[Premium-Rx] Tuning problem with Racal R1772 / R1776, a bit of progress:

[Nico Dongen]

As was suggested by some I have cleaned the Switch contacts and I have adjusted the encoder balance but this did not cure the problem. When I unscrewed the front to get access to the Display Board I found that the board has been butchered in the past when someone was making “repairs” On two IC positions the copper wiring was destroyed when they had unsoldered the IC’s. To repair the damage the new IC’s where connected with blank wires directly from the IC pins to the place where the wire in the PCB had broken off. It is a total mess. On another position they removed the IC and installed a socked with a new IC. There is not too much damage on this place except that I see dry soldering joints at the socked pins. 

In the third variation they had cut the pins high at the old IC, placed a new IC over these pins and soldered the pins together. In this last IC I found the problem that the out of range H and L lamp where lit at the same time. It turned out that one pin was not soldered so this is solved now.

The main problem with the frequency jumps when switching Tune Slow to Fast to lock still remains. I noticed that sometimes the tuning is frozen disregards if the switch is in fast or slow mode. The problem comes and goes at will so it more looks like a problem in the logic and not so much in the switch or adjustment.
The IC’s that where replaced and have such a poor soldering quality are in the encoder input circuit (ML-8, 27, 28). This can imply two things, the circuit was defective and they replaced the IC’s to get it going and introduced another problem during the repair. Alternatively the fault my have not been solved by replacement of the IC’s and the fault is caused by some other component they missed. I will try to trace the problem with the scope but these kinds of problems are difficult to get visible. 

Any suggestions?  Does someone perhaps have a spare display board available?   

Best regards,

Nico