RT-3 with 120 cycle hum

[ChristopherDB113]

This problem developed over the weekend. The organ played normally on Saturday but when I switched it on Sunday there was a hum present. Note the organ is in my house and not a church so there was no disaster, and the organ is still playable but the underlying hum is annoying. I'm good with a soldering iron and multi-meter, have a schematic of the preamp, and a copy of the service manual. Just looking for some direction on which component may be bad and how to test it so I can replace it. Here is possibly too much information:

RT-3 built in 1960 with original capacitors on tone generator. All tube sockets cleaned about a year ago. Vibrato scanner cleaned and restored to proper operation and oil reservoirs filled to normal about a month ago. The pre-amp is an A0-28-1. The organ is played through an A20 tone cabinet built in the early 1930's, and its tubes are decades old. The organ has seen daily use of 30 - 60 minutes run time.

The tone is 120 cycles and not 60. Confirmed because the hum is close to the C one octave below middle C on my piano, whereas 60 cycles would be about two octaves below middle C.

The following things will lower the volume of the hum: Expression pedal moved to low volume. Normal / Soft volume switch set to soft. Even with percussion turned off, the following will lower the volume on the hum: Percussion volume set to soft. Percussion decay set to slow. Turning percussion on and pressing the same tabs would get the same results. Just to clarify, you get results doing any of the previously listed things, and the more you do, the quieter the hum will get.

When the organ is played, the hum is in the background and does not increase in volume. Presets cancelled or adjusting drawbars does not affect it.

I assume someone has seen this in the past and may know the cause and cure. I assume all responsibility if I get in there with some tools. Thanks in advance.

Christopher

[David Anderson]

With all drawbars on the upper manual in and on the B preset, turn percussion On, press down a key and let the percussion tone die away. Keep the key down. Release the key. See how that affects hum. You may have hum getting into the signal path via the preamp's percussion input stage. The 12AU7 in the preamp acts as an audio gate and will mute hum introduced via the percussion input circuit.

[ChristopherDB113]

David, thanks for the testing procedure. Here are the results:

When a key is pressed and held, the percussion tone strikes and the hum quickly fades away completely, and will stay gone for as long as you want to hold that key down. As soon as the key is released, the hum returns.

Let me know if this indicates the 12AU7 tube is bad, and if so, which V# socket it is in.

Thanks,
Christopher

[geoelectro]

My guess is the filter for the 205Vdc supply C60.

geo

[David Anderson]

I suppose it could be a 12AU7 (V7) with badly unbalanced sections (I saw that one time), but that's not my first choice. V5 (6C4) is the percussion input tube. What's common to both these tubes is the power supply node for percussion. To check if it's a tube, I would substitute a known-good tube, 12AU7 and/or 6C4. I have seen 6C4s develop heater-cathode shorts, but that would tend to give you 60Hz hum, not 120Hz.

Given that you say you're sure that you're getting 120Hz hum, that points to the high voltage power supply, so C60 (50uF) may be failing and not filtering the B+ supply adequately for the percussion section of the preamp. Of course, C60 is part of a multi-section can capacitor. You could replace it with a single discrete capacitor, but that might be a sign that you need to recap the whole power supply. The preamp is 60 years old at this point, and electrolytic capacitors don't last forever.

I would get out my ESR meter and check the power supply capacitors to verify, but I'm guessing you don't have one.

I hope you appreciate that we can't always tell you exactly what the problem is and tell you exactly how to fix it from an online description of symptoms. What I've tried to do is to help you eliminate what it isn't. We now know it's coming in through the percussion circuit.

A worst-case scenario is that you've had some kind of failure of one of the three transformers involved in the percussion circuit, but that would be unusual.

[ChristopherDB113]

Thanks for all the suggestions. While I don't own an ESR meter, I know someone that does. However, they said I would need to de-solder that capacitor to check it, and if I was that far in, I might as well just replace it.

My plan is to order those tubes, since that could be a quick fix, and if not it is never a bad idea to have spares on hand.

Regarding C60 I will have to check my schematic at home later tonight to see where that is. I suspect it is one of the two large cans on the top of the chassis between the big transformer and expression pedal housing. In that case, this kit from Tonewheel General should do the trick: https://ssl.tonewheelgeneral.com/bui...em_no=AO-28CAP

Christopher

[David Anderson]

C60 is one section of one of the multi-section can capacitors mounted on the chassis. The can capacitors usually contain between 2 and 4 capacitors, all sharing the same common ground connection. The capacitor location is on the schematic.

My ESR meter (Anatek Blue) can be used in-circuit. It has paid for itself many times over.

[ChristopherDB113]

I looked up the schematic online and saw that I would need to replace the can closest to the expression control. I also looked up your ESR meter and see it is affordable, though at this time I would only use it on rare occasion.

[Jaim]

If you go under the hood to replace the filter caps, I would be tempted to replace the 6AU6 screen and plate resisters (R7, 8, 19 & 20) as well. These are prone to failure.

Jim

[Jaim]

My ESR meter (Anatek Blue) can be used in-circuit. It has paid for itself many times over.

I used my ESR on a Farfisa that had some hum issues where several boards had bad grounds. Made TS easy.
I seem to recall David suggesting that use for an ESR sometime back.
Thanks David

Jim

[ChristopherDB113]

UPDATE: I finally got around to pulling the organ away from the wall to get in there. Pulling either V5 or V7 will stop the hum. When replacing these with new (different) tubes the hum returns. Therefore it seems that C60 needs to be replaced.

I rarely use the percussion unit so my solution for now was to pull V7. Tonewheel General sells replacement can capacitor kits for under $100 but purchasing one and doing the install is a low priority for now. Just glad to have the diagnostic work done and the hum silenced.

Thanks all for the advice. I may post another update when I replace the cans but that will likely be months from now so don't anyone hold their breath.

Christopher

[DrGeorge]

Rather than disconnect wires to measure ESR, just clip lead a 10 or 20 uF, 450 volt cap across the suspect can section and see if hum goes away. Lot easier.

[David Anderson]

Rather than disconnect wires to measure ESR, just clip lead a 10 or 20 uF, 450 volt cap across the suspect can section and see if hum goes away. Lot easier.

You do not have to remove any wires to use the Anatek ESR meter, so it is, in fact, easier to use the meter. It tests caps in-circuit.

[ChristopherDB113]

I thought that something like that might be possible. If I understand 'clip lead' correctly, you are saying to cut the leads off the existing C60 and solder in a new capacitor inside the chassis.

While theoretically easier and certainly cheaper, I had thought that replacing the entire can would possibly head off any future problems, since I'd be replacing several capacitors in one shot.

[tpappano]

No! Don't cut anything yet. When someone refers to 'clip leads', they usually mean this sort of thing :
https://www.amazon.com/TOTOT-Inches-...lip+lea&sr=8-5
Wires with 'alligator clips' on each end used to temporarily connect all sorts of things. Traditionally cheap and shoddily constructed, but very handy 8) In your case you would use two 'clip leads' to connect a known good capacitor in parallel with the existing one to see whether the original is doing its job.

[Drawbar Dave]

Modern radial capacitors are manufactured to a higher quality and are cheaper than the can capacitors. On my own collection of gear I prefer to replace caps in this way these days.