Hammond B3 tech help needed. Distortion only on certain notes/drawbar frequencies. - diyAudio
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Old 13th April 2011, 11:22 PM   #1
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Default Hammond B3 tech help needed. Distortion only on certain notes/drawbar frequencies.

Trying to troubleshoot a friend's B3 and we're experiencing an unusual problem. W'ere getting distortion only on certain notes/drawbar frequencies. We found some settings which isolate the distortion. One was to pull down only the 3rd(from the left)drawbars for both manuals and play the 4th or 5th G's above middle C on the lower manual and the 2nd, 3rd, 4th, and 5th G's on the upper manual. No other notes distort with this setting. The distortion is somewhat intermittent and overtakes the note sometimes to the point where the note is gone completely and there's only distortion. This problem affects other notes with different individual drawbars as well but this one setting will perhaps help to pinpoint where the trouble is? Any suggestions? Thanks!
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Old 14th April 2011, 02:27 PM   #2
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Have you checked the individual generator for the particular tone you have isolated? Each generator coil has a filter on it to shape the timber of the tone it produces. These filters are often just a capacitor across the coil, but can also include other components on some generators. If the filter components are failing, that can produce the symptoms you are describing.

Dave
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Old 14th April 2011, 03:56 PM   #3
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Quote:
Originally Posted by dcgillespie View Post
Have you checked the individual generator for the particular tone you have isolated? Each generator coil has a filter on it to shape the timber of the tone it produces. These filters are often just a capacitor across the coil, but can also include other components on some generators. If the filter components are failing, that can produce the symptoms you are describing.

Dave
That's the area I was headed having fixed dead notes in other organs but I couldn't locate my service manual file with the frequency chart. I just found it this morning. What's seems strange is that if the upper manual's 2nd and 3rd Gs are affected with 3rd drawbar pulled out, it should be the same for the lower since each manual key shares the same frequency set. Also each G note uses a different frequency for the 3rd drawbar so its strange that almost all G notes would be affected at the same time. Problem is I have to drive a 1/2 hour to check this thing out cause lugging it to my place is not a possible for me or the owner. Guess I'll print out the chart and start checking the filters when I return some amps I fixed for him next week.
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Old 14th April 2011, 06:02 PM   #4
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Hard to imagine the internal amplifier causing this problem. Checking at low volume could help in that regard. What is the B playing through? Any chance this is a problem within the external amplifier/driver system when that frequency is reproduced? A long shot, I know, but it sounds like that's the category you're in now. Does it do it with the vibrato turned on? If so, that at least eliminates the manual preamplifier as the vibrato circuit uses a separate dedicated preamplifier for that function.

Dave
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Old 14th April 2011, 07:44 PM   #5
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Quote:
Originally Posted by dcgillespie View Post
Hard to imagine the internal amplifier causing this problem. Checking at low volume could help in that regard. What is the B playing through? Any chance this is a problem within the external amplifier/driver system when that frequency is reproduced? A long shot, I know, but it sounds like that's the category you're in now. Does it do it with the vibrato turned on? If so, that at least eliminates the manual preamplifier as the vibrato circuit uses a separate dedicated preamplifier for that function.

Dave
The filter caps have been changed and the tubes were all substituted with known good ones and no difference. This is a late 50's organ which was preciously serviced by Ken Rich. It has a 1/4" line out box so of course the first thing I did, which I forgot to mention, was disconnect it from the Leslie and run it into a couple different amplifiers which yielded no difference.

I'll have to check with vibrato on because I think we only ran it with it off.
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Old 14th April 2011, 10:25 PM   #6
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I would suggest the prob may be dust/humidity on some areas.
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Old 15th April 2011, 01:25 AM   #7
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theatre-organ.com in their faq has a a/b/c frequency and foldback chart online.
You say the filter caps were changed. There are also axial type electrolytics under the deck of the Ao28. These are mostly cathode resistor bypass caps. Were these also changed?
I would take the top off and blast out the sliders with contact cleaner through the slots before getting too involved with a scope. Warning, lumberyard contact cleaner is gasoline in a spray can, very flammable. Stay 10 m from open flame, pilot lights, smoking, electricity turned on or off. Set up a fan to dissapate the fumes before starting. If using a professional non-flammable contact cleaner from CRC or LPS (bromine compounds) check for compatibility with plastic before use. You don't want to dissolve the plastic parts. This stuff is $20 a can and is mostly borrowed, not bought.
Some people disassemble and clean the sliders/drawbars the old fashioned way. For details see bobmann videos on U-tube, or check organforum.com hammond forum for many related threads.
My 1964 A100 has one paper TG cap, the rest mallory red plastic dialectric. You say this organ is late fifties. Is it possible some of the paper caps on the TG filters are way off? Any break in the wax cover on a paper cap subjects it to deterioration from the moisture in the air.
Some people buy M's for a pittance as a source of proper value red caps.
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Last edited by indianajo; 15th April 2011 at 01:33 AM.
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Old 2nd June 2011, 03:51 PM   #8
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Got it solved. Open solder connection on one of the caps above the tone generator. It was tough to find because it was so sensitive to vibration it seemed like it could be anywhere! Amazing, there were actually several wires/connections here and there on the organ that were not soldered but were just floting in the eyelet where they were supposed to be soldered. And this organ was purchased from one of the "top" organ techs in LA! Attention to detail and thoroughness are a must with repair work unless you want this stuff to come back and bite you later on.

Thanks for all the tips/advice forum friends!
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