Hammond BCV with Vibrato/Chorus volume drop

[David Anderson]

I was working on a ~1939 Hammond BCV today, and the organ's volume drops significantly with the selector knobs in the Vibrato positions. The "wet" signal has a perfectly normal, even vibrato/chorus effect, just at greatly reduced volume.

To those who haven't run across one of these, a BCV is a Hammond BC that was retrofitted with the post-WWII vibrato chorus system, so it has the CV preamp, line box, scanner, and vibrato selector switch from the BV/CV era. They can have both scanner vibrato and chorus generator, but in this one, the chorus generator is missing.

This organ has lots of problems, so I didn't even have time to get into the volume drop issue. I thought I'd run it by the group to see if there are any common causes of vibrato volume-drop in V-series organs. The fact that this organ had the vibrato system retrofitted raises more possibilities. Was it all installed correctly? Did it ever work right?

I'm thinking of looking the area of the circuit where the Normal/Vibrato signals are equalized by padding down the Normal signal to match the series losses of the line box. This organ had a lot of mods over its lifetime, including a Glenn-Tone tube percussion unit and a "Nova Bass" module, both currently disconnected.

Ideas? I've been putting out fires left and right all week and am too tired to think.

Edit: Has anyone ever accidentally reversed the Ground/Drive wires to the line box on a BV or CV?

[enor]

I have indeed reversed those wires by accident once. The result was, if I recall right, a pretty vicious John Lord-like distortion effect.

The only problem I can think of in a -V organ that would lead to reduced volume with vib/chorus on, is some sort of fault in the delay line. An open resistor or a bad solder joint, most likely.

[David Anderson]

I'm trying to come up with a list of things to check. With a -2 or -3 series organ, there are enough differences in the signal path to provide a number of suspects, but in the -V organs with the vibrato system between the two halves of the preamp, I'm having a harder time deducing what would cause these symptoms, especially since the Bypass audio signal comes from the same resistor divider string (R40, R41, R42) as the input to the phase delay line. If the scanner bypass audio signal is good, it seems like the input to the delay line would have to be good as well. Faults in the delay line, in my experience, usually produce some audible symptoms, like choppiness, but here, there are none.

I'm starting to think it could be something in the preamp. The source impedance of the scanner is much higher than the source impedance of the dry signal. That's why the input impedance of the second half of the preamp has to be so high. If, somehow, the input impedance of the preamp was reduced, that might affect the scanner signal more than the bypass audio signal.

Normal volume is very strong.

Vibrato/Chorus sounds very nice with no choppiness or motorboating sounds in V1/V2/V3, but at markedly reduced volume.

[muckelroy]

I think checking the resistors in the 6J7 tube shield board is wise here. You're absolutely right that since the dry signal is strong and derived from a point even lower in the divider string than the “wet” in the line box, it isn't likely a line box problem. However I do wonder what the line box would do if R45 had drifted? It seems that could influence the level of the whole line box chain, unless I'm reading it incorrectly.

[enor]

How about a short in one of the caps?

[muckelroy]

If one cap is a dead short, I suppose there might be some decent but quiet sounding vibrato if it is on the far end of the line (C42 or C43.)

Test the signal level at each terminal to see what kind of losses you have. I'd be curious what the spread of voltages is.

[David Anderson]

Solution: The Vibrato Line and Vibrato Ground wires were, in fact, reversed at the preamp terminals. This organ is also missing the Vibrato/Chorus toggle switch, so you only get Chorus, not Vibrato. The Chorus effect is pretty mild, so it might benefit from tweaking the 22k resistor value.

This organ had decades of sloppy work in it.