Hi all,
I traced the source of the high pitched noise to my Maplin modulator. The racket is there even with my CF player switched off. I did a search on 'noisy Maplin modulator' and I found a link - back to this forum! :
https://www.vintage-radio.net/forum/...ad.php?t=13712
The mentioned modification only actually requires the smaller of the two capacitors. It appears the larger capacitor is inserted in a cut wire link, which serves no purpose - a piece of wire is a better low impedance than a capacitor, by and large. A 10n ceramic seems a good choice for the second capacitor.
I notice too that my modulator has a heatsinked 7805 while the one at the link has a 78L05. I have found several references to these modulators shutting down after a period of operation. The conclusion in these references is that the 78L05 is over-heating and going into thermal shutdown. Their recommendation is to replace the 78L05 with a 7805. If you follow that advice (which incidentally, is not mine) then watch the orientation!
Peace at last
