Bush AC1 1947

[ValveMan]

Hi fellow radio restorers,

For the last few weeks I've been working on a Bush AC1 radio receiver from the late 1940's. I've replaced all wax and electrolytic capacitors and finally decided that I'd power it up (through the use of a variac) and see if my work had paid off. The radio warmed up however almost instantly the speaker began making a bub bub bub...... noise which varied in speed as I adjusted the variable tuning capacitor. I went through all three wave bands SW MW LW and the radio didn't pick up a single station. Why is this happening? Is it a failing valve or something to do with wiring? I honestly have no idea. I've been repairing radios for a short while however I've never come across this up until now.

Any help or advice would be greatly appreciated,

Regards,
Dylan

[paulsherwin]

The first thing to do is to carefully check your work for wiring errors, then check again. If that doesn't reveal something obvious, take some voltage readings and compare them to the service data.

Bad valves are rarely the cause of old radio faults.

[Boom]

I can't see the circuit on the BVWS cd but has the AC1 got an EF39 in the IF? If so what condition is the red coating in, this is notorious for causing instability when it falls off.

[ValveMan]

Hi Dave,

Funny you should mention this, the radio does have an EF39 in the IF and it is in poor condition to say the least. I've been in contact with my good friend James who has been able to supply me with a NOS EF39 which has tested good on his tube tester. I've heard these valves are notorious for their red coating flaking off and causing instability. That's just how it goes huh?
I'll hopefully receive the valve by the weekend and will give it a test. I'll be sure to keep you posted. It's rare that us radio restorers are able to blame a problem on a valve however finally the moment has arrived!

Kind regards,
Dylan

[paulsherwin]

You can wrap some kitchen foil around the EF39 to see if it makes any difference.

[ValveMan]

Thanks Paul,

I'll be sure to give a try in the morning. I'm hoping that this is the problem because I simply don't know what I'll do if it isn't! Suppose I'll work it out in the end, I hope.

Kind regards,
Dylan

[Radio Wrangler]

The bub-bub-bub sound is usually 'motor-boating' or 'squegging'

In a mains set it points to capacitors gone open or low value, possibly a weak rectifier.

It could be incited by RF instability, so the foil screening is a good idea. If that doesn't work, then you need to look for dodgy capacitors, or also any resistors in power feeds or screen grids that have gone high.

Motorboaring is when the power consumed by an audio power amp pulls the supply down and the local oscillator stops, the power recovers, the oscillator starts, reception resumes and the whole thing keeps repeating.

Squegging is when an oscillator does the same sort of thing to itself.

David