W8WWV - ICOM 756 VFO Encoder Saga

Greg Ordy

Note: This story twists and turns at the end, so please read all of this page,

The rotary shaft encoder in my ICOM 756 developed a problem. This is the primary VFO control. A portion of its rotation would not change the displayed frequency. It had a dead zone. The problem increased in severity until approximately 90 degrees of rotation were dead.

It seemed as if the obvious solution was to replace the part. ICOM (Bellevue, WA) does indeed sell the part (their number 91510606). The cost, however (at the time of writing this page), was $130 (USD). Since that seemed rather high, I decided to explore alternatives.

The first alternative was to repair the defective encoder. It appeared as if the problem was due to some sort of contamination. Many encoders are based upon optical slots in a rotating wheel. A small piece of lint or even hair might disrupt the normal operation of the encoder. I decided to take mine apart.

Getting at the encoder is not fun. Even after the front panel is unscrewed from the chassis, it is still connected by several delicate multiwire cables. It's very easy to damage the radio while removing the encoder.

Finally, the encoder was hanging from the radio by its 4 wire interface cable. The encoder looks much like a common potentiometer. In fact, as I pried open the metal case, I fully expected, even hoped, to see a slotted wheel with a LED/optocoupler assembly across it. I wanted to find some lint, or other contamination, clinging to part of the wheel.

I was more than surprised when the contents of the case turned out to be a solid wheel surrounded by a printed circuit board with two integrated circuits, and a number of surface mount passive components, including miniature trimmer pots.. This was no simple encoder. It was a highly integrated and complex subsystem.

While the circuit board and its supporting structure obscured most of the rotating wheel, I was able to gain limited access to it. I used a combination of cleaning sprays and compressed air on the wheel. When I next turned on the radio, the encoder worked perfectly. After about 6 hours, however, the encoder again started to fail (the dead zone returned). I resorted to more spray and air. While the dead zone did permanently go away, the control did develop jitter in the least significant digit. This is much better than the dead zone problem, but it is not perfect.

So, with only partial success in repairing the encoder, I again looked into obtaining a replacement part.

The ICOM schematic showed the part number to be RMS20-250-201-1P. I didn't know if this was an ICOM number, or the number of the manufacturer. The encoder metal case duplicated this number, along with the name Copal. I was fairly confident that I now had the manufacturer's name and their part number. I got on the Internet, and in less than two minutes, I was looking at a specification sheet for the encoder.

The encoder is indeed an impressive part. Please check the specification sheet for details. It is a magnetic encoder as opposed to optical. Each single revolution of the shaft produces 500 output pulses. At the standard tuning rate of 10 Hz/pulse, one knob revolution changes the frequency by 5 KHz. I talked with a factory representative and learned that there was no specific or recommended cleaning/adjustment procedure. I also learned that the part is not stocked in the United States (except by folks such as ICOM, who purchase the part for their own use and consumption).

It is possible, however, to order the part directly from their (Copal) California office. In quantities of 10, the price per part is $26. In quantities of 100, the price per part is $22. The bad news is that there is a 12 week delivery delay.

Given the lack of success in cleaning the original part, and the replacement cost from the manufacturer, I decided to order 10 parts. I am writing this page while awaiting delivery (estimated at June, 2000). I will be installing one encoder in my 756, and keeping a spare one on the shelf for my 756Pro. It appears to use the same encoder. The remaining parts have been promised to other 756 owners with similar encoder problems.

NOTE: The information on this page was gathered in March, 2000. Prices and other data may change over time. I will not be held responsible for damage done to your radio as a result of reading this page. I have provided this information so that others with the same problem can have access to my experiences. If you decide to open your radio, at the least, obtain the appropriate ICOM service manual. I've also been told to work with one hand in your pocket...

EPILOG: Copal did indeed send me 10 encoders a few days ahead of schedule. I installed one in my 756, and it worked, electrically, perfectly. While putting my radio back together, however, it became clear that the new encoder shaft was 1/2 inch too short! I rechecked that I had ordered the part number obtained from both the ICOM schematic and actual encoder case. They matched. I then started to more carefully check the Copal data sheet. Indeed, their data sheet described a part with the shorter shaft. I had been so concerned about the electrical interface, and because the part numbers were an exact match, I just didn't pay attention to the shaft length.

I contacted Copal, and it turns out that they made a special batch of encoders just for ICOM. These encoders have the same Copal part number but a longer shaft. In addition, Copal could not even sell me the part, claiming that they sold all of the stock, and were not set up to make any more. I suspect that ICOM specified a special variation on a standard part and then Copal built them a custom batch. So, as of this writing (September, 2000), the only place where you can purchase a long shaft encoder is from ICOM. I have been working on schemes to extend the encoder shafts I have, and I believe that the shorter or normal shaft encoders are used in the ICOM 706. Still, the lesson here is that you can never do too much checking...

[I have since acquired a 706MKIIG, which is a fine radio. It does not use the short shaft encoder. I do believe that the 756PRO and 756PROII use the same encoder as found in the standard 756]

Update: July, 2002. I was contacted by Wolf, DJ3TZ concerning this issue. Apparently the Kenwood TS-850 uses a Copal encoder which is nearly identical to the ICOM 756. Wolf was also experiencing erratic frequency tuning,  which became worse over time. Wolf was clever enough to experiment with the trimmer pots in the encoder. He found that his problem was due to a marginal alignment of the encoder. By adjusting the trimmers within the encoder can, he was able to regain complete functionality! So, if you are experiencing frequency jumping and jitter in the last digit, perhaps you have an encoder which has drifted out of alignment. Wolf gave me permission to reprint his experience on this page. Here it is. Thanks Wolf, this sounds like it might be the real problem, and it's easy to fix, if you're willing to open up the radio and spend some time.

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Last update: Monday, September 30, 2002 02:09:28 PM
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