Help with Hum in Right Channel Fisher 400

This S/N 35,xxx unit has been restored with all of the following work done:

New rectifier
Replace all electrolytics including can caps
Replace film caps including coupling caps
10ohm fuse resistors added
Screen resistor mod
New power switch at the rear panel
2 Thermisters
New diodes
X-Y cap
IBAM
Grid resistors swapped for 200k

I have a mild hum in the right channel only. Volume and balance knobs have no effect on the hum. I'm using the AUX input. Hum also on the other inputs.

Pulling V16 (R channel output tube) stops the hum.
Pulling V15 (other R channel output tube) does not stop the hum.
Pulling V12 R Driver and Phase Inverter DRAMATICALLY INCREASES THE HUM.
Pulling V11 L Driver and Phase Inverter, hum remains the same.

I've done all the usual tube swapping and no tubes are bad/noisy.
I'm assuming the problem is localized to the V16 socket and/or V12 the R channel Driver/Phase Inverter.

I've checked all the grounds for continuity with the chassis.
I've cleaned and tightened up the V16 socket.
All resistors that were marginally out of spec were replaced.
I've swapped the coupling cap for V16 and the hum remained.
I've re-routed the IBAM bias wires away from high voltage lines.
I've probed with a ceramic screw driver for badly placed leads or AC wires out of place.

L Driver Phase Inverter Pins / R Driver Phase Inverter Pins
1) 280V 1) 288V
3) 138V 3) 132V
6) 135V 6) 129V
8) 1V 8) 1V

I've spent about 15 hours troubleshooting and have arrived at a frustrating dead end. Any suggestions?

Argy -- Very thorough troubleshooting!

Normally I would suspect a bad coupling cap for V16, but you say you've tried changing that out with no success. I would also suspect the power supply decoupling cap for the B+ tap serving the phase inverters -- but that would produce the same effect in both channels.

The clue is that the hum increases when the driver tube V12 is removed. It would be normal for the hum to go up somewhat with the removal of that tube -- but not dramatically so. With the tube installed, the impedance levels as seen looking back into phase inverter are relatively low, while with the tube removed, those impedance levels go up -- hence the ability for the output tube grids to pick up noise in that condition.

One thing to try is to use a small decoupling cap -- say 33 or 47 uF at 35 volts, and place it right at the bias source end of the grid return resistor for V16 (R101). I'm wondering if noise is not entering that tube via the IBAM by way of a ground loop?

Finally, can you see the noise while probing with a scope? Also, is the R power amplifier in general working properly otherwise? That is, will is develop (about) 30 watts RMS with low distortion?

Keep us posted!

Dave

Thanks for the suggestions Dave. I'm encouraged that THE Fisher Guru has offered his help.

I've also thought the IBAM might have a ground problem and double checked the wiring and tried a different ground, but that did not help.

The unit is working well and sounds very good, aside from the hum.

Unfortunately, I'm at a dead end here, because my scope is broken.

I'll be out of town on business for the next few days so will try your suggestions next week and update then.

Anyone else, please chime in with your suggestions.

Also, the hum is "MMMMMM" 60hz.

60 Hz implies it is not power supply related. What happens when you short the OUTPUT side of the two coupling caps together? Any noise from the PI stage is then being injected common mode into the output stage, and therefore SHOULD cancel out for no hum.

Dave

Shorting the output side of the two coupling caps results in INCREASED hum.

:scratch2:

You shorted them TOGETHER, correct? (not to ground!!). If that resulted in increased hum, then the PI stage is completely eliminated, and the problem must be in the output stage, because THEORETICALLY, no signal should be able to get through the output stage with the control grids (basically) shorted together.

Here's a likely possibility then:

Fisher uses 220 ohm heater balancing resistors from each side of the heater winding supplying the output stages, to ground. But these resistors are often grounded by connecting them to the cathode terminal of an output tube, since the cathode is grounded in the original design. But you have now installed cathode current sampling resistors. If you did not disconnect those heater balancing resistors from the cathode terminal -- if originally grounded there -- and have now installed the current sampling resistors between the cathode terminals and ground, then 60 Hz heater voltage is now modulating the output tube where the resistor is "grounded" at.

When installing cathode sampling resistors in Fisher equipment, it is important to make sure that those resistors are properly re-grounded as required, so that cathode modulation cannot occur. If this has happened, it would in fact create all the symptoms you are describing.

I hope that helps!

Dave

That's BRILLIANT Dave!

Hum free now

I cliiped the cathode end of the resistor and re-grounded.

I can leave town now and not have that hum at the back of my mind all weekend.

You've saved me a few grey hairs Dave, thank you very much!

Glad it helped you out!

Dave