McIntosh 4100 cap question

Discussion in 'McIntosh Audio' started by eedork, Aug 24, 2017.

  1. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Hifi engine has 2 versions. The earlier one seems to be yours and shows what i said. I'll glance at the other. Just out of curiosity what voltages are you seeing at pins 13, and 14, (to ground) and what do you read pin13 to pin14?
    The second manual is just an addendum which doesn't change anything in the PS.
     
  2. eedork

    eedork AK Subscriber Subscriber

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    Hi nj -

    I read about 48V from 13 to 14 after a few minutes of warm up. From 14 to ground is about 49V and from 13 to ground starts at ~6V and then drops as the unit warms up to almost 0V.

    How do you interpret the schematic? The weird part is that pin 14 says 40V on it, but pin 14 is also directly connected to the 48V supply rail. So that is just inconsistent, and I'm not sure how to interpret it.

    -Matt
     
  3. eedork

    eedork AK Subscriber Subscriber

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    I should clarify why I've started looking at the relay circuit in the first place. The whole 4100 has been recapped with the exception of maybe five or so caps that I did not get in my original order.

    In any event, when I first turn the unit on after about 30 seconds the right channel (only the right channel, the left is fine) DC offset goes to 48V and trips the protection circuit. After a few seconds the protection circuit turns off. This happens three times and then strangely the channel stabilizes with the protection off. At this point the DC offset is about 13mV on the right channel and both channels seem to work as expected.

    Something is obviously not right, because the protection circuit shouldn't be tripping and resetting - that alone is very weird. The fact that it does this three times and then seems to stabilize is also weird. When this first happened I pulled the right channel driver board, removed the output transistors, tested and verified that they were good (using the simple diode test on my DMM), cleaned them and reinstalled with new sil-pads. So I'm really stumped here, and suspect that the issue may actually be with the protection circuit itself, not the right channel driver. If the right channel driver had a fault (like rail voltage on the output) I would expect to see it *all the time*, not just in short bursts.

    What's also weird is that after the unit stabilizes, everything seems to work fine. I verified this with a pair of test speakers playing at moderate levels for two hours. It played and sounded excellent - DC offset remained steady and both channels ran cool.

    This is what led me to the relay. I was using it as a point for measuring DC offset of both channels and noticed the strange coil voltages that did not line up with the schematic's 40V (pin 14) and 16V (pin 13) markings. Is it possible that the protection circuit itself is causing this problem?

    On the relay the right channel output from the driver board and the right protector input on the power supply board (pin 15) are connected together (the same is true for the left channel as well). This is where I was measuring DC offset. The fault could therefore be from either the power supply or driver board.

    -Matt
     
    Last edited: Sep 4, 2017
  4. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    I'm not going to lie to you. This is somewhat above my pay grade. Hands on i could do some tracing and testing and maybe come up with an answer. Again not having it in front of me makes that impossible. (Truth be told , i might be as stumped as you after a bit. ) i don't really have time to dig into drawing tonight but i'll try soon. i suspect there are more than a few here who can help you past your issue before i might figure it out. i'm thinking out loud and guessing at the same time here. is it possible a cap went in reverse polarity during recap?
    i try to take pics of before, in case mistake in silk screening on board. the three times thing makes me think something not right is going on as well.
     
  5. eedork

    eedork AK Subscriber Subscriber

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    Thanks nj, and I understand your position completely and please don't feel any pressure to dig into the schematic. I'm just looking for a second opinion since the schematic seems to contradict itself.

    I do have pictures of all boards prior to the recap. I am careful when doing the work, but have not yet verified all polarities on the new caps. I'll try to do that soon.

    -Matt
     
  6. eedork

    eedork AK Subscriber Subscriber

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    I've attached a screenshot of the schematic just to illustrate the discrepancy I've found with pin 14 on the power supply board. I've circled the two points that are causing my confusion.

    If anyone can make any sense of this, please let me know.

    Thanks!
    -Matt
     

    Attached Files:

  7. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    If it makes you feel any better (probably not , I see what you see. I'm not sure how it can be 40v and 48v (unless its measured from another reference point). (i did look at the manual, i was hoping screenshot didn't tell the whole picture) More likely a typo.I suspect if you worked on these for the past 30 years or so , you just know. I'm surprised Chris or Patrice hasn't chimed in on what you're seeing.
     
  8. eedork

    eedork AK Subscriber Subscriber

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    Thanks nj - I'm glad to hear that I'm not crazy. I also just double checked and am definitely measuring 48V across the coil, so I'm extremely surprised to see a 24V relay there, and even more surprised that it is still actually working. I'd love to get to the bottom of this - hopefully one of the experts will provide some insight.

    Speaking of experts, I actually talked to Patrice on the phone today (thanks for the call Patrice - I really appreciate it!). He gave me some good advice and also recommended that I call the factory and make sure to get the correct service manual for my serial number, which I did. Brian at McIntosh parts was kind enough to email a PDF to me (thanks to Brian at McIntosh too!). The bad news is that the service manual that I received from McIntosh appears to be the same one that I downloaded from hifiengine, so no clarification of the schematic there.

    During our conversation, Patrice felt from memory that a 24V part was used in the 6200/4100, but of course he stated that I need confirm this by getting the correct McIntosh part number for the relay from the correct service manual for my serial number and asking the factory.

    Another idea Patrice had was to swap the right and left channel driver boards to see if the protection issue I described earlier followed the board. It did, which confirmed that the issue was on the right channel driver board and not in the power supply. I removed the right channel driver board, took a closer look at it, and identified a handful of sketchy looking solder joints. I reflowed these joints and also worked the bias trim pot back and forth a few times being sure to set it back to the original ohmage. After installing the board, I no longer have the weird protection issue. So that's a win, and I'm crossing my fingers that the real issue was a bad solder joint that I missed during my initial recap.

    I'm still baffled by the relay circuit however. I don't want to put a new 24V ($10) relay in and see it burn up the minute I apply power because the coil is getting 48V.

    -Matt
     
  9. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    I have a few thoughts. Not the end all answer but maybe help fill some of the holes. First, did you measure the coil voltage when its energized?
    What resistance do you read through the coil of the one you pulled. What is the resistance of the new relay? Does the relay you pulled have standard part # that you can cross reference to a known part?
     
  10. eedork

    eedork AK Subscriber Subscriber

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    Hi nj - yes, I did measure the coil voltage while energized and playing music.

    I have not measured the coil resistance - I'll do that when I pull it again. The new relay should arrive tomorrow, so I'll compare the coil resistance of the old and new relays and report back.

    The old relay is a Magnecraft 78RCSX-11 which states 24VDC right on the case. Another AKer posted earlier in this thread that this is the same part that they have in their 4100, and CohibaJoe also referenced this part in his 4100 recap thread:

    MAC 4100 Recap

    This relay issue is absolutely baffling!

    -Matt
     
  11. eedork

    eedork AK Subscriber Subscriber

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    Did some more testing tonight. Everything is now working great on this 4100 with the exception of the FM right channel. All other inputs are working, including phono, AM, AUX, etc. I originally thought this was due to a bad LM301 on the preamp board. It is not. The right channel for FM is not making it off the tuner board (and not even off the TCA4500a decoder chip). So at this point I suspect the unit is either seriously out of alignment or the TCA4500a chip is bad. Note that I am getting a stereo light, so the TCA4500a is working to some degree.

    -Matt
     
  12. eedork

    eedork AK Subscriber Subscriber

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    Just replaced the TCA4500A with a brand new National Semi LM4500AN decoder chip and I now have FM stereo. The TCA4500A (a Motorola part) is no longer available, but the LM4500AN is available and is a direct drop-in replacement based on my research and understanding.

    It should probably be aligned at some point, but for now this 4100 is sounding great!

    -Matt
     

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