What steps to diagnose loud hum in a 21H?

[mpeets]

Found a 21H, and plugged it in (26-1 Connector that worked with a 125). Hum started and got louder as tubes warmed up. Ground loop? Bad tube? Not sure of steps to diagnose/fix problem. Disconnected from 26-1, provided AC power directly to pins 3/4. Same hum. Any ideas?

[geoelectro]

A 21H is not compatible with the 26-1 connector. 21H is a 6H type while the 125 is a 6W type connection.
It's possible some damage from the mismatch occurred. Otherwise, it's likely a bad filter capacitor.

http://www.captain-foldback.com/Lesl...matics/21H.GIF

The power supply section shows rectifier tubes (5Y3) and two filter caps with a 30 by each one. 30 microfarads. They are in fact located physically in one silver can. I would measure AC volts on each terminal looking for any excess AC voltage. Usually the first stage, the one closest to the rectifier tubes, should read no more than 10Vac. 2nd stage should be around 1Vac. Like the schematic shows there are high DC voltage present so use caution here.

Geo

[mpeets]

The organ is an X-77 that I found a couple of months ago. There was no X-77 Leslie and the output connectors had been bypassed. I took the four signals from the X-77 console and ran them to a small mixer, then routed the output of the mixer to an old Leslie 26-1 connector. The output of the 25-1 is AC on pins 3 & 4, signal on pins 1 & 6. All worked fine with a Leslie 125. So, when I found a reasonable 21H Leslie, I checked the pinouts and found the power and signal pins were exactly the same, I thought I'd just plug in the cable to the 26-1. Mistake, apparently. Disconnecting from the 26-1 right away, I checked the speaker and then horn by connecting them to a small amp without the Leslie even turned on. The speakers are fine. Also, the motors work perfectly when AC applied directly with the Leslie completely off. So, the hum. Without anything connected, I apply power to pins 3 & 4. The hum starts and builds as the tubes warm up. I'm hearing the problem is probably a capacitor, but I don't really have a clue about caps, filters, capacitors, resistors, or much else. However, I have a good tester and I can follow instructions.

[Wes]

Personally, I wouldn't start "fixing" anything until you can actually hook it up properly and see what it sounds like.

[David Anderson]

How can you test the speakers in a Leslie 21H without the amp powered up? The 21H is supposed to have a field-coil woofer that needs to have its coil energized by the amplifier to produce sound. Has it had a permanent magnet woofer installed?

[mpeets]

Exactly. Someone swapped out the field coil speaker. Anyone's guess as to what else has been done to the unit over the years. The challenge is to connect an X-77 console to a 21h Leslie that the X-77 was never designed to connect to. There is no ootb connector that will work, as far as I know. The X-77 has four channel output, and the oem 12 pin connector in the X-77 was torn out and some other wiring has been done that I have not yet been able to restore. The console does work. What I was able to do was find the four outputs and solder on a guitar cord for each channel. I run the four channels into a mini-mixer and connect the mini-mixer to a RotoChoir Leslie simulator and finally connect the RotoChoir to a master mixer, which also hosts a Korg i2, SM58 mics, and an instance of Cubase on a PC that has a sound card with twelve ins and outs. The master mixer outputs to two Bose L Compact amp/speaker systems. Sounds great. But, since I acquired the Leslie 21h real cheap on a whim, I would like to get it working and set up an alternative output from the X-77 or from the mini-mixer. So, I'm thinking a) run AC into a connector box, b) run the "mixed" output into the connector box, and run an on/off half moon switch into the connector box. I had this all working with an old Leslie 125 I've had for years just sitting out in my shed. I used an old 26-1 connector box I found. All working. But the sound from the 125 is not awe-inspiring. I now realize the 26-1 connector is not going to work with the 21h, so the challenge is what to use to get the "mixed" X-77 signal, the AC, and the switch to the 21h. I do know that the motors run properly when plugged directly into an AC outlet, the speakers work when hooked up directly to an amp, and the tubes light up. If the 32h amp in the Leslie worked properly, shouldn't I just be able to alligator clip the AC to pins 3 & 4 and the signal to pins 1 & 6 to get amp'd sound as a test to verify the amp is working? Then, if the amp is ok, I can focus on a permanent connector configuration. If the amp is not ok, I can pull it and take it to someone who can make it work. I have an amphenol I can wire whatever way is going to work, and I have a 6 pin cable from the 21h that I can't customize. The schematics of the console and the Leslie are Greek to me. In one hand i have 2 AC wires, 2 signal wires, and 2 switch wires. In the other hand I have 6 pins at the end of the Leslie cable. Once i know the 32h amp actually works, I need help to figure out how to the connect what's in my right hand to what's in my left hand.

[muckelroy]

It sounds like you are trying to home brew a Leslie hookup kit with some kind of tremolo control.

I'm going to guess you don't want to sink a whole lot of cash, but otherwise, I would suggest you purchase a Trek II UC1-A combo preamp.

If you don't want that expense, you have two strategies:
1.) Using a genuine Leslie hookup kit of some sort (like a 428 kit, or a 6122 kit)
2.) DIY

The 21H Leslie actually uses B+ voltage fed in VIA the signal pins 1 and 6, which feeds a 6J5 tube, which is responsible for either opening or closing the motor speed control relay. The relay is the thing that physically "clicks" on and off, and applies, or does not apply AC voltage to the motors. Working backwards, you see the coil that actuates this relay is attached to the plate of the 6J5 tube. The grid of the 6J5 tube is fed from pin 6.
-- It may seem foolish to apply a high DC voltage to the signal itself, but this works because the signal chain BLOCKS that DC before it can go to the input signal tube (the 6SN7), and that blocking of DC is accomplished via two 0.1 uF capacitors just ahead of the input grids of that tube.
-- This is not too complicated, but making it work requires a hookup kit, and a matching transformer to do the job, both of which can be expensive.

You could bypass the relay, the need for B+, and all that jazz by simply connecting the AC motors directly to a switched AC outlet, and switching that AC power on and off at the console. The only downside to doing this is that it could introduce a bit of noise if that switch is living near the X-66 organ, or your mixer. But then again, I think that's a lesser concern, given how unorthodox this setup already is so far.

If you connect a balanced output to pins 1 and 6, signal ground to pin 2, AC to 3 and 4, ignore pin 5 like fire, and hook up the motors to a separate power switch, you should be in business.... however an amp of that age will need attention and/or a rebuild if it's not had any servicing in more than 20 years or so.