Amplifier motorboating vs. scanner motorboating

[Bob-i]

In my recent AO-28 troubleshooting thread it was mentioned that the motorboating was more likely the scanner than the preamp. In this thread I will describe my troubleshooting and how I reached the conclusion that the filter caps in the AO-28 were the likely cause.

Amplifier motorboating, aka parasitic oscillation (P.O.) is typically caused by electrolytics drying out. It sounds like this...

https://youtu.be/iEj3ngPD9N0

Scanner motorboating is something I haven’t experienced yet but my understanding is that it’s caused by dendrites in the scanner shorting out. Here’s what I found on YouTube...

https://youtu.be/LRoK3eNc5pk

So when my A100 started motorboating what I heard was very typical of older guitar amps I’ve serviced. I searched this forum and some threads suggested filter caps, so no surprises.

I pulled out my audio stethoscope, just a 1/4” jack, the ground connected to an alligator clip, the hot connected to a .01uF cap then to an alligator clip. I connected the 1/4” to a powered studio monitor, clipped the ground to the AO-28 chassis, put a weight on a key and started listening at various points in the signal chain.

First I validated that I could hear the motorboating at the speaker out. Just touching the speaker terminals briefly was enough, very little organ sound and lots of thumping.

Next I went to the input of the power amp. Same results so that eliminated the power amp.

I skipped the output of the preamp since electrically it’s the same as the input of the power amp. I went to the input of the preamp. Both the normal and vibrato inputs produced clean organ tone.

Conclusion. The problem is in the preamp, most likely culprit would be the can caps. So I ordered a cap set from tonewheel General. After a few hours of tracking down some crossed up wires, the organ is 100% again. Not bad for a 54 year old organ. From what I can tell only these caps and the reverb pan are not original.

[David Anderson]

Turn on the organ without starting the tone generator. (Run switch only.) The scanner will not be turning.

If it still motorboats, it's oscillation caused by dried up capacitors, most often a cathode bypass. If it doesn't motorboat under these conditions, check the scanner. A shorted scanner pole is easy to find with an ohmmeter.

Diagnosis shouldn't take more than 15 minutes.

[Bob-i]

Turn on the organ without starting the tone generator. (Run switch only.) The scanner will not be turning.

If it still motorboats, it's oscillation caused by dried up capacitors, most often a cathode bypass. If it doesn't motorboat under these conditions, check the scanner. A shorted scanner pole is easy to find with an ohmmeter.

Diagnosis shouldn't take more than 15 minutes.

Thx, very helpful, however my diagnosis only took about 15 minutes too.

And yes the cathode bypass was the root cause but I replaced all of them while I was in there.