I recently helped Andrej from Emšo Blues Band to repair his Viscount organ. Well, actually he did all the work, I just gave him some tips to figure out how the problematic chorus / vibrato section of the organ works. Eventually he found the issue and managed to get the organ fully working again.
The main issue was that, since some repair work on the other boards of the organ, the vibrato/chorus effect was having issues. Without using the effect the organs sound was correct, however with any of the two effects being used the low tones were heavily attenuated and also the sound was plain wrong.
Viscount organs are not that common so their repair manual / schematics are a bit harder to find (hint: You can try contacting the original manufacturer.). And even with the schematics in hand one still needs to track the signal down its path and make assumptions on what may be wrong.
The organ uses a TDA1022 BBD delay line to generate the two effects. Depending on the effect selection, different deviation frequencies and different mixing modes of delay line output and input signal are used.
Manual effect switch controls both the deviation frequency and the mixing of the original signal and TDA1022 output signal. One of the lower opamps is used as a LFO. CD4046 is used as a VCO to turn the slow LFO deviation frequency into higher frequency control signal used for driving TDA1022. CD4016 is used as analog switch to set mixing of the signals and to modify the frequency of CD4046 depending of the active effect.
After quite a bit of troubleshooting he found out that the control signal from the effect (vibrato/chorus) selector switch was not getting to the CD4016. Actually the signal from the wire was not getting to the PCB although the wire seemed soldered properly to the board.
He even made this nice image of the fix he made:
I guess there are two options why there was no connection – either a cold joint or corrosion of the joint and copper over time.
Cold joints usually happen with improper soldering techniques (wrong temperatures, poor wetting, unclean soldering surfaces), but the problem may not develop immidiately. Sometimes the defective solder joint may even mechanically hold the component in place while still not being conductive. This was also the case with Crumar Organizer organ I repaired some time ago – the wire started conducting only after the board warmed up…
Corrosion of the copper may happen because of too hot soldering temperatures or even slowly over time due to air and humidity in contact with two different metals. The PCB trace may sometimes develop hairline crack / corrosion directly adjacent to the soldering pad. The crack may be thin enough to be almost impossible to see without magnification.