Locating testing points on a schematic

Discussion in 'Radio Circuits, Repair & Performance' started by KD2NOM, Mar 7, 2018.

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  1. KD2NOM

    KD2NOM XML Subscriber QRZ Page

    Hello,

    I am in the process of restoring a Hallicrafters S-85 receiver. I have replaced all the capacitors and am preparing to align the coils, but before I do that, I want to verify that power is moving thru the radio properly after my capacitor replacement.

    This is my first real electronics project and I am not well versed in reading schematics - I am using this project as a learning opportunity. In looking over the schematic and the service manual there does not appear to be a section that discusses measuring voltages at different points within the radio.

    Are there special designations on the schematic that show where to measure?

    Thank you for your responses.

    Mark
     

    Attached Files:

  2. KD0CAC

    KD0CAC Ham Member QRZ Page

    My thought is that with mostly point to point like in that area , the test points are in the mind of the tech - using wiring diagrams .
    Test points came in , sometime after circuit boards , maybe multi layer , more than just top & bottom ?
     
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

  4. KD2NOM

    KD2NOM XML Subscriber QRZ Page

    Thank you - however this is for the S-38 and not the S-85 - I do have the service manual for it with the alignment procedure.

    I'll look at the tube data though - that is a good place to start!
     
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Sorry. My mistake. The theory is still the same.

    Enjoy.
     
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    There are some general rules of where to measure voltages and what to expect.

    First, the plate voltages should be in the order of 250 V
    Screen voltages in the order of 150 V
    Grid voltages in oscillator and mixer circuits in the order of -10 V.
    Must be measured with a resistor of about 1 Mohm in series with the DMM or VTVM otherwise the oscillations will stop
    Cathode voltages of RF, IF and AF stages, in the order of 5 V

    Everything measured with the RF gain fully on.

    More guidance can be found in tube data manuals.

    73/
    Karl-Arne
    SM0AOM
     
    W7UUU likes this.
  7. KD2NOM

    KD2NOM XML Subscriber QRZ Page

    I just downloaded the tube spec sheets for each tube and am reviewing them now.
     
  8. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    On page 10 of the S-85 manual there is a diagram showing every tube, in the approximate position on the chassis, from the underside of the receiver, with the nominal voltage given. The tube pin numbers are given in the schematic on page 11. You can always write the voltage, for each tube pin, on the schematic using the data on page 10.

    If you don't have this in your manual, send me an E-Mail at

    k9sth (at) sbcglobal (dot) net

    and I will forward a PDF of the entire S-85 manual.

    The voltages should be within about a 15% to 20% range, probably on the high side if different due to the higher line voltages of today. Remember, most of the resistors, etc., in the radio had a +/- 20% tolerance when new.

    Glen, K9STH
     
  9. KD2NOM

    KD2NOM XML Subscriber QRZ Page

    Thanks Glen - I do have the complete manual and have also downloaded all the tube references sheets from online. I didn't even notice the voltages on that page since the printing was so small. I have been testing each tube / pin now using a dc voltmeter with one probe to chassis and the other to the pin - so far most are within tolerances! Some are high so I am noting those for later investigation.

    Thanks for the tip
     
    Last edited: Mar 7, 2018
  10. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    I can forward the PDF and you can "blow up" page 10 so that you can see the voltages better.

    Since I work on "boat anchors" for others, although I do have a number of original / reproduction manuals, I have even more PDFs of manuals on my computers that I "print off" whatever information that I need at the time to work on the unit.

    Glen, K9STH
     

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