ICOM IC-R7000 Radio tuning wheel only tunes down

From Frank
frolektrics electronics notes & projects

When I bought my second hand R7000, it also came with a few problems thrown in for free. Most were fixed by replacing all the electro’s but by far the most annoying was the fact that the radio only tuned down when using the encoder wheel!

The encoder itself puts out an I-Q signal, that is to say an “In phase” signal that oscillates between logic high and low as the tuning encoder wheel is turned, and a “Quadrature” signal that does the same but is offset 90 degrees with respect to the in phase signal (why is it called Quadrature? – think of a square, 4 * 90 degree corners).

Now normally, you would have the I-Q signal going straight to the microprocessor within the radio to effect tuning. The frequency of the I or Q would determine tuning speed, and the order of the IQ edges would determine direction. I.e If I is before q then tune down – else tune up.

The ICOM engineers had different ideas however. They wrote the MCU code such that it accepts a Step & Direction signal from the encoder rather than I&Q. Dunno why, it may save a few program cycles – you only have to interrupt on the step signal, then read the direction signal to determine which way to tune.

The fallout from this design decision is that now some glue logic must be implemented between the IQ output of the tuning encoder and the Step-Direction input of the MCU.

As my radio only tuned down, the best place to look for a fault would be around this glue logic – as you can get at the IQ signal from the encoder and the STEP-DIRECTION to the MCU all in one spot. I I have pasted in the relavent snippet of the logic board below.

I & Q signals from the encoder come in on IC4 pins 13 & 8 respectively. The STEP output to the MCU comes out from the right hand side of C9. And finally the direction output of the MCU comes out from pin11 of IC5.

IC5 implements a D-Flip Flop (the crossed gates of IC5d & IC5a is a classic DIY S/R flip-flop) and so if say the order of the IQ edges is I-Q I-Q then ones are clocked through to pin 11 and the MCU counts step pulses up. If the order of the IQ edges is the other way, Q-I Q-,I then zeroes are clocked through to pin 11 and the MCU counts step pulses down.

On my radio Pin11 was always low no matter which way I turned the wheel, thereby telling the MCU to only tune down!

The fix was easy in the end and just involved the replacement of a $2 part on the logic board, IC5 – A CMOS series quad NAND gate 4011.

Note in this shot, the new IC5 installed on a good quality machined pin IC socket. You could solder it directly onto the board if you wish, but if you do use an IC socket – make sure it is a good quality machined pin type and not one of those cheapie dual wipers, which have a tendancy to induce chip creep as the temperature cycles. This radio was a work of art at the time when it was made and should be respected.

I hope that this article may be of use in your R7000 repair endeavours.

Cheers