Rodgers 750A Issue

[Ajtheorganist]

Decided to start a new topic in this forum since it now falls more under repair than schematics

Our church's Rodgers 750A has a high pitch cipher-like noise that constantly sounds. It's not terribly loud, but loud enough to be annoying for the congregation. Originally I thought that this was a feed-through issue, but after adjusting the feed through and investigating some more, I believe it's some sort of cipher. I discovered that it was affected by the Foundation Chorus and Main Tremulant, so I decided to look toward the oscillators. Sure enough, note 69 (on the principal I believe) is ciphering through somehow because I was able to tune it sharp and flat without any stops on! I went back through every stop and investigated if it was on any specific stop, but all the stops added to the sound/none were the same like a normal cipher.

Does anyone have any ideas? All of the components near note 69 looked ok, but I didn't have a multimeter on me at the time.

[jbird604]

Could be a leaky diode in one of the keyer boards on the circuitry related to that pitch. It wouldn't have to be fully shorted, just passing enough current to be audible. You might tinker with the level controls on the various stops and find out which one affects it. Then find the associated keyer board, trace the #69 pitch to where it feeds onto that board, then check the diodes that gate that pitch.

[Ajtheorganist]

Thanks for the suggestion! I tried adjusting all of the level controls, but none of them affected the sound of the high-pitched noise. I even went back through them all hoping I missed it on the first time.

What else should be checked? I was hoping to not have to go through the entire keyer board assembly checking diodes, but it might come to that. Is there something I could unsolder so that the entire circuit for that note would be open? I hate not having a note on the organ, but it's high enough to almost never be used and when it is, I think I could manage without.

[jbird604]

If you are sure that it is coming from that one oscillator you can of course disable that single pitch by unsoldering the tiny enamel wire that carries that pitch on to the various keyer boards. But it would be better to try to determine which keyer board is letting it pass through so you would still have that pitch on some stops.

Did you try turning down each of the four main level pots on the output board? I think they are labeled Main, Flute, Pedal, and Swell or something like that. If just one of those affects the volume, you would have a clue as to the source of the tone.

[Ajtheorganist]

I did forget to mention that the main volume pot was the one thing that affected the sound, I apologize. What all does it affect?

[jbird604]

The "Main" pot adjusts the volume of any stops that have not been split off into the other channels. So if your 750 has all four separate channels implemented, you can rule out the flute keyers, the pedal extensions, and the various swell keyers. It may well be just the Unit Diapason keyer that is the source of this cipher. Try lifting the enamel wire that brings that pitch to the board from the oscillator rack. Or, if you are careful, you can ground that pitch just past the gating diode on the diapason board. If you can stop it that way, you'll know that the gate diode is probably leaky.