B-3 momentary drop in volume when switching speeds to Leslie 122

[Roundsound]

My Hammond B-3 has a momentary drop in volume when switching speed from slow to fast with my Leslie 122. The switch is new. What could be causing this?

[Roundsound]

Actually, the volume dip happens when I switch from fast to slow speed, if that helps to clarify. Any input would be appreciated.

[Hamman]

This is on Geo's business site "Benton Electrics (or is it Electronics?)" site under 122 leslie switching
Hope this helps and Geo I hope this was OK to post:-)
Relay switching in a 122 is carried out by applying a DC voltage to the balanced input of the Leslie Amp. This voltage is applied equally to both input pins. Pin-1 and Pin-6. The source of this DC voltage can come from the Organ or from the Leslie itself.

Here's how it works. The DC voltage goes into a "kit" that divides the voltage down with resistors and selectively applies it to the Leslie input through the control of the Leslie switch. The heart of the system is a 1:1 matching or isolation transformer. The divided voltage is applied to the center tap of the transformer thus applying it to both input pins equally. Since the input stage of the Leslie is capacitor coupled it effectively blocks the DC and only passes the AC (signal) to the 12AU7a tube. There is a 1meg resistor connected to pin-1 of the Leslie input that goes to the relay switching tube. It senses the DC and turns on or off the switching tube which in turn turns on or off the relay.

This voltage should be around 50 or 60 Vdc. This can vary due to the source of the voltage and is not critical. Some older kits can actually deliver up to 100Vdc. However, there is a common failure in the above circuit here that can cause the voltage to be too high. The 47K 1w resistor in the kit may open up after many years and cause the divider to not reduce the source voltage enough. This higher switching voltage can actually cause a momentary dip in volume at the time of switching. This is because the coupling caps in the Leslie input are briefly coupling the large voltage swing into the 12AU7a tube and changing it's bias. You can put the Leslie switch in the chorale position and measure the output DC voltage with a meter. Since it only requires around 35Vdc to actually switch the relay, then for the least adverse effect on the audio, the lower this voltage, the better.

The second possible problem is the 10 mfd 450v cap inside the kit which can short. This will cause no DC to exit the kit and make the Leslie only operate on fast speed.

If this capacitor were to open up and effectively not be in the circuit, the volume dip at switching would be more obvious because the switching voltage would rise too fast.Good Luck!

[Roundsound]

Thanks Hamman. Although this seems beyond my scope at this point, I will look into measuring the output DC voltage and replacing the 47K 1w resistor in the Leslie.
You are a great resource!

[Chris Keyman]

Can also happen if your house AC is lousy at the box or wiring. In all seriousness...try running an extension cord to some other outlets. Rare to happen, but I have seen/ heard it myself. Those switches draw a lot of current.
Chris

[Roundsound]

Problem solved! It turns out that when I cleaned the dust off the Leslie amp, I noticed that the brown motor plugs were in the white sockets and the white plugs were in the brown sockets. When I switched them to their proper spots, everything worked just fine. Ya gotta love simple solutions!