[sq65]

Hello, I have a 46-1209 I am restoring. I was always told the radio didn't work, however when I got it home and powered it up it seemed to work fine (with a bit of filter hum of course). I have replaced all paper caps and electrolytics, and now it doesn't work properly. When you turn it on it makes a loud click and 60hz hum. When dim-bulbed at 200w the problem goes away about 5 seconds after all the tubes are warmed up, but plugged straight into the wall it doesn't stop humming.

My observations:
- When the 7AF7 or the 56K resistor on it's 3rd pin is removed, the problem is gone, but the radio must be turned up to max to hear anything.
- When the 7AF7 is plugged in and the problem is occurring the AC volts on Pin 3 is 120, but when removed none of the pins supply 120.
- The volume control does not affect the humming

I have double checked the wiring of the new filter caps, especially around the 7AF7.

Any help is greatly appreciated.

[morzh]

Have you touched the speaker wires, either speaker plug or the socket?

[sq65]

About all I've done is unplugged them to take the chassis to my workbench. Anything I should check with them?

[morzh]

It is kinda impossible for the plate to have 120V with the tube in and have it not with the tube out, short of the tube having some shorts inside in a very improbable way.

The B+ is supplied through the field coil of the speaker, plugged between the plusses of 10uF and 25uF caps.

[sq65]

Yeah, I have no idea what's going on there. I will re-check it in the morning, and also trace out the B+...

[sq65]

I double checked everything around the filter caps and it all seems to be hooked up right...

Here are the voltage on the 7AF7 when the tube is removed (measured to chassis):

Pin 1 0 DC 2.9 AC Heater
2 0 DC 0 AC Cathode
3 260 DC 0 AC Plate
4 0 DC 0 AC Grid
5 0 DC 0 AC Grid
6 245 DC 0 AC Plate
7 0 DC 0 AC Cathode
8 0 DC 2.9 AC Heater
Some pins had 0.1 or 0.2 VAC but I rounded down

Sure enough, when the 7AF7 warms up my meter shows 80-120vac on pin 3!

[morzh]

What kind of meter is it?
Relative to what is the voltage being measured?
Are the AC line filter caps C53A/B hooked up right?

[sq65]

Meter is a DMM (Brymen BM-257 to be exact)
Voltage being measured to the chassis
Yep, just double-checked 'em

[MrFixr55]

Hi SQ55,
A strange issue indeed!  Just so I have you straight, on a dim bulb you get normal operation after the set warms up.  Do you get hum before it warms up when on the dim bulb??I am not surprised that you have AC on pin 3 of the 7AF7 when the radio is playing, along with the hum.  The tube has 2 triode sections.  One is the phono preamp, only used for the phono.  This triode is used as a "cathodyne" phase invertor and provides signals to the grids of the 2 6V6 output tubes.  One 6V6 grid is fed from the plate.  The other 6V6 grid is fed from the cathode.  The signal from the cathode is in sync with the signal going to the Phase Invertor grid.  The signal from the plate is 180 degrees out of phase.  You can expect both an AC and DC component on the plate and cathode of this tube if there is audio or a hum.

Does the hum coincide with the volume control's position?  What happens if you leave the 7AF7 in but pull the 7X7 Detector / 1st Audio Amp.  If the hum disappears with the 7AF7 in but the 7X7 out, then the hum is coming from that stage or before that stage.  If the hum remains the same then the issue is with the phase invertor. It is possible that there is leakage between the filament (heater) and the cathode.  If the convertor is also a 7AF7 then swap the tubes and see what happens.  If the convertor is a 7F8, then you will need to find another 7AF7.

Wou can also check both DCV and ACV on the power supply.  At the junction of C102A and the speaker field, the AC "Ripple" should be less than 2V.

Hope this helps.