Replacing pots, I think service manual is backwards?

Discussion in 'Radio Circuits, Repair & Performance' started by KE7ZNV, Jun 25, 2013.

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  1. KE7ZNV

    KE7ZNV Ham Member QRZ Page

    Working on an old Icom IC-28A radio, in need of replacement pots for both the volume/power, and squelch controls. Looking at the service manual, the volume/power control lists a RK0941114 (10KB) part number, and the squelch/check lists a RK9A12 (10KA) part number. This appears to specify a 10K Ohm linear taper for the volume/power, and a 10K Ohm audio taper for the squelch, which seems backwards to me. Wouldn't I want an audio taper on the volume, and linear taper on the squelch?

    These appear to be Alps pots, and the closest I can cross reference would be

    Power - RK097111202P, 10K Ohm, push on/push off switch
    Squelch - RK0971114D0B, 10K Ohm momentary push (or RK0971114Z07, which would be a better fit physically, but I'm not sure if the 1B taper would be better than the 3B of the D0B pot)
     
  2. KE3WD

    KE3WD Ham Member QRZ Page

    Check for part numbers on the actual pots themselves, might be a typo in the manual.

    If that doesn't pan out, try using your ohmmeter on the existing pots, a linear taper pot will be very close to half its full value when the shaft is turned to the halfway point.


    73
     
  3. KE7ZNV

    KE7ZNV Ham Member QRZ Page

    only numbers I see on the pots themselves are 103 on both...not terribly helpful, and the reason I'm replacing the pots is because they both only work in one spot, with juuuuusst the right, tiny, amount of pressure sideways on the knobs. They're dead everywhere else, thus measuring isn't of much help :p
     
  4. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    103 = 10k

    Take them apart and measure them. If they don't work anyway, you have nothing to lose.

    If you can't do that, just replace them whichever way you think will work and see if it works. I can't imagine how having an audio-tapered pot for squelch would matter. You might never even know if it was that way or not.

    You can connect them temporarily with jumper-wires to see if it works the way you think it should.
     
  5. KE3WD

    KE3WD Ham Member QRZ Page

    The convention is used on pots, capacitors, coils and is simple enough once you know the secret handshake.

    "103" = a 10 with 3 zeros after it, or 10,000

    With small capacitors, the decimal just goes in the other direction to indicate the fractional decimal values used in such caps.

    About your pots - they may not even need replaced, from your last description here, it sounds like a condition of dry or dirty brush wipers internal to them.

    A quick shot of Deoxit or equivalent control cleaner/lube, then rotate the pot from end to end ten or twenty times while still wet with the good stuff in order to mechanically WIPE the contact areas often restores these dry pots to like new operation.

    Well worth trying before ordering up new pots. Most of the time it solves the problem, the exception being pots that have broken carbon trace or have been mechanically worn by being turned without lube for too long.

    (In a pinch, you can shoot a wee bit of WD-40 into the pots to see if that's the problem. While the WD-40 won't do as good of a job as a formula such as Deoxit, and may not last as long, it can indeed clean and lube a dirty pot and make it work like it should again.)


    73
     
  6. KE7ZNV

    KE7ZNV Ham Member QRZ Page

    Ooooohhhhh! That makes perfect sense now! Thanks! :)

    Although, that said, it wasn't the values I was questioning, but rather the type - linear or logarithmic/audio. That was my big question there, whether it made sense that the squelch control would be an audio taper, and the volume being a linear taper, as the service manual specifies. That part seems backwards to me, with logic telling me that squelch should be linear, and volume being logarithmic.

    I have a ton of the Techspray contact cleaner here (http://www.techspray.com/product-info.php?pId=15&cId=26) here, and have soaked the pots down a few times from the shaft end, and from the tiny openings on the side, and that's how I got the minimal operation from them. Ordering/replacing them isn't a big deal to me, as they're only about $15 shipped from Mouser. I just want to make sure I'm getting the right ones as the pins are set up slightly different, and one is a push on/push off, with the other being a momentary switch, so they're not directly swappable. Thus I didn't want to buy both the linear and log versions of both to see what works.
     
  7. K8ERV

    K8ERV QRZ Member QRZ Page

    I agree the log should be the audio. Do you have some pots to try even if the value is different? Probably not critical.

    TOM K8ERV Montrose Colo
     
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