Audio trafo open cct? any fix?

[snowman_al]

Originally Posted by Cathode Ray

... But the radio howls with feedback when both the volume plus tone are turned up, low tone with high volume seems ok though!
I see two options:
a. Leave the radio as it is, and perhaps come add some capacitor / resistor feedback circuit. It only seems to be unstable under certain conditions, so it might not need much negative feedback.

b . Have the trafo re-wound, but I'm not sure the radio is worth spending much more money on to he honest, it's not especially rare or sentimental.

Anyone got suggestions for option a?
Ray

a - I think there is nothing to loose by trying a resistor in place of the o/c winding. Ultimately it is only a path to ground for the tone network? (But I do not think like Philips!)
You could try 100 ohms to 1k ohms and see what happens.

b - Knowing when to stop or take a step back is always tricky...
Alan

[Cathode Ray]

Thanks Alan & Frank for your good suggestions.

However before going down the alternate feedback route I asked Ed Dinning about an idea to add the missing turns on the outside of the trafo, he gave it approval and suggested the number of turns to use (cheers Ed!)so that's what I did.

I stripped the trafo core and added 50 turns on the outside, tapped onto secondary winding.
It was guesswork to figure out the winding direction, get it wrong it would have provided positive feedback instead of negative.

Anyway I got lucky in the coin toss and it worked, no more howling feedback and it can take full volume.

I love this stuff, it's especially satisfying when something like this happens.

Cheers

[linescan87]

Well done Ray, that's a very good work around.

John Joe.

[Cathode Ray]

Thanks John Joe, I'm very happy with it.

It was a tight squeeze to get the E laminates back in place over the core, had to gently squeeze the core in a vice to make it 1 mm thinner !

[snowman_al]

Chapeau Ray!

[TrevorG3VLF]

A howl indicates positive feedback and the volume control is in the loop. The feedback could be audio or RF/IF with rectification in the audio amplifier.

I would look at the bypassing of the HT, screen and agc lines.
If the feedback is RF/IF then a small capacitor (1nF) across the volume control and another between grid and cathode of the EL84 may work.

I do not have a BX548A diagram on the CD to look up

Check the voltages on the new EL84. Previously there was a positive voltage on the EL84 grid, this is wrong. Oscillation could give a negative EL84 grid voltage, at least there is a grid stopper.

[Cathode Ray]

Thanks Travor,
The feedback has stopped since I repaired the secondary winding.
But regarding the positive voltage on the EL84 grid, that was measured with a faulty EL84 in pace. But now I have replaced the valve I will re-check the voltages and see how it's looking.