I've got a rebuilt 200A on the bench which has a decent hum when running.
The hum disappears by about 80% when I break the earth ground. It goes away even more when I touch the chassis.
I have a feeling this is a circuit problem and not due to the pickup because I've disconnected the reed bar preamp output & 15V/150V supply lines. I've also ensured that the power supply is wired to mains voltage correctly, and have tested the circuit with both the neon bulb and capacitor across the AC input removed from the circuit.
Any thoughts?
thanks!
The hum disappears by about 80% when I break the earth ground. It goes away even more when I touch the chassis.
I have a feeling this is a circuit problem and not due to the pickup because I've disconnected the reed bar preamp output & 15V/150V supply lines. I've also ensured that the power supply is wired to mains voltage correctly, and have tested the circuit with both the neon bulb and capacitor across the AC input removed from the circuit.
Any thoughts?
thanks!
Is it mains born or twice mains frequency? I fancy it is mains born and would go for wrong or poor grounding. The original was isolated from the mains.
Hi Jon, thanks for the reply.
I believe the hum is mains 60Hz, plus what seems to be a higher frequency buzz as well.
What do you mean by 'newer one'? 200 v.s 200A circuit upgrade?
I started unplugging transformer taps to try and isolate the problem - Blue (x4) for the 150V supply does not affect the hum. The hum goes away when I unplug X1/X2 secondaries for the low voltage supply, +/- 22.5vdc and 15vdc.
I believe the hum is mains 60Hz, plus what seems to be a higher frequency buzz as well.
What do you mean by 'newer one'? 200 v.s 200A circuit upgrade?
I started unplugging transformer taps to try and isolate the problem - Blue (x4) for the 150V supply does not affect the hum. The hum goes away when I unplug X1/X2 secondaries for the low voltage supply, +/- 22.5vdc and 15vdc.
Last edited:
I suppose C6 and C21 are new? Are the solder joints good?
What is the AC ripple at the collector of TR6?
What is the AC ripple at the collector of TR6?
Hrmmm. Those cap and transistor numbers don't quite look right. Let make sure we're on the same page (literally)...
Here's the schematic:
http://home.earthlink.net/~monsanto/200A2.jpg
Here's the schematic:
http://home.earthlink.net/~monsanto/200A2.jpg
The big 15 v power supply electrolytic, also the one for the regulator driver transistor, the 15 v supply after the regulator transistor. I'm not going to download another schematic, these instruments are $600 broken and I'm not buying one.
These "pianos" are 40 years old; all the original electrolytic capacitors go in the trash before I start using my brain very much. I put 71 e-caps in my 1967 Hammond organ and just doing that fixed all the problems except a loose tube socket in the reverb. Are you on the same page?
I make a few bad solder joints when I do things, frequently I find if I made it worse instead of better replacing a part that is what is at fault: what I just did. Fortunately the Hammond had plug in chassis, so I could operate it after every couple of new parts and see if I made it better or worse. That focuses your eyes right where the problem was just made.
Good luck.
These "pianos" are 40 years old; all the original electrolytic capacitors go in the trash before I start using my brain very much. I put 71 e-caps in my 1967 Hammond organ and just doing that fixed all the problems except a loose tube socket in the reverb. Are you on the same page?
I make a few bad solder joints when I do things, frequently I find if I made it worse instead of better replacing a part that is what is at fault: what I just did. Fortunately the Hammond had plug in chassis, so I could operate it after every couple of new parts and see if I made it better or worse. That focuses your eyes right where the problem was just made.
Good luck.
Sorry, "rebuilt" from the OP meant to suggest that all the electrolytic caps are new, should have specified. Gone over all the solder joints twice...double and triple checked them. Could a transistor be faulty as to cause a hum when the instrument is earth grounded?? Its like ground-loop, but...there is no loop.
- Status
- Not open for further replies.