Leslie 900 with combo preamp II

[Thomas204]

So I recently got a great deal on a leslie 900, Which had been standing in someones garage for about 9 years. When I went to the seller to get it, he surprised me with a box of replacement parts, a rotary effect pedal, a frequency inverter (50/60Hz) kit and a preamp II.
It was an imported leslie, so I had to get a transformer to get the 117V required for the leslie and the preamp. Yesterday I gt the transformer, but when I tried everything out, nothing happened :(.
The bottom drum motor made some humming noises when I turned the preamp on, but after a second or two, the breaker on the pedal pops out. I think the problem is in the leslie? because when I haven't got the 9-pin cable attached, the curcuit won't short.
I'm quite sure the leslie is modified, it doesn't have the reverb unit nor the reverb preamplifier, and it also has a two-speed motor on the bottom drum, which is connected via a 6-pin connector on the power supply, with only 4 pins attached to cables (I believe from other posts that the original only has two pins connected). The top part doesn't do anything, no spinning and no sound. When I disconnect the motor connector from the bottom drum, the preamp won't short, but I don't get any sound either. I checked the motors and speakers on their own, and they worked fine, so the problem is either in the leslie power supply/amplifier or in the preamp pedal. (after some googling I also found that the preamp II was originally intended for a leslie 825, so maybe it's just the wrong pin layout? The only difference I found in the two pin layouts was 29V DC on pin 5 for the 900, where the 825 had no connection.
I contacted the previous owner, but he said he never used the pedal and couldn't help me with those technical questions.
Does anyone have any idea where the problem could be?

[Jyvoipabo]

Good evening,

The appropriate pedal is the Deluxe with 4 footswitches. A type II have same pin-out bu only 2 footswitches excluding Reverb. This is why the reverb is discarded. This is not a problem because on stage you can add reverb through the mixer. And when everything works, i think it is possible to modify the pedal.

If a motors growls and trip the Circuit breaker, you have a problem with a thyristor. Then ,the motor is only controlled with a half wave. What he doesn't like because it needs a full wave to work properly.

JP

[Thomas204]

Thanks!

Indeed sounds like that's the problem, I'll check it out in a week or two, exams have t go first :(

Thomas

[Thomas204]

Sooooo...

I finally have time and my hands are aching, but I still can't find the problem.
I've looked into the schematics and found the thyristors, but I can't really find them on the board.
I checked the backside of the board and there are some markings (A, C, G) for the anode cathode and gate of the thyristors. I've looked up how to check them with a multimeter, but when I try to, nothing happens. So either the thyristors are bad, or I'm doing something wrong (which feels to me like the more likely scenario).
To try to check if I am doing something wrong, I checked the voltages on the connectors to the upper motor, but I get nothing, on every point 0V.

I really don't know what to do next, should I just buy new thyristors? I don't think that would totally solve it, and I'm still not entirely trusting the pedal.

If anyone has any suggestions, please leave them as I want to let this thing rumble :P

Thanks in advance!

[Jyvoipabo]

> Thyristor or Triacs : They are difficult to test off line with a multimeter. But between A and K they work like a switch. In case of a defect, they can often work strangely. So to test them, it is more easier to put the amp on the table and connect a filament lamp instead of the motors. It should lit brightly when On and remain dark when OFF.
> If you find zéro Volt everywhere on the control circuit, there is a problem. Look at the 900 diagram and more precisely : 'Q9,Q10 Transistor Voltages DC Table'.


You can use it to check that depending on the modes you get correct voltages.
Remark : They were always negatives or zéro.
The pedal connects pin 1 (Gnd) to pin 6 (or 7). Voltages on this pins should be ~-12 / zéro at work.

JP

[Thomas204]

One little noob question; what do I measure when measuring those voltages in the table? I connect one lead to the given leg of the transistor, but the other lead? Or do you just measure the difference of the voltages on the different legs?

Also when removing the amplifier from the leslie itself and inspecting it a but further, I noticed a kind of burn spot beneath one of the resistors, so I guess that'll need replacement. (unfortunately I can't upload the foto so it seams, so I'll try that another time)

I'll try the light test with the triacs this afternoon and reply afterwards.

Thomas
Sorry for the late response, I'll try to be online more frequently.

[Thomas204]

So, I (kind of) tested the triacs, and while I do get continuity on the cathode and gate, I don't on anode and cathode on any of them while switched on any speed. Also tested the burnt-looking resistor, and it still measures 18 ohms while on the diagram it says ohms. I also tested the relays and I don't measure continuity on any of them either with the leslie switched on any speed.

Thomas

[Jyvoipabo]

Ah Ah !

How to measure voltages.
Set the meter on volt DC and appropriate scale.
Connect the black wire (Com) to ground. It is often the metallic chassis and don't move it.
Connect the red Wire to each point to be measured.

Don't worry about the resistor value into the +/- 20% range is OK.

JP

[Thomas204]

Hmmm, that's what I thought and tried, but then all the legs of the capacitors measure 0V. The connector that goes to the motor does measure 117AC though?

btw I put the black lead on the chassis of the ampifier itself (the metal housing)

Thomas

[Jyvoipabo]

No negative voltage on the transistor legs = KO.

Négative suppy include a transformer(T1) 2 rectifiers (D4,D5) the filtering capacitor (C46) 2 power resistor (R56,R49=130ohm/5W) and a diode (D10).

The 2 power resistor can't be cold. They must warm up a little at work.

Check R56 ~130Ohm.
Check -33V supply on one side of this resistor and -12V on the other side.

JP

[Thomas204]

I checked the resistors, they are both fine (between 130 and 140 Ohms), but there is no voltage on either side of R56.
could it be the transformer?

Thomas

[Jyvoipabo]

I don't think so.
A burnt transformer does not go unnoticed because it smells very bad.
However, check fuse and winding are not cut.

JP

[Thomas204]

I'm back!

Sorry for being away for so long, I thought I would have some time on hands in the vacation, but... well not.

I checked continuity on the transformer, and it all seems to be right, so no broken wires. Maybe it is the pedal after all?

Thomas