need help finding replacement capacitors, also resistor value

Discussion in 'General Electronics Chat' started by Kremlar, Apr 16, 2012.

  1. Kremlar

    Thread Starter New Member

    Apr 16, 2012
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    First of all, I apologize in advance for my ignorance on this - I've got pretty much no electronics background.

    Trying to repair an old Commodore computer I have here and looking for some advice in getting replacement parts.

    I need to find replacements for 2 capacitors, both here:
    http://www.oldschoolcomputing.com/images/c128/capacitor1.jpg
    http://www.oldschoolcomputing.com/images/c128/capacitor2.jpg

    Capacitor 1 seems to be a ceramic disc capacitor, .22uF, +80%/-20%. The service manual says it should 25 volt, however the "16V" on it makes me think it's 16 volt. Can anyone confirm if "16V" means 16 volt? If so, does the discrepency make any sense?

    For capacitor 2, the manual says it should be .1uF, +80%/-20%, again 25 volt. Can anyone confirm if it looks right?

    In both cases, replacements of those values seem tough to come across but I want to be sure I know what I'm looking for before I dig too far.

    Finally, have a question on this resistor:
    http://www.oldschoolcomputing.com/images/c128dcr/resistor2.jpg

    The service manual says this should be a 27 ohm ceramic resistor, but the color bands look brown to me. Could this be because of age/heat? Or is it possible the service manual is incorrect?

    Thanks in advance for any advice, and again sorry for the newb questions!
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
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    All of these components are common and easily sourced. They also are not likely to fail unless physically broken. The values on the capacitors match your other data. You may be having some difficulty only because you're trying to find an exact match to the voltage and tolerance numbers. Newer components will generally have better ratings (higher voltage and narrower tolerance) since technology has improved since that computer was built. You may not readily find a ceramic capacitor with a voltage rating below 50 but the component in the circuit will never experience more than 12V. You can always use a higher voltage rating than is required, you'll have lots of margin. Don't be concerned about the tolerance or that replacements may look physically different.

    The resistor in your photo appears to be 110Ω but the three most ambiguous colors when degraded by heat are brown, red and violet. Red and violet tend to turn towards brown with heat and age. You could measure the resistor or try taking a close up photo of it in good light with a white piece of paper on each side to indicate correct white balance.
     
  3. Kremlar

    Thread Starter New Member

    Apr 16, 2012
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    Thanks for the reply! One of the capacitors is physically missing, so that's a no-brainer. The other one is in an area where there was some corrosion. It's probably OK but thought I'd just replace it while I was replacing the other one.

    The resistor is a tougher one. It looks like it's slightly busted open on one side, and then there are the 2 marks underneath. It may be just heat marks but thought I'd replace it anyway and while I have it out check the board underneath and see if there's any trace damage.

    You are right - I was trying to match exact specs. Thanks for that info!

    So, for capacitor 1, this would be a suitable replacement?
    http://search.digikey.com/us/en/products/RPEF51H224Z2K1A03B/490-3862-ND/946498

    And capacitor 2?
    http://search.digikey.com/us/en/products/RPEF51H104Z2K1A03B/490-3859-ND/946495

    I will try to take a better picture of that resistor and post back.

    Thanks again!
     
  4. Kremlar

    Thread Starter New Member

    Apr 16, 2012
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  5. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Yes, those modern MLCC types are perfect for your application. They're being used as bypass capacitors on your board to pass unwanted high frequency noise on data lines to ground.

    Even a resistor that's cracked in half can often be measured by probing from the break to a lead on each end and adding. I wouldn't bother replacing it unless it failed or the element was exposed which yours appears to be but if I didn't replace it I would make sure there's no corrosion contacting the leads.
     
    Last edited: Apr 16, 2012
  6. MrChips

    Moderator

    Oct 2, 2009
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    Can you provide the model of the computer? It might be possible to find the schematics.
     
  7. Kremlar

    Thread Starter New Member

    Apr 16, 2012
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    Actually, 2 computers....

    The first (with the 2 capacitors I'm looking to replace) is a Commodore 128. I have the service manual for that along with a Sams Computerfacts book. Here are PDFs for those:

    http://www.oldschoolcomputing.com/documents/c128_service.pdf
    http://www.oldschoolcomputing.com/documents/c128_sams.pdf


    Problem with this one is it simply does not boot in any mode (powers up, but black screen). If you're not familiar with this computer it has a Commodore 64 mode, a Commodore 128 mode (40 column composite output), and Commodore 128 mode (80 column RGB output). All modes produce the same result. I guess the C64 and C128 modes are pretty independent, so its not uncommon for the computer to be able to boot in 1 mode but not the other. This one does not boot in either mode.

    When I took it apart it was apparent that it had been worked on before. 1 chip half out of its socket, broken solder joint holding/grounding the metal shield to the motherboard, etc. I pulled and checked all the socketed chips and there were multiple bent pins. I also noticed that missing capacitor by chance.

    I actually have a fully working C128 here as well and I swapped all socketed chips between the 2 computers - the chips from the bad computer work fine in the good one, so it's not a bad socketed chip. I also tried swapping the power supplies. So, I feel the issue is a motherboard component or other damage, or a non-socketed chip.


    The other (with the resistor) is a Commodore 128DCR. This was sold as a Commodore 128D in the US, but inside the motherboard is labeled 128DCR (cost-reduced). Apparently the original 128D sold overseas used a normally Commodore 128 motherboard, but when sold in the US the motherboard was a 128DCR. I could not find the service manual for that. I DID find the Sams Computerfacts book for it however. Here is the PDF:

    http://www.oldschoolcomputing.com/documents/c128dcr_sams.pdf

    This one actually works OK mostly, but its C128 mode (40 or 80 column) stops booting after it has been on for a while (warmed up maybe?). If I power it down for a while it again starts working. I have never gotten it to lock up or cause issues while its on, just when I power it off it will not power back on in C128 mode until cooled down. C64 mode always seems to work fine.

    I havent troubleshooted much on this one other than reseating the chips, but I did see that suspicious looking resistor which made me consider replacing it.

    Any helps is appreciated!!!
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    If this is a commodore c128. It is quite old. So I would have replaced every electrolyic caps. Regardless of status. As they may have dried out. Or at least in the process of doing it. Then go through the power supply unit. To check if it is working as it should. Next why do you think the caps pointed to in the picture are gone.
    In C64 mode it works on a slower clock frequency, and hence draw less current. So it can be that the old electrolyte caps can keep up with load. But not with the full load in C128 mode.
     
  9. Kremlar

    Thread Starter New Member

    Apr 16, 2012
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    There is one that is missing. The picture is of one that is not missing. Was just trying to show the markings on the capacitor that SHOULD be there. :)


    I thought about replacing all the capacitors and probably will. None are leaking that I can see, but sure they could be dried out.

    Just thought I'd try things that showed visibly first.


    C128 mode also uses chips/etc. that are inactive in C64 mode (and vice versa).
     
  10. P-MONKE

    Member

    Mar 14, 2012
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    Personally, I would. There is a chance (small, because the circuit board is lacquered) that the resistor is making some contact with the tracks that run underneath it.

    At the very least do a continuity check between either end of the resistor and the tracks that it crosses.

    This actually happened to me recently on a 1974 TTL logic board - I spent hours tearing my hair out trying to work out why the output of an inverter was always high irrespective of the input. It turned out to be a pull-up resistor that had been slightly squashed by a previous tinkerer, cracking off some paint, and the metal skin was shorting out to +5V.
     
  11. t06afre

    AAC Fanatic!

    May 11, 2009
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    You will probably not need to change all the caps. But changing all the electrolytic ones(those with polarity) may be a good idea. The resistor looks to me that the first three rings are brown(110 ohm). From your pictures it looks original (not replaced). And I can not see any signs of overheating around it. But it could be that the picture fool me on that fact
     
  12. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I second the suggestion to replace all electrolytic capacitors and would go a little farther. I'd make sure the replacements were 105° rated and buy them from an authorized distributor like Digikey or Mouser to avoid counterfeits.

    Have you been able to get any output from the power supply?
     
  13. Kremlar

    Thread Starter New Member

    Apr 16, 2012
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    Then what do you think those brown marks are on the board? Just marks from the normal heat produced by the resistor?


    Yes, power supply works fine on a known working C128...
     
  14. t06afre

    AAC Fanatic!

    May 11, 2009
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    I can not see any usual signs of overheat on the PCB or the resistor. From your picture. You can try to desolder the resistor and use some alcohol to clean up the board. I think the brown stains will come of then. Then the resistor is out of the board try and measure it.
     
  15. Kremlar

    Thread Starter New Member

    Apr 16, 2012
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    Thanks guys... I pulled the resistor and it's all burnt looking and flaked off on the bottom. I cleaned the board with some alcohol and there are no burn marks there. I measured the resistor and it comes in at 110 ohms.

    So, now I want to get a replacement. I had bought some 27 ohm 3W resistors and they are FAR larger than this one.

    So, I'm pretty sure it's 110 ohms and 5%, but what about type? Carbon film, something else? How can I tell and does it matter?

    What about wattage? How do I know what size I need?

    Thanks again guys...
     
  16. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Any type will work but most you'll find will be carbon film or metal film. For wattage, just put in the largest that will fit conveniently and leave a small air gap between the resistor and the PCB..
     
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