I bought my IC260 back in 1979. It was my first multi-mode rig and duly replaced the notorious Belcom Liner-2 that I had previously used for 2m SSB. Originally designed as a mobile compliment to the IC251 base-station, the IC260 quickly got a reputation as a high (telecoms) quality transceiver, with it’s characteristic compressed audio and ‘clean’ signal. The build quality is truly fantastic. Take a look under the top lid of an IC260 and you will see what I mean. Early on in it’s life it went back to Icom for a repair associated with the control logic. However in the intervening 20 years, it has given sterling service. However, last year I noticed that the SS/MW button (Start Scan/Memory Write) was intermittent. Finally, it failed altogether. Although I now have an IC706 MK2, I still use the IC260 for 2m occasionally and as the Prime Mover for my selection of transverters. Even though the IC260 only has a modest three memory channels, it is often convenient to set up a scan between two of them (ch2 & ch3). Not being able to write to memory made it not possible to scan portions the band when looking for beacons on 23cm and 13cm. I therefore decided to investigate the SS/MW issue. Below, is a diagram of the circuitry associated with the SS/MW button. I was fully expecting it to be a lengthy diagnostic process.
Icom IC260E (purchaced 1979)
Keeping the Prime Mover Alive
or. . . .
TLC for the Icom IC260
However this was not the case and after confirming that the circuit was not producing the appropriate pulses, I began to check voltage levels. It was then that I discovered that the voltage at the junction of C11 and R34 was 8V when S1 was open. This implied a resistance of 125K across the open switch. This too was confirmed, but where was this resitance? It had to be the switch itself! The time constant set up by R32/C12 is not critical so I simply wired a 100K resistor across R32 to prove my theory. This resolved the issue and the SS/MW button was again effective. Adding the resistor was far simpler than replacing the switch, so I have left the 100K resistor in place. Then a couple of weeks ago, whilst setting up my 13cm 60W amplifier, I became aware of a possible instability in the system. At first I put the blame on my 13cm transverter. The problem manifested itself as a noticeable residual, sometimes intermittent Drain Current. However the problem was not with the transverter but with the IC260.
IC260E SS/MW (S1) circuitry
I knew that the IC260 was in need of a tweak in relation to the SSB unit which involved resetting the USB/LSB/CW and FM frequency offsets. Get this wrong and it is possible to transmit a small level of carrier on SSB! Having reset the offsets, I did a test on 23cm to learn that although there was no vestige of residual carrier, there was something wrong which on occasion made me sound a bit like ‘The Crazy Frog’. This was traced to the 1750Hz tone unit which was ‘stuttering’ even when the ‘Tone’ button was NOT pressed. Solution? Throw out the tone unit! I can count on one hand the number of times I have used a repeater in 30 years!
IC260E 1750Hz Tone Generator (S3) circuitry
I ran a test on the bench with the removed tone-unit. Sure enough, supplying it 13.8V via a randomly selected 68K resistor produced the same ‘stuttering’ output that I had seen when it was installed in the IC260. I can only conclude that over the years, dust has accumulated inside the switches and over time has formed tracks of resistance which ultimately become significant