[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?? Did you use correct "working voltage(WV) on the electrolytic caps? Consider using a minimum of 350 WV for C101. 400 or 450V may be better. the reason for this is that the 'no load' voltage on C101 (for example, if the set was powered up without the speaker plugged in) could be much higher than 350V. If the set was, in fact, powered up without the speaker plugged in, that cap may be damaged. C102 A and B should be 300 or 350 WV.

I am not surprised that you have AC on pin 3 of the 7AF7 when the radio is playing, along with the hum.  The plate of an audio tube has both an AC and DC component when it is amplifying an audio signal. In the case of this stage, there would be an AC component on the cathode of the invertor stage also. The schematic states that voltage readings are taken with the volume control at minimum. In a good set with no hum, there would be little AC superimposed on the DC readings on the pates and cathodes in the audio stages.

The 7AF7 tube has 2 triode sections.  One triode is the phono preamp, only used for the phono.  The other triode, the one that we are interested in, is used as a "cathodyne" phase invertor and provides signals to the grids of the 2 6V6 output tubes.  Because these 6V6s are a "push-pull output stage, the signals feeding one 6V6 must be 180 degrees out of phase with the signal feeding the other 6V6. 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 phase invertor 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 (1st AF) 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.

You should also check both DCV and ACV on the power supply with and without the 7AF7 in place.  At the junction of C102A and the speaker field, the AC "Ripple" should be less than 2V.

Hope this helps. Keep us posted nd good luck!

[morzh]

MrFixr

The chassis, if there is no ground pin in the AC plug, due to the Y-caps from L/N to the chassis, is at 60VAC relative to the L or N, or Earth.
If there is a leakage from L/N to B+, this could result in some AC measured during warm-up until the tubes start conducting and present some load to lower the AC component on B+.

This is a far-fetched theory, of course.

And it could be refuted by temporarily removing the filter caps from the circuit and re-measuring.

[sq65]

Thank you for the detailed response MrFixr.

To clarify, the hum starts when the tubes are fully warmed up, and goes away after about 3-5 seconds. The hum is not affected by the volume control, and still happens when the 7X7 is removed. Also, I replaced all the filter caps with 450v.

[MrFixr55]

And no hum without the 7AF7? Not common but I am thinking heater - cathode short. This would introduce hum in this stage and the output stage. The hum would be unaffected by the volume control, as this stage is after the volume control. try tapping the tube to see if this changes things. Filaments on this tube and the det / 1st AF tube runs off a different winding than the other tubes. This winding is center tapped, which is why you read 3VAC between pin 1 or 8 (filaments) and chassis ground.

Hope this helps.

[MrFixr55]

Hi Morzh,

Dunno if the AC EMI caps are an issue. I never liked the concept but never had an issue with these causing an issue when looking only at the radio. Hopkins filters used on commercial electronic equipment have the same caps but also inductive filters.
Modern TVs use the same concept. When adding an external input that is at ground potential (grounded chassis), that would be a different issue, as there would be current on the shield.

[sq65]

I pulled the tube out and it's shorted from pin 1 to pin 2. I suppose that's the problem...

[morzh]

No, it's not a problem, just during certain condition it could show up.
Yes, an internal tube short could do that too.
I am not sure why the AC amplitude is 120V whereas the filament voltage is low. And connected to Chassis via centertap.