H100 Hum Mystery

[Randall Voth]

Been replacing the e-caps in my H100, starting top down. There's always been a hum since I got the organ last year -- louder with expression pedal -- twice as loud with celeste tab down -- and I figured it would go away after replacing all the capacitors. Haven't done the Amp yet, but have replaced the e-caps in all pre-amps and upper level boards and the 2 beneath the power supply.

So I hooked up my new scope to muck around and see what different readings were from here and there so I could compare for changes as I worked on the power supply and pre-amp. (Don't know what to do with a scope but want to learn.)

*** I attached the ground lead of a probe to the pre-amp and the hum just disappeared! ***

Can someone explain this?

Obviously the scope is grounding the pre-amp to external earth because the scope has a three-prong plug and the Hammond has a two-prong plug.

Nice, but I don't want to just ground the organ to get rid of the hum because I haven't done an ac leakage test -- I don't have a 10K 2 watt resistor. I can scavenge a 33K 2 watt. Can I do the test with a 33k resistor and some math?

Is it dangerous to ground the organ? (I've searched the forum and get conflicting opinions).

Since grounding eliminates hum, what kind of problem does this indicate in the organ?

The touch percussion doesn't work, if that indicates anything. I can't find the big cap that apparently solves this problem because I have an updated tubeless pre-amp and the schematics I've seen don't show any that I haven't replaced. (There are two square cans hanging upside down just above the updated pedal pre-amp -- are these capacitors?)

Also there is severe distortion and low volume in the B channel of the amp, if that is a further data point. I haven't touched the parts under the amp yet but I have swapped the tubes for the channels and it doesn't change anything.

Sorry for my inexperience. I just got the organ bug last year and collected a bunch of organs. I want to restore them all to original condition and this is the one I'm starting with.

Thanks in advance for any help some kind soul can provide.

[Jaim]

After you get done with the H, the rest of the tonewheel Hammonds will be a breeze. I would check the ground connections on the preamp boards. The cheesy stake pins tend to corrode. After I rebuilt my preamps, I soldered the connections.

Jim

[nonreverb]

Indeed....Today I was faced with an H112 that I was told was a C3. What a let down. Really dirty drawbars belt driven scanner missing one belt, screaming genny and non-working presets. Got some of it working some not....:-P

[Randall Voth]

Okay, I found a 10K resistor (used a 1/4 watt) and did an AC Leakage test between the earth and the organ. It measures 0.8 volts ac so I'm no longer worrying about the transformer. I hope a 1/4 watt resistor doesn't invalidate the test.

Can I just use a three-prong plug and ground the organ to help with the hum? I'll obviously still replace all the e-caps and check for bad resistors in the amp.

I also rechecked the ground wires between the pre-amps and they all measure next to no resistance. I'll certainly solder them in the near future.

[geoelectro]

Okay, I found a 10K resistor (used a 1/4 watt) and did an AC Leakage test between the earth and the organ. It measures 0.8 volts ac so I'm no longer worrying about the transformer. I hope a 1/4 watt resistor doesn't invalidate the test.

Can I just use a three-prong plug and ground the organ to help with the hum? I'll obviously still replace all the e-caps and check for bad resistors in the amp.

I also rechecked the ground wires between the pre-amps and they all measure next to no resistance. I'll certainly solder them in the near future.

The 1/4 w resistor doesn't invalidate your test.

When adding a grounded power cable, the possibility of something shorting to the chassis becomes more than you possibly being shocked. So, it is highly recommended to add a fuse to the hot side of the AC. That way, a short to the chassis would just blow the fuse.

When adding fuses to Hammond's it is recommended to only fuse the electronics and not the motors. This is because we want the fuse to be close to the current of the electronics so that it would blow under a high current condition. Adding motors to a fuse means the fuse value must be higher making the protection less effective.

I take my meter and measure the current draw at the point where the fuse is going to be installed. If I measure 1.2A for example I use a 1.5A~1.75A fuse. I try to have the fuse about 1/2A higher than actual current drawn. Use a slo-blo whenever it's for AC power.

Geo

[Wes]

I had this exact same experience with an X77 the other day, except it happened when I connected my scope to a speaker lead in the X77G! (I couldn't tell for sure which speaker group was active).

If anybody knows WHY grounding these chassis makes the hum go away, I'd love to know. There shouldn't be any path between ground and the organ as-built, and yet, we hear a difference. I wonder if it's because it makes the chassis a more effective RF shield. This X77 tested at 0.05 VAC on the leakage test (!!!) so it's transformer was in very good condition.

Wes

[Randall Voth]

Geo, thank you so much for the info for both the leakage test and the fuse. You provided me peace of mind and it's a long drive for me to buy a 2 watt resistor.

The organ has a 3.5 amp re-settable circuit breaker installed. I doubt it's original... don't see one in the schematics. The breaker runs to the blue wire in the connector near the motor capacitor which leads to the fluorescent light ballast, which is after the run motor in the schematics.

The organ has screws holes for a Leslie switch and a bunch of wire nuts in the speaker wiring so I presume it ran a Leslie and the 3.5 amp power draw would have included that amplifier? I'll test the current draw while running and reduce the amperage as you recommend because I don't intend on adding a Leslie to this organ -- the celeste vibrato (without hum!) sounds really nice to my ears. Besides, I only have a 110 and it's hooked to the M101 at my parent's house. (My mom was thrilled to get "a real Hammond!")

The main problem with this organ is the low output distorted B channel and this hum. Otherwise mechanically just needs cleaning of drawbars and
re-lubing the preset keys and maybe that trick of adding ribbon to the keycomb felts. (Thanks Bobman for the videos!).

Oh and the touch percussion doesn't work. Where is that capacitor C124 on a non-tube updated pre-amp H100? Or what is its equivalent?

Oh and one solenoid in the bass pedals sticks. Is there an "in situ" way to fix this?

I should probably be starting new threads for every question...

[geoelectro]

The re-settable breaker is just for the generator. A fuse should be added to the power transformer individually. If a Leslie is added, it has its own fuse.

Common problems for the H-series is bad plate load resistors in the amp chassis. There are a few small E caps that I would also replace. Get your DC voltages back to spec and it should run fine.

Geo

[Jaim]

The main problem with this organ is the low output distorted B channel and this hum.

Have you tried swapping the A & B channels into the main amp?

Jim

[Randall Voth]

Have you tried swapping the A & B channels into the main amp?

Jim

Thanks, Jim. I just tried swapping the two clear coating wires (1 & 7) that feed the pre-amp signals A & B and there was no difference. Channel B still quiet and distorting. I'll narrow it down further in the amp. I'm replacing only a few parts at a time and then testing the results hoping to gain some experience in what causes what.

Minus the hum and with the pre-amp e-caps replaced this organ really sounds fantastic -- powerful bass and bright tone. My wife is looking more for a classical sound than jazz to suit her teaching and compositions. The M101 this replaces has wax caps in the TG and is really dull compared to this. I guess that one and my recently rescued dump-bound RT2 will need those replaced... might take me a while!