Leslie 122xb lower rotor speed issues

[John East]

I have a 122xb which I use with a Hammond XK3c. I don't play it that often as I prefer to play my A100s and vintage Leslies, but it may as well work properly - which at the moment, it doesn't. The problem is this. As soon as I put my foot down on the expression pedal, or turn on the vibrato chorus, or the tube overdrive on the organ, the lower rotor switches to fast and stays there unless I brake to stop completely. Not the horns, just the lower rotor. On advice I've replaced the 6550s, but it makes no difference. There are a couple of engineers here in the UK trying to suggest a solution remotely, but to no avail. Has anyone had the same problem and, if so, how did you solve it please?

Hoping for words of wisdom!
John

[David Anderson]

My guess would be that the problem is on the motor control board -- perhaps a bad Triac or bad solder joint. Some earlier Leslies with this motor control board have a single-sided board that's prone to cracked solder joints. Later boards have through-plated holes. Through-plated holes are more reliable, but it's a lot harder to remove components without damaging the board. You need excellent soldering tools and skills.

That's just a guess based on symptoms.

[John East]

Thanks David,

I'll pass that on to those who may be able to help me actually fix it.

[Jyvoipabo]

Hello,

The motor drive of the 122XB is a little bit complicated.
It consist in a servo loop including a tachometer.
This is a shutter disk and a fork opto coupler.
I suggest to clean this opto and the disk for dust and check the disk is secure with the motor.

Because upper and lower use the same circuit, you can try to swap the fork if it is possible.

JP

[Frankquip]

I have a 122XB and had a simular problem with both horn and rotor going to max speed periodically, I found there was an oscillation in the wire harness on J3 (the slim connector towards the front of the unit on the motor speed board with the speed sensor and motor drive wires). I installed a 5uf cap to ground on the "control" pin on that slim connector, I think it was pin 7 the violet wire? The horn and rotor speed should be adjusted to 400 rpm with the onboard speed pots.

[David Anderson]

[QUOTE=Frankquip;n715912]I have a 122XB and had a simular problem with both horn and rotor going to max speed periodically, I found there was an oscillation in the wire harness on J3 /QUOTE]

Had you already replaced the electrolytic capacitors on the motor control board? I ask since oscillation can be a symptom of a supply problem.

[Jyvoipabo]

Hello,

Try to replace C11 200µF/25V cathode of the 6550 pair.
Check 25 Volts accross this capacitor.

JP