[Lowfer] Icom R75 Frequency Jumps

[John Andrews]

I have two Icom R75 receivers, both more than 10 years old. They have 
pretty much run 24/7 between here and Maine, a sort of service that they 
probably weren't designed for. In the last few years, I have replaced 
most of the electrolytics.

But starting a couple of years ago, I began noticing small frequency 
jumps on the order of 0.1Hz when looking at QRSS screens. Didn't happen 
all of the time...one or the other receiver might be OK for weeks. My 
first approach was to look at regulated voltages, but that got nowhere. 
The troubleshooting was made more difficult by the slowness of a QRSS30 
or 60 screen -- a jump wouldn't be noticed until a minute or so later.

To make the long story short, the culprit turned out to be the passband 
tuning control on the front panel. That's a dual 10k pot with concentric 
shafts and a detent in the center of revolution. The pot feeds 0-5V DC 
signals to A/D inputs on the microcontroller. That detent position had 
become very noisy, and the controller was programming the DDS circuits 
accordingly. The pots were made by Toko, and have been discontinued. But 
Icom seems to have a good supply at 16 bucks apiece. Since I had also 
been having issues with audio volume jumps, I picked up replacement 
AF/RF gain controls as well.

Anyway, all seems to be well now. The line on an Argo display now looks 
like the EKG of a patient that assumed room temperature some days ago, a 
good thing in this case.

John, W1TAG