I have tried capacitors in series, at values between 200nf and 1nf. Caps may block the DC bias, but they also seem to leak part of the tuned audio signal from IFT2 to ground, or reduce amplification in some other way.

I suppose the audio is AC at that point, and chassis ground is common. You can't tap into the audio circuit without making the ground connection. Trying different value caps, I found that larger caps reduced the tuned audio by a larger amount, and smaller caps blocked the external signal by a larger amount. The capacitance in series on the external source controls the balance between the two signal sources, but does not allow both to be present at full source level.

Using a switch, the external signal is clean and well-balanced, but about 5db lower speaker volume than the internal tuner.

Maven

Looking at the circuit, you have around 250k of resistance between the ext input to the radio and chassis (500k pot and approx 500k in parallel).
Any external input will need to have an impedance much higher than this to prevent loading down the signal from the radio. At a guess it would need to be around 1M to achieve this.

So ideally, your external signal would be applied to the radio via a capacitor (to provide DC isolation) and a 1M resistor in series to achieve the correct impedance level i.e. reducing the amount the radio signal is being loaded down. Using a very small value capacitor would achieve the same outcome as you have found by experiment.

The signal being applied to the radio would also need to be at a fairly high level to match the radio signal level. The standard line output signal from audio equipment will be too weak, you'll need the output from a headphone socket to provide sufficient signal.

There is no quick easy way of doing what you want to do. The radio was not designed to do it and unless you use a switch to swap between signal sources, you will always have problems trying to get things to match.

I guess it's stay with the switch then, for the time being. At the moment I have a jury-rigged switch on a loose lead outside the cabinet.

If I want to implement this neatly, I might look for one of those units that combines a rotary potentiometer with a push-pull switch - as used on some generations of TVs.
Instead of using that to switch AC on/off, I could use it to switch inputs from tuner to external pickup.

Issues with that - 1) I'd have to find a 500k pot with that switch, and 2) I'd have to do another conversion to add a 40k tap to the switched pot. I'll let that thought compost for a while..

Thank for all the advice so far.

Maven

Might be time to consider an AM transmitter. Then you can listen to whatever you desire on all of your radios.

Ben

+ 1 for the AM transmitter.

Does anyone sell kits for micro-power AM transmitter? I built the Jaycar FM transmitter some time ago to extend my TV's audio, but I haven't seen AM versions around.

Maven

There is a discussion on AM transmitters here. All of these household transmitters are 'flea power' to avoid a visit from the Commonwealth Government.

The one I built runs two valves with a very low HT rail so it is safe whilst maintaining the valve theme.

Silicon chip described a flea power TX a few years ago.
No one produced it as a kit, but the PCB is available for SC shop. It uses readily available parts.
I think mine came it at under $50 with the article back issue, the PCB and parts.

I did discover an etching issue with the PCB as mine didnt work straight up.

I will see if I can dig some more info up tomorrow

Of course a valve TX would be more 'period correct'