amp configuration with MC30s and a MC2100 driving a pair of JBL 250TIs. It’s a spectacular combination.
     

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