Hammond A-100 Microphonic feedback

[Von R. Colborn]

The continuing saga of getting my 1960 A-100 working properly continues. About three weeks ago I started getting microphonic (acoustic) feed back through the speaker system. It is a lower frequency, very close to the 'F' below middle-C. All that I have to do is to depress the swell pedal more than half way and it will start to feed back, softly at first but over a few seconds increase in volume until I reduce the swell pedal. The feedback then subsides.

I can get this feedback without the tone generator actually working, simply by turning on the run switch and letting the amplifiers warm up.

I've tried all of the usual stuff such as tapping on various tubes to see if some are overly sensitive. All were okay. I did find that the matching transformer is VERY sensitive to the tapping, almost like flicking a microphone to see if it is on. I have found that while the feedback is happening I an firmly grab the housing on the matching transformer the feedback subsides; When I let go the feedback starts up again.

I had extra sets of tubes for both the pre-amp and the power amp. Replacing the tubes made no difference.

I would appreciate any insight that you folks can provide or additional things that I can do to isolate the source of the problem. Is it possible that the matching transformer, itself, could be the problem?

Thanks

[Jaim]

Are the screws tight on the MT cover? The ground wires on soldered on the starter end of the TWG. Check the joints and the screw connections on the preset panel for tightness. I have heard of instances of the MT cover getting magnetized.

Jim

[Von R. Colborn]

Thanks. I'll look into that. The strange thing is that had been working fine for months and it started up all of the sudden.

[geoffbrown]

I took the MT cover off my organ two years ago, forgot to put it back. no hum issues developed over that period, makes me think that blaming hum issues on that cover is wrong and likely caused by something else,

[David Anderson]

It's not common, but matching transformers have been known to become microphonic. I've never run across one, so I don't have the full range of solutions fresh in my mind. I would start by playing the offending tone with the MT cover off and damping various parts of the MT windings (there's no voltage on them) with your fingers like you would with a tube to see what makes the feedback stop. Once you've found it, take steps to try to lock it down. There are transformer varnishes you could squirt into gaps if, say, a winding is loose on the laminations.

[tpappano]

I wonder if the transformer core also might have become slightly magnetized. If so, and if the windings or core could move at all, you would actually have a dynamic microphone. Maybe try degaussing it with a bulk tape eraser?

[David Anderson]

That is another possibility.

[Von R. Colborn]

Thanks David, I'll try the finger poke'n testing first. Certainly, if I grasp the MT housing and pull on it slightly I can disrupt the feedback.
Another piece of info - If I plug the Leslie 21H in and then remove the amplifier stage tubes from the power amplifier I do not get the feedback.

Thanks tpappano, I don't have a tape degausser. I'll see if I can find one. It's hard to find an old TV shop these days.

[geoffbrown]

I would look at the obvious and easy thing first like the power supply capacitors. then look for bad grounding and/or bad connections.
The matching transformer would not suddenly cause hum issues unless it has been damaged or there is an insulation break down. but then this situation would show up as no sound or scratchy sounds

[Von R. Colborn]

So far I'm not having any success in finding the source of the hum but it certainly seems to center around the MT. The slightest tap on or near the MT is picked up and amplified. Are MTs readily available, if so from whom, and will they break the bank?

[David Anderson]

I would do some more careful checking of the MT, including the solder joints on the terminals. You should be able to damp the problem by stabilizing the various parts of the MT with your hand.

[tpappano]

I can't help but think the transformer is probably OK, but it, or pieces of steel near or around it, have become magnetized. Can you do sort of "precision" tap testing to find which piece causes the most noise? Unfortunately I don't have a good mental picture of how the A100 transformer is mounted and what exactly is near it.

[David Anderson]

An A-100 MT is mounted just like any B or C case matching transformer. On 3-series organs, there is a 470pF capacitor in there, and ceramic disc caps have been known on occasion to become microphonic, although it's more likely with an applied DC voltage, which isn't the case here.

[pspees]

I'm having the same exact issue. The steel parts and core of the transformer are indeed magnetized. I'm going to try a tape head demagnetizer...