Hi

This came into my workshop needing servicing, the stations were really crackly and hard to tune.
The first thing I did was replace all the wax paper capacitors with "orange drop" capacitors, and replaced the electrolytics with new ones.
All the valves were tested and they are leaking/bad. However the radio worked beautifully on the bench.
I then replaced the old figure 8 electrical flex with three-wire flex 240V AC and earth to the chassis.
Now the radio hums.
Customer doesn't want me to replace the tubes (but they need to) - any suggestions about the hum?
Thanks

To be clear, are you saying that the hum only appeared when you earthed the chassis? That is, if you lift the earth the hum stops?

It would be unusual for an earth lead to cause hum. I would be first checking the electrolytic caps that you replaced, starting at the filter cap. Are these caps brand new?

Yes, insufficient filtering is the usual suspect, although he seems to be saying all was okay on the bench until he replaced the figure 8.

From memory the first filter cap on the 573 in not grounded to the chassis but goes through the bias resistor. If it's been grounded to the chassis it might cause a hum.

^ Yes, C25 (24 μF, 350pv) finds ground via R17 (150 ohms, 1 watt).

I will have a look at the circuit if I have it. But is the hum from the transformer, or the speaker?

I normally earth leakage test, the transformers. My concern if it is it rattling after earthing it may have a short. I would recheck the cable wiring for an open earth and also take a serious look, or get an electrician, to take a serious look at the wiring of the earth circuit. Energy Safe Vic was actually promoting insulating testing the house wiring every five years.

You can get non contact devices that plug in and will give an idea if a socket is correct. I had an incident Tuesday where in the course of doing another electrical project, the CB on the A/C was heard to be faulty (less than two years in new house).

It may be a cathode heater issue, but in itself is one I have never had.

Marc

Thanks for all your advice. I have a happy ending for this story. After checking all new capacitors, including the filter ones which can induce the hum, I also thoroughly cleaned the tuning gang. Amazing what a dust brush and a little iso will do.
I restrung the dial, and the little feller gave off a magnificent hum-free reception on not just my local AM station, but ABC Sydney!
The upshot is, the radio picks up more stations than previously, and one happy customer!
And it lived happily after after....
PS. If you're game, ask me what I used to restring the radio dial!

Let me guess - fishing line? If so, the problem is keeping enough friction for a secure transmission of knob torque. The critical point will be the smallest pulley - usually the one at the knob. If fishing line or some other low-friction cord is slipping there, increase the friction of the pulley by lining the groove with a small, wide rubber band or a plumbing O-ring or two if a suitable size is available that will grip the pulley and give the line some purchase.

Maven