[Collins] Cure for Noisy PBT on 75A-4 (long)

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Sun, 10 Nov 2002 13:27:22 -0500


Noisy 75A-4 Passband Tuning Control: a  Quick Cure



The  Passband Tuning control on my A4 was noisy. As I turned it, I could hear static that sounded like a noisy AF gain pot. Also, sometimes the BFO pitch would jump around in frequency.

I determined that this was caused by a dirty connection from the shaft of the Passband Tuning Capacitor C117. I confirmed this by temporarily jumpering the insulated shaft coupler, which made a good ground connection through the PBT control shaft at the front panel and of course through the metal belt that turns the PTO housing.

I knew exactly what was needed - a tiny drop of Deoxit in the right place. The problem is, the ‘right place’ is inside the BFO can - or so I thought.

I contacted Howard Mills who kindly gave me instructions on getting into the BFO can. He told me that the way to do it is to first loosen the shaft coupler and slide it towards the front panel. Then clip the three leads to the can and remove the two screws holding it to the chassis. The can can then be removed from the radio and worked on separately. Remove the pan head screws from the outer cover, four #4’s around the base and three #6’s in the front side. Then you can carefully slide the cover off.

I did all this, and applied the Deoxit to the wiper of the shaft of C117. Then, upon close examination I learned something which might have obviated the necessity to remove the BFO from the radio.

The shaft of C117 transitions into a bushing right at the front of the capacitor. The bushing and the shaft are the same diameter and color, so it’s easy to assume it’s all a solid shaft into the capacitor frame. You probably wouldn’t notice it unless you looked very closely with a bright light - I certainly didn’t. Anyway, the joint at the bushing is just barely visible with the cover on. I applied a tiny drop of Deoxit to the joint between the bushing and the shaft, and the shaft immediately worked much more freely.

If your 75A-4 exhibits this noisy Passband Tuning behavior, you might try to fix it the ‘easy way’ without removing the can. Loosen the shaft coupler and slide it off the capacitor shaft. Then apply a tiny drop of Deoxit to the joint between the shaft and the bushing, right at the plane of the metal cover, and work it into the joint by turning the control back and forth. You can turn the capacitor through 360 degrees with the coupler removed. It’s worth a try, and if it doesn’t work, you can still fix it the ‘hard way’!

73,

Tony, K1KP