Keeping the Prime Mover Alive
3 minute read
or TLC for the Icom IC260
July 2006
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 (2005) 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. I was fully expecting it to be a lengthy diagnostic process.
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.
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!
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.
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.
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!
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.