Hi!

I've just started into the world of Tube radios and was wondering if someone could point me in the direction of a small fault I can here.

I have a little Nipper like this one http://www.radiomuseum.org/r/hismasters_little_nipper_62_52_ch_62.html?language_id=2.

Its working but you can hear a high pitched noise over the top of the audio. The noise does not change when moving the volume up and is there on any frequency. it doesn't start straight away when turning the unit on but slightly after the unit warms up. I have re-capped the beast and changed over a few resistors that had gone out of tolerance. I have not changed out any of the Mica Caps and am assuming it may be one of them.

Could someone point me in the right direction before I start changing out the Micas?

Thanks.

On the face of it, you have some form of unwanted feedback oscillation occurring. Could be a bad capacitor, but first what about the aerial?

Does it have a long aerial on it? If so, does removing it make a difference to the problem?

By long aerial you mean a length of wire attached to the back aerial wire? If so then no it just has the internal one. That's one thing I have not examined closely.

Yes, I mean a long external aerial. Sometimes, in certain locations, a long aerial can overload the front end with high signals, causing whistles.

Interesting. I could see it being this. I have 2 little nippers and the one with the problem picks up stations a lot better than the other one. They both do not have an external wire attached. Perhaps I should look more closely at the internal antenna

As I understand the symptoms described, the sound does not change with the volume control, suggesting an audio problem.
Is there a negative feedback connection from the speaker that is reversed, causing positive feedback?
Harold

Edit.
I have just looked at the circuit and there is indeed neg. feedback from the speaker. I suggest reversing the feedback connection and see if it changes. It may be easier to reverse the transformer primary connection leads.

You sir are a master! That was exactly the problem.

The unit had had some work done on it previously. I had replaced a few off value components already from this repair. The small transformer had also been replaced and wired wrong!

Thank you very much! I'm pretty sure I would not have been able to work that out without some expert help! Noise has completely gone.

A good result, happy to help.
Harold