M3 Swapping Amps to Diagnose lack of Percussion?

[mwd]

About to pull my hair out trying to get the M3 to play percussion. I've zapped, cleaned, replaced 4 tubes and still no percussion. I measure 19.5 dcv on K terminal when percussion switch is on and it drops to zero when I press a key. I am advised that this would indicate, most likely, that the problem is in the amp, but I also read this is supposed to be 25 or more volts.

I am not an electronics wizard but I can solder and run a volt meter. The nearest technician is probably 500 miles from me, if I could find them, so I'm all I got. I bought a spare AO29-7 amp that matches what I have and is supposed to work. This is what I would like to ask.

Is the 19.5 vdc on the K terminal too low and if so does that indicate a problem inside or outside the amp?

Would it be feasible to swap the power amp to determine if the problem was actually in the amp?

The power cord on this amp needs to be replaced in that I think internally it has broken down in areas near the plug end. It is not exposed or cracked but I've had the organ stop a couple of times. I moved the cord and it ran fine for as long as I wanted. Is it possible the organ would run but not get enough power for the amps. I have a brand new cord from Tonewheel Hospital but I want to put it on the amp I ultimately wind up using.

[Sweet Pete]

Holding down an upper key,middle C for instance,does the perc go plunk when the on/off tab is engaged?
No? Not enough voltage, most likely still shorting out in the perc switchbox!
Clean the switches before changing amps.
With the bottom fasteners removed the cover will pry off easy.
There are still 'hairs' in the switches most likely.
Careful, the metal rocker springs fly across the room when disturbed sideways!
Apologies if you have already cleaned them! Spare tubes would also help you diagnose the issue.

[mwd]

Thank you Sweet Pete. There is no plunk. I've had the box apart twice, once to clean, once to double check. I have already done the zap procedure and replaced V4, V7, V8, V9, cleaned and rotated the percussion cutoff control. Now kinda' stuck. Wondered if there was anything else in the diagnostic tree that I could do prior to changing out the amp.

[Sweet Pete]

To my surprise the cutoff dial was shorted.Once it was worked relentlessly back and forth? Been working ever since! 1960 M3.
After confirming the transformers had continuity I was stumped.Gave the dial another complete turn or 50 and voila!

[Drawbar Dave]

It could be one of the three percussion transformers. Have you checked them for continuity?

[Sweet Pete]

Unlikely but possible.Short in the cutoff dial is most common.

[Drawbar Dave]

Happened on an M102 I had once. Just the once though!

[mwd]

Regarding transformers... no I haven't. Do I check, terminal to terminal, for continuity with no power? How do you test the cutoff rheostat? 100% green here.

[David Anderson]

I'm a little leery of continuing to engage with this problem, but I would point out that there are people to whom you could send you AO29 to be refurbished. They could check to see that everything inside the amp is working.

I would also consider whether or not there might be some problem in your percussion switches or wiring harness, like a broken or missing contact or a wire that's come off. Faulty triggering is one cause of percussion not working, but there's also an audio signal path that could potentially be interrupted or shorted out. The relevant harmonics do have to get to the input terminal for percussion on the amp, and there are switches involved in that. The triggering part of the percussion circuit is an audio gate, but if there is no audio, there is nothing to gate.

A tech's worst nightmare would be to refurbish your AO29, have you reinstall it, have percussion still not work because there's a problem that's not in the amp, and have you get angry at him for something over which he had no control. It's not common for the audio signal path to be the problem, but I think you're outside the usual box.

[mwd]

Well David I wholeheartedly encourage you to engage this problem and I appreciate it greatly :-) I finally found a repair shop today and wrote them for a quote to get a ballpark figure. I ask point blank questions here, that are probably second nature and obvious to many of you, so that I am following procedures that those more experienced, like yourself recommend, to the T. I will say unequivocally some technical info flies right over my head but I'm definitely not going to go poking around in dangerous areas and wreak more havoc. I would love to run this over to a electronic repair shop or have a Hammond tech come over to the studio but those options just don't exist where I live. One of the reasons I keep prodding you folks is so I don't spend 100 bucks shipping, plus service fees, only to hear 'well the problem is not in your amp Mike'. I come from a technical world, just not this one.

The switches look good, they are clean, they were clean when I cleaned them but I did it anyway. No wires broken, none appear shorted. I am guessing the contacts are correct, although I have not seen in side another percussion box. I did notice one contact had 3 arms and one had 1. As I mentioned in the original post I have a spare, stated working, exact amp replacement I could exchange and at least see if might be amp related prior to sending anything off.

[DannyH]

Hi, MWD,

did you fixe it already ?
If yes, it would be great to hear how and what the problem was !
I got a M100 with percussion problem, too... So I am very curiose about your solution.

Best regards,
Danny

[mwd]

No Danny I'm still broken. I do have a spare amp so I know I have some good transformers if that is the problem. I got a beautiful Hammond A102 that functions correctly, as well as a couple of virtual organ programs, that have been occupying my Hammond cravings. I haven't given up on the M3 repair though.

[Jyvoipabo]

Hello,

I suppose you did this :
http://www.dairiki.org/HammondWiki/P...roubleshooting.

19 volts is too low for correct triggering.
Remove V8; percussion off (and/or disconnect K if necessary).
Voltage should be 32 V on K terminal (Decay fast).
~ 27 V (Decay slow)

JP