I know this problem may be a common one, but any help would be greatly appreciated. I have a valve console radio from the mid sixties. Trademarked "Belatone", a house brand of Knock and Kirby's, it was made by A.W. Jackson Industries and is the "Precedent" model (thanks to the forum member who provided this info). The rdaio picks up strong stations clearly (mainly the ABC stations here in Sydney) but between stations it squeals, with the pitch of the squealing sound lowering itself when a station is reached. Playing with the tuning knob can achieve a variation of what sounds like several octaves in pitch. Can anyone offer suggestions as to what causes this, and possible remedies? Thanks in advance

Assuming it's not RFI from a local source, it appears that the radio is oscillating. One common cause of that is the negative feedback circuit (if present) wired back to front creating positive feedback. Other causes can be bad bypass capacitors and/or chassis earth connections gone bad.

Thanks GTC. It does appear to have a NFB circuit, with the tone pot and a cap to ground as part of it, the wiper and one leg of the tone pot wired between the OT secondary and the cathode of the triode section of the 6BM8 valve. I'm no expert, though. I'll be replacing the filter caps, and I may just go through and do all the caps in the amplifier section while I'm at it. I'll leave the receiver section untouched since I have no idea how that section works. I'll also check earth connection to the chassis. Cheers.

Edit .... there doesn't appear to be any cathode bypass caps. I also assume the NFB connection is ok since it's been untouched since the 60s.

I'll also check earth connection to the chassis.

By that I mean the various grounds within the circuit -- that is, any solder connections to the chassis.

I also assume the NFB connection is ok since it's been untouched since the 60s

In any case, NFB is easy enough to check. Just reverse the connections and see what difference it makes.

Thanks for clarifying. By reversing the connections of the NFB, do you mean connecting to the other side of the output transformer? Thanks again for the help.

Edit, I mean moving the NFB connection to the other side of the OT.

Sounds like feedback around the IF stage, especially if the volume control can turn it down.

Look for bad caps in the AGC circuit, or missing shielding. Perhaps even just bumping the IF amp might change the sound.

By reversing the connections of the NFB, do you mean connecting to the other side of the output transformer?

Yes. That reverses the phasing of the feedback.

Thanks for the suggestions. This gives me a point of departure for troubleshooting.

One of the chief area's of problems when new caps are used occurs around the second detector & first Audio Frequency valve. 6AV6 being one of the most unpredictable. If there is some sort of fault on the chassis It will often go "Microphonic".

The newer caps are not marked "outside foil" and often lead dress and proximity in that area become an issue. This being due to the amount of radiation & inductance that can happen in that area. I have often had to improve, or fit shielding and that incudes shielded wire.

Wires in particular away from the chassis can radiate, or become antennas and that will destabilise the whole show & manifest as oscillation. Do also check any valve shielding and for wiring mistakes.

One quick way to often find the oscillation is to probe the components & wiring with a chopstick. Any change in the coupling will normally cause a change in frequency of the oscillation.

Did you recheck the IF alignment after changing components, or check the resistors as you changed caps?

Marc

"The newer caps are not marked "outside foil" "

Can anyone suggest a way of detecting which lead is connected to the outside foil where it is not marked with the usual black band?

These are a different construction and without actually ripping / dissolving the cover off it, then it may be a case of placing it in an inductive position and alternatively earth an end and see which end produces the least signal on an oscilloscope.

In an outside foil the outside foil encapsulates the entire outside of the cap as the name suggests. That makes it less susceptible to signal penetration, like a shielded cable. Outside foil goes to the closest, or nearest to earth potential.

Most of the time lead dress can fix the issue. Parallel wires are also a trap and will induct into each other. That's what causes CFL lights to flick.

Another often-overlooked problem is that early electrolytic capacitors were apparently better at RF bypassing than modern types. Consequently many manufacturers relied on the filter electrolytics to also handle RF bypassing, and replacing them with modern higher-inductance types can result in instability.
The simple solution is to fit a 0.1μF polyester capacitor across each filter electrolytic.

I really early time there were no such things as electrolytic caps and they did not need RF bypass, Once we had electrolytic caps then we saw sets in the later part of the 30's onwards with them. One early HMV I serviced actually had an electrolytic on the screen & that was decoupled with a non polarised cap. Most of the Autodyne era often had 0.25mfd for decoupling on B even with the early wet electrolytic caps.

If you put polarised electrolytic caps in a set that never had them, then the decoupling cap is needed.

What have you actually done to the set? By the late sixties wax paper caps were becoming rare and Polyester, Stryroseal & Ceramic caps were taking over.

What needs to be done is to find if the source is internal, or external. NBN boxes, & switchmode power supplies being major causes. Altering things, like changing components and wires, can add issues.

Wires not crossing at 90 degrees, or away from the chassis can result in instability. Have a bit of a probe at the wires & components with a chopstick, or something else non inductive & see if you can change the frequency of the whistle.