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Voltage and Resistance Measurement Tolerance?

Discussion in '"Boat Anchor" & Classic Equipment' started by KJ6SVX, Jul 19, 2013.

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  1. KJ6SVX

    KJ6SVX Ham Member QRZ Page

    As mentioned in a previous post, I've recently acquired a Heathkit SB-101 station. The amp is in good operating condition and the transceiver works but needs some TLC. I've been going through the final test procedures specified in the construction manual and am wondering what tolerances are OK with respect to the various resistance and voltage measurements. Clearly the components have aged and the measurements aren't going to be spot-on so I was wondering how far off they should be before I go looking for a faulty component.

    The major issue with the transceiver is that the 28.0 - 28.5 mHz crystal is bad, however the quality of the reception seems to fluctuate. I've made a couple of contacts on 20m so I know that both XMT and RX do work. Output is 100w +/- into a dummy load on all band switch positions except 28.0 - 28.5. Also, the meter needle only wiggles a little when in the s-meter function.
  2. KO6WB

    KO6WB Premium Subscriber QRZ Page

    A lot of questions in one posting...But the voltage and the resistance readings will vary and if
    they are within 10-15% that should be good enough.
    If you have other 10 meters portions work and just the 28-28.5 is bad then I'd suspect a bad hetrodyne crystal.
    Perhaps a bad solder joint or two or a dirty switch.
    As for the s-meter being a problem I'd want to know if the AGC was turned off and also if received stations have
    a large difference between the strong ones and the ones just above the noise. If that's true and the AGC is enabled
    then I would start looking at the AGC circuit in the 101.
    Hope this helps
  3. K7MEM

    K7MEM Ham Member QRZ Page

  4. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    The tolerances for components and alignment measurements should be exactly the same as those specified by Heathkit in the manuals! Why would they be any different?

    With a radio this old, looking at output deficiencies and then deciding to check component tolerance is a 'backwards' approach. It really is worth the time to go thru the radio first. In the end it will give you a reliable radio, prevent catastrophic blowouts and save grief.
  5. KE3WD

    KE3WD Ham Member QRZ Page

    *Don't attempt any adjustments to cure problems before first performing the following:

    *Visual Inspection of solder joints, connections, cleaning up anything that looks like a dull solder joint and if any plugin connects are found inside, at least remove and replace the plug (ONE AT A TIME, to avoid mistakes) once in order to mechanically wipe the connection points clean.

    *Go over all circuit board hold down screws, terminal strip hold down screws, basically all screws, and loosen a partial turn and retighten, by hand, to make sure that all Grounds are good.

    *Clean and lube all controls and switches except for any main power switches, using Deoxit or equivalent.

    Only then can you make any reliable Voltage or Continuity measurements, as any of the above could be getting in the way of the proper measurement by adding resistance, or even in some cases not even completing the circuit being measured. Dry switch and control contacts, bandswitch wafer contacts, etc. can do that easily.

  6. KE3WD

    KE3WD Ham Member QRZ Page

    The post earlier, "10 to 15%" is a good tolerance for those measurements.

    Important thing here is to not convince yourself to go chasing after what I call "red herring" readings. For example, if the Plate Voltage is supposed to be "360VDC" and you are reading something lower or higher but the voltage is still there basically, it is within the above tolerance or even a bit outside of that, it is not likely to be the show-stopper.

  7. KM1H

    KM1H Ham Member QRZ Page

    What the above continuously misses is that 50-60's measurements may not be the same today.

    Carefully read about the instruments used as well as the AC line voltage. An old VTVM in good calibration will likely read close to a DVM but a VOM of 5000 to 20000 Ohms/volt sensitivity will read way different, especially grid voltages. Some real old gear used 1000 Ohms/V meters to create the charts.

  8. KH2G

    KH2G Ham Member QRZ Page

    I noticed in days of old and today, Heathkit volt and resistance measurements tended to be pretty realistic. I agree that a 1000 Ohm/V meter will not be as accurate as a 1 meg/V meter in the scheme of things.
  9. KE3WD

    KE3WD Ham Member QRZ Page

    Check schematics and sometimes written spec literature for what was used to make the readings for the charts. That is supposed to be included, but sometimes isn't.

    On schematics, the ohms/volt is often listed.

    If it isn't, I think it is safe to assume "vacuum tube voltmeter" input impedance, which should be 10Mohms or better.

    If you have a VOM and you start to encounter ALL voltages being lower than the printout, suspect the meter to be loading down the circuits.

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