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Thread: Help identifying a resistor please.

  1. #1
    Join Date
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    Default Help identifying a resistor please.

    Hi Folks,

    In this recent thread I mentioned this I'd got a Canadian PDP-8 clone.

    I am in the process of restoring it and have checked out the power supplies and given the boards that make up the CPU and clean and visual check.

    I have now moved on to the core memory boards and have run across a snag I hope someone can help me with.
    IMG_20191102_144851803.jpg
    Inspecting on of the two core memory boards I noticed a broken resistor on the board.

    In the hope of finding out what value it is I turned to the other identical core memory board and it has the same fault on the same resistor. Talk about identical.

    IMG_20191103_121524180-crop.jpg
    So here's is my problem. How do I identify this resistor? I have a photo but I can't see a tolerance band so I'm not sure which way to read it. Could it really be a 50 Ohm resistor?

    Can someone suggest a way forward?


    Thanks,

    Andy.

  2. #2
    Join Date
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    Default

    It looks like the next door resistor has the same band structure.

    If it was me, I would use a multimeter first to measure the resistance of an intact device and see where that takes you. Measure twice, with the multimeter probes one way them the other.

    It is either a very high value or a very low one!

    Green, brown, brown = 510 Ohms.

    Brown, brown, green = 1,100,000 Ohms = 1.1 MOhm.

    Dave

  3. #3

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    I completely agree with Dave. It looks like a 510 ohms one, but I'd not take chances and just measure the twin one.
    Frank

  4. #4

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    I vote for 510 Ω.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  5. #5
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    My vote is for 510 ohm, 5%, carbon comp. A way to verify is to ascertain its function in the circuit, of course.

  6. #6

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    I have never seen green used as a tolerance band on a carbon comp resistor. It looks like a 510 ohm to me but I can't tell if it is a brown or a faded red.
    Both boards likely failed because of the thermal stress. It is likely on a 12v rail. That would be a little over 1/4w for a 510 ohm. The resistor is only a 1/4w resistor.
    Anyway 510 is a standard value and 522 is not so I'd say within 99%, it is 510 ohms.
    I don't recommend replacing it with another carbon composition. They are poor resistors and drift a lot when run near their rated wattage. They do have low inductance compared with carbon film but then your not running at 1 GHz either. If you are an exact type freak, then use a carbon comp.
    Dwight

  7. #7
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    "If you are an exact type freak, then use a carbon comp."

    That's why I mentioned the fact that it was composition. I suspect that there are still people on this forum looking for vintage Rifa line filter caps as well.

    I note that the 68 ohm resistor to the left of the intact 510 ohm is a film resistor, so it's a big shrug. This must be a piece of industrial kit--given the Teflon sleeving and washers on each resistor. You wouldn't find this in consumer electronics.

  8. #8

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    I might still have some. IIRC though it wasn't depicting 0.5%, it had some other significance.

    But on no three band resistor should one be tolerance, I think.

  9. #9

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    Quote Originally Posted by KC9UDX View Post
    I might still have some. IIRC though it wasn't depicting 0.5%, it had some other significance.

    But on no three band resistor should one be tolerance, I think.
    No, no tolerance band means 20% or 15% as I recall. Silver means 10% and gold means 5%. I don't recall ever seeing a 1% carbon comp let alone a 0.5%.

  10. #10
    Join Date
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    Location
    Leicester, UK
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    Default

    Thank you everyone for your replies.

    I read its companion both ways round and it is indeed 510 Ohms (minus a smidge) so I have replaced it with a new one.

    IMG_20191103_140133702-crop.jpg
    Looking at the size or the original compared to the 1/4w replacement, I wonder if it was 1/8w. anyway, it's gone now!


    Thanks again all,

    Andy.

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