I have restored two AWA 429MA Champions, both of which drift high as they warm up, to the degree that the station is no longer heard. This particularly at high frequencies; not really noticeable at low frequencies. I haven't bothered with this as retuning is simple. However now I want to fit FM converters which work by using oscillator frequency tracking to tune the FMconverter, so that the normal tuning knob can be used.

https://www.ebay.com.au/itm/146458817120

I suspect that AM drift will also cause FM drift with the converter activated, so need to address this drift.

I haven't changed any of the micas in these radios so a CHANGE here is not responsible, and I doubt that they would cause drift anyway. That probably leaves an RF transformer. Any ideas on the cause, and a fix?

The drift is upward in frequency, ie the radio needs to be tuned higher to bring the station in again.

Assuming it's the local oscillator that's drifting, there's not many parts in that circuit.

R1, L4, C6, C7, C8, and the 6BE6 valve. The valve is fed by 2 voltages, so they should be checked for drift as well (pins 5 and 6).

The drift in the local oscillator can be confirmed by using a communication receiver on SSB tuned to the receiver frequency plus the IF frequency. So for SEN it will be 1170+455=1625kHz.

I remember fixing Astor car radios as a kid with this same symptom.
Replacing the 6BE6 always fixed it.

Thanks for the suggestions gents. Will see what 6BE6s I have and do some swapping.

I would check the components around it. However, Pentagrids generally drift upwards, until they stabilise (RCA). Some are also very sensitive to voltage change, which is another reason for checking, or monitoring "B" voltage (analogue meter).

OK will go for Ian's easy fix first and try a new 6BE6. If that works then resistors/coils/whatever not at fault. If it doesn't then the real work starts. I suspect in the end though it will be 6 of one and half a dozen of the other, given the radios were made to a price at the lower end of the market.

Waiting for the radios to turn up now.

Further to this modification, the supplier has indicated that the output of the converter is only 1V whereas the 6AQ5 output valve requires about 6V. So an output with an inbuilt preamp would be needed eg 6AV6, 6AT6; or maybe a solid state preamp. Before I go this effort, will look at other options eg bluetooth adaptor, and whether they have the same issue.

Is that model a reflexed set? If so, just insert the audio at the detector instead of the 6AQ5 grid.

Most of the non-reflexed models used a 6BV7.

Drop me an email and I'll send you the audio amp circuit from the AWA P4 TV, which uses a 6AQ5 and an NPN transistor. Very simple and it works well with plenty of gain.

It's not a reflex set.

There's 2 versions of the 429MA, some have a 6AQ5, while others have a N78. My radio has the N78, and on recollection it drifts pretty badly too. Never looked into the problem though.

If the radio is never to be used for AM again you could repurpose the IF bottle as an audio preamp. That would work and require surprisingly minimal changes.

If you want to do this (I think it's necessary for reasonable volume to be achieved) email me for a circuit.

I don't know if its reflexed but I think not

Re 6AQ5/N78, you have reminded me that at least one is N78, I'll just have to wait until they are back in my hands. But both valves require similar input voltage, as far as I can determine from looking at valve specs.

I've got a couple of NOS 6BE6s so will soon find out if that is the problem. Thanks Robbert, interesting to here that you seen this drift too.

Its also worth mentioning that some Champions had a KT61 output.

My inclination is to leave the radios able to receive AM as I'm considering other options such as using an AM transmitter with FM receiver built in or bluetooth. Or maybe, shock/horror, a solid state preamp. So I'll take a raincheck on your offer, thanks Ian.

Often just dirty valve sockets will cause drift. As they heat up the contact changes . Seen it many times.

Could be valve sockets but not that long since I cleaned them, but both radios? Time will tell.

The gentleman that sells these FM converters locally on eBay says he can offer them with 2.5V output to suit direct connection to high Mu output valves. It might be worth enquiring about this option.

Thanks for that info. Only 1...1.5V output mentioned in the listing given in post 1 here. Website mentioned but address not given, will search. I have been corresponding, will enquire.