Conn 580 Rhythm section

[Freddy, UK]

This is a Conn 580 Type 1 with the manual rhythm section which, over the last two weeks, has slowly faded out. The rest of the organ works perfectly.

I have the service manual which shows the input voltage to the pcb as 29 and the test meter shows 30, so that's a good start. However, after that, the actual voltages, and all seem to be present, are slightly less than those on the cct diagram, eg 17v is reduced to 14. There is an output voltage at the pot that controls volume and the last cap in the circuit pings the esr meter so is OK. I think the next pcb in the line is the post amps to which the Min-o-matic unit is connected and that works fine.

The fact that the whole unit has failed rather than say, the tambourine or snare drum, seems significant but my limited electronic knowledge cannot proceed further.

I know you can't realistically fault find without seeing the organ: if anything immediately springs to mind, please respond.

Thanks

[Drawbar Dave]

I am not familiar with this organ or particular circuit, but if I were trouble shooting it I'd be looking for electrolytic capacitors that may have failed. After that I'd be looking at any divider circuit that split down the dc supply to the various areas. If the organ is over 20 years old I'd replace the electrolytic caps anyway. You could check the resistor values with a multimeter although you might have to lift one leg of the resistor to get an accurate reading.

[Freddy, UK]

All the dc voltages seem to be present. Replacing the caps is easier written than done as there are 50+ and very crowded on the pcb.
I am also advised to make up a signal chaser (lead with a low value cap), find a known audio point (don't know what that would be) and work back from the post amps.
Thanks for your response.

[Freddy, UK]

How lucky can you get? A friend of a friend knows a retired electronics expert who once worked for Wurlitzer and lives only a few miles away. He agreed to come and have a look and eventually narrowed it down to either a cap or transistor. I had already checked this cap with the ESR meter and it showed only slightly leaky. It was replaced and once more I can do drum rolls and cymbal crashes and the rest. It seems that the slightly leaky cap was dragging down voltages across the board. Happy ending.