75A4 PTO losing full MHz range?

Discussion in '"Boat Anchor" & Classic Equipment' started by W1BR, Jan 14, 2021.

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  1. W1BR

    W1BR Premium Subscriber QRZ Page

    Has anyone else with a 75A4 have the same problem with an aging PTO losing tuning range? Mine has a cumulative error of about 800 Hz per each 100 kHz. Enough to require constant recalibration across the dial, but still within correctable range using the graticule.

    Aging caps or permeability changes in the tuning core?
     
  2. W1NB

    W1NB XML Subscriber QRZ Page

    I’ve read about this and experienced it on my 32v-3, although not to the extent that you have. According to what I read, the ferrite slug degrades over time causing permanent change in its permeability. My A-4 has not experienced the same issue.
     
  3. W1BR

    W1BR Premium Subscriber QRZ Page

    Received a few answers on the CCA reflector. It sounds like I'd have to pull the PTO to access the trimmer inductor. If that doesn't have enough correction it will require removing turns. Not sure I want to go through that much work.
     
  4. W1NB

    W1NB XML Subscriber QRZ Page

    Peter,
    You’re probably already aware but I have occasionally seen PTOs for the A4 come up on eBay. Of course sellers will rarely indicate the tracking accuracy.

    I have a basket case A4 in pieces waiting to be reassembled. I’m wondering how hard it would be to build a test jig. I’ve disassembled and reassembled a PTO from an S Line but I don’t know how different the A4 is. I’d be curious about what you ultimately do and how you make out.
     
  5. W1BR

    W1BR Premium Subscriber QRZ Page

    CCA guys said to put it in a vice and power it off the radio or with an external supply. You'd need a large knob with a pointer and a cardboard back for the graticule.

    This PTO is not bad, just the end to end cumulative error is annoying. Fortunately none of the ham bands cover the full MHz tuning range, LOL. Next time it is on the bench
    I'll dig into the PTO. There is an adjustment between the two tubes for a trimmer inductor to set the range. I've been told that one may have to remove a turn, or going further,
    there is a 5 pF cap across a large value cap in the tank circuit (560 pF????) that can be clipped if needed.

    https://antiqueradios.com/forums/viewtopic.php?f=5&t=389108
     
  6. W1BR

    W1BR Premium Subscriber QRZ Page

    Another gent is telling me the 75A4 PTO can be adjusted without removing it. A bit of conflicting info, and I haven't had time to look inside.
     
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    Located the adjustment... it looks like it can be done without removing the PTO, assuming the slug hasn't aged beyond the trimmer's correction range. It is right to the left of V-14.
    Between V-14 and the hex metal spacer.


    75A4 PTO.JPG
     
  8. W1BR

    W1BR Premium Subscriber QRZ Page

    Fixed. About three turns clockwise and she came in with perfect endpoints. 100 kHz intermediate points are within a few hundred Hz. Had to move the shaft setting to get the graticule centered at zero beat. Also found the W1LSB AGC issue was caused by some bad notes I had written down... corrected the errors and the AGC is very acceptable on CW and SSB.
     
    W4NNF and KP4SX like this.
  9. W1NB

    W1NB XML Subscriber QRZ Page

    Congrats, and thanks for the info. I’ve saved it for future reference in case my project A4 has similar issues. Thanks for the updates.
     

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