A100 and 122 - major distortion

[Lazlo]

Hi All. A friend of mine asked me to sit at his recording studio's A100/122 and determine why the whole set-up was distorting. Here's what I heard:

With the Leslie under half volume (i.e. not loud at all) and the organ set to an 888-type registration, there was indeed major distortion on both the Leslie and the A100 internal speakers. With the 122 disconnected and using just the main speakers, there was major distortion with the swell pedal having hardly any gain. The whole rig was just unplayable, a real miserable sound. He's done a 122 amp rebuild, so which part of the A100 internals should we look at next? I'm not technical when it comes to electronics, but my friend is, so any advice greatly appreciated.

Lazlo
UK

[David Anderson]

Most 122 kit installations on an A100 use the output from the preamp as the source, so, in that case, your friend should look at the preamp -- which he should already know if he is technical when it comes to electronics.

People sometimes take the signal from the internal power amp speaker output for 147 hookups, but rarely for 122s.

[Lazlo]

Thanks David. Would issues in the preamp also explain the similar-sounding distortion in the A100's internal speakers?

[David Anderson]

The signal goes through the preamp before anything else, so yes. Distortion upstream --> distorted signal in everything downstream.

[Guest]

Hello all, I am the friend in question with the organ. I thank my friend for introducing my to the forum to try and solve my A100 problems. I am very experienced in electronics. Iv rebuilt all the amplifier circuits in the organ (and leslie). All valves tested for good emission and no shorts (on my AVO CT160), all voltages test good. Bench tests perform to service manual specifications. Still, the organ distorts severely on the leslie and its internal speakers.

Iv, for test purposes, bypassed all circuitry between the preamps output and the Leslie's input and still, distortion and low volume. This eliminates any switching and muting circuitry.

Any help greatly appreciated. I don't know what more test to perform!?

I bought the organ and leslie from Whitefiled street studios in central London some 15 years ago when they had a closing auction. They were previously Sony and before that CBS studio.

Thanks a lot, I really would appreciate any help and be eternally grateful!

Cheers, Lewis

[David Anderson]

The strategy is like any standard diagnostic process: narrow the possibilities until you find the problem.

The AO28 preamp has two input channels, Non-vibrato and Vibrato, so switching between these should tell you if the problem is in one input channel or the other. If both sound the same, then you have a problem somewhere in the 12AX7/12BH7 stages. Low voltage could cause distortion in all stages, but I assume you've checked for proper DC voltages inside the preamp. I suppose you could have a bad output transformer, but that's a rare problem and somewhat hard to test for without a scope or substitute transformer. A scope would also be useful in showing where the signal becomes distorted.

I also assume that since you said you've rebuilt the preamp that it did work properly when the work was completed and only later began to malfunction. If not, I would suspect some error in the rebuild. One of my repair principles is always to look carefully at any prior repairs done to a piece of equipment.

[Papus]

When I first read the title of this thread I said to myself "A100 + 122 major distortion, sounds ideal - no problem."

Big Jon Lord fan here.

[Tonewheel]

OK, Papus, loved that comment.
But it is nice to have some choice in the matter. My B3 is okay, but my A 100 is doing the same thing as Lazlo's.

[gtojimmy]

I have an A102 which had the same issues...recapped the preamp due to a major 60hz hum....hum's gone, organ sounds like it should now!