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Thread: Bad Filter Cap in a 5150

  1. #1

    Default Bad Filter Cap in a 5150

    This weekend at VCFMW I had someone with MUCH better hearing than my own finally track down the source of the annoying whistle/whine in my 5150. Turns out it's the speaker! He suggested that there must be a bad filter cap somewhere on the motherboard that is causing feedback on the speaker. So my question is, what's the best way to tackle this? I'm not much of a hardware guy, but I'm willing to try and troubleshoot this. Do I just start replacing every cap I see that might be connected to the speaker until the noise stops? Where are all the filter caps on the 5150 located? This is the latest revision of the board that supports 640K.

  2. #2
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    There are lots of capacitors on a 5150 an 5160, but they come in three varieties, the ceramic capacitors, the tantalum capacitors and the variable capacitor. The ceramic and the tantalums look similar, but the tantalums are usually larger and use three legs while the ceramics use only two legs. I'd start with replacing the tantalums first, since they carry much more capacitance than the ceramics and are more likely to fail. The tantalums can explode when they fail.

    However, this does not definitely mean that the capacitors on your board are the culprit. It could easily be a capacitor or some other component on an expansion card, a disk drive or the power supply. Marginal power supplies are a prime source of hum.
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  3. #3

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    I suppose I can try removing all the cards and see if that helps. That's an easy first step.

  4. #4
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    Quote Originally Posted by tempest View Post
    This is the latest revision of the board that supports 640K.
    With the 10/27/82 revision BIOS fitted, both the 16KB-64KB motherboard and 64KB-256KB motherboard support 640K total.
    But it sounds like you have a 64KB-256KB motherboard.

    Quote Originally Posted by tempest View Post
    This weekend at VCFMW I had someone with MUCH better hearing than my own finally track down the source of the annoying whistle/whine in my 5150. Turns out it's the speaker!
    Presumably proven by removing the speaker.

    Quote Originally Posted by tempest View Post
    He suggested that there must be a bad filter cap somewhere on the motherboard that is causing feedback on the speaker.
    Because the other end of the speaker is connected to an an open-collector driver (the 75477 chip below), I would not have thought that above-average ripple on the +5V line would cause the symptom.
    I think that added to the possible-cause list should be:
    * Faulty capacitor C9 (breaking down)
    * Faulty 75477 chip



    Quote Originally Posted by tempest View Post
    Where are all the filter caps on the 5150 located?
    +5V filter caps on 16KB-64KB motherboard - see photo at [here].
    +5V filter caps on 64KB-256KB motherboard - see photo at [here].

  5. #5

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    Quote Originally Posted by modem7 View Post
    But it sounds like you have a 64KB-256KB motherboard.
    Yes it's the 64KB-256KB motherboard.

    Quote Originally Posted by modem7 View Post
    Presumably proven by removing the speaker.
    No we didn't remove the speaker. But it's pretty obvious the sound is coming from the speaker.


    Quote Originally Posted by modem7 View Post
    Because the other end of the speaker is connected to an an open-collector driver (the 75477 chip below), I would not have thought that above-average ripple on the +5V line would cause the symptom.
    I think that added to the possible-cause list should be:
    * Faulty capacitor C9 (breaking down)
    * Faulty 75477 chip
    I can look at those. I assume the 75477 is soldered in? Removing that may be beyond my skills, but I can probably get the capacitor out.
    Last edited by tempest; September 12th, 2017 at 08:54 AM.

  6. #6
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    Quote Originally Posted by tempest View Post
    No we didn't remove the speaker. But it's pretty obvious the sound is coming from the speaker.
    But not obvious enough for you to have tracked down the whistle/whine source by yourself. Removing the speaker is simple. Imagine removing the speaker and discovering that the noise is still present.

    Quote Originally Posted by tempest View Post
    I assume the 75477 is soldered in?
    It is on the two boards that I have just looked at.

  7. #7

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    Quote Originally Posted by modem7 View Post
    But not obvious enough for you to have tracked down the whistle/whine source by yourself. .
    That's because my hearing is crap. I'm honestly amazed I heard the noise at all. Years of working in a server room will do that to you.

  8. #8

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    I've finally gotten some spare time on my hands to work on this. What should be my first step in attempting to diagnose the problem? I can start by removing all the cards and see if by any chance the noise disappears (unlikely as I think I had it on with only the disk controller at one point and the noise was still there). But after that I'll have to start using my multimeter. Any tips on how I should go about testing things without blowing things up?

  9. #9

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    I dont suppose you know someone with an oscilloscope you could borrow?

    First thing I would check is for ripple in the +5V line at the power supply -> motherboard connector. If there is excessive ripple there I would start by replacing the smoothing capacitors in the power supply.

    Next I would examine the motherboard closely and make sure nobody has snipped off a filter cap. Usually the tantalums short out when they fail; this can cause the tantalum to explode or it can trip the power supply overcurrent feature (and the computer turns off). A common diagnosis for the latter scenario is to snip off the capacitor and see if the computer turns on. A lot of people don't bother replacing the capacitor and leave it at that as the machine still works.

    Lastly check for ripple on the +5V pin of the 75477 chip. You can the use the schematics to find the closest filter caps and replace them. If that doesn't work check for ripple on the inputs to the 75477 chip. If it's clean then replace the 75477; if not you will need to probe further back in the circuit to find out what is going on.
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

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