[Lowfer] Icom R75 Frequency Jumps

[Howell, Laurence J]

Mine follows the same pattern as John - the rare jumps in vol level on one unit (actually your listening at one level and suddenly no audio; turning down and up the pot brings back the audio but no "scratchiness of old" noise from the pot); failure of the -14V reg, a few caps in the +5 and -5V lines had dried out and been replaced, but Ive not had the "jumpies yet" - 

I had to get Icom to completely replace one board in one of the receivers as I and Icom WA repair shop  couldn't find a noise issue that stumped us both so went the "easy" but fiscally painful route - this was after replacing a few devices after a nasty DC voltage surge.

As ever the issue seems to be when you switch the unit off and on again :-)

Still nice receivers and both are now back on line :

Laurence KL7 L 

To: Discussion of the Lowfer (US, European, & UK) and MedFer bands
Subject: [EXTERNAL][Lowfer] Icom R75 Frequency Jumps

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
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