37-116x - Oscillator trimmer instability - bad ground - Printable Version

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+--- Thread: 37-116x - Oscillator trimmer instability - bad ground (/showthread.php?tid=2449)

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37-116x - Oscillator trimmer instability - bad ground - spfradios2 - 06-15-2010

I've never seen this issue come up before, but if it has, pardon my taking your time.

I've been completely rewiring my 37-116. During the restoration of the RF section, which include coils for the expanded bands along with the associated trimmers, I was able to diconnect and test the trimmers (call compensators) for connectivity and operability. Some trimmers are connected to the chassis ground via the frame. To do this, various style of rivets were used.

Some of the rivets used appear to be made of aluminum (not sure about this). For those trimmer, I found that the ground connectivity was poor, perhaps from 70+ years of oxidation/reduction between incompatible metals. The resistance was fluctuating wildly every second, from a few ohms to hundreds of ohms. Such poor connectivity is not obvious, and most people may not take time to check it. But it seems to me that this would cause a very unstable radio, especially in the oscillator section.

I also had a similar experience with the IF expander pot. The center lug had poor connectivity to the metal strip going inside the pot. Again, a bad rivet perhaps caused by incompatible metals.

To remedy the situation, I simply hard wired from the chassis frame directly to the spring arms (top) of each affected trimmer. For the pot, I wired from the lug, through the rivet hole, to the internal wire strip exposed on the outside of the pot.

If you can't stablize your oscillator, may a bad trimmer ground is the issue.

John
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Re: 37-116x - Oscillator trimmer instability - bad ground - Ron Ramirez - 06-15-2010

Excellent post, John, and thank you for sharing this with us.

That reminded me that I once had a similar issue with a poor ground in the low frequency padder of a 37-650. I think I ended up replacing the padder...and then running one wire to ground as you suggested.

I still think that Philco's "Unit Construction" was one of the worst ideas they ever came up with. The man who designed it, David Grimes of Inverse Duplex fame (or notoriety), should have been given a life sentence of having to service the things.

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Re: 37-116x - Oscillator trimmer instability - bad ground - hotrod - 06-17-2010

Thanks for the tip John. I didn't check any of the trimmers when recapping my 38-116 and still have an alignment to do this fall. So far I haven't noticed any problems with the oscillator but i will keep this in mind. I also like how you labeled the trimmers and I think I will do the same when it is time for alignment.

Rod

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