Strange HMV 1121 behaviour

[boater sam]

On the bench, an HMV 1121 radio, the one with the 4 separate glass tuning scales and the speaker behind.

Odd behaviour is that in the first, "Hi Fidelity" position of the tone control the set sound is good and loud. Turning the switch to any other position causes the tone to alter as it should but also the volume to drop substantially,  but only after a couple of seconds. Reverting to position one restores the volume instantly and the whole sequence can occur again.

Looking at the Trader sheet, 987, the tone switch contact S23 is closed in position one and open in all other positions. This would seem to point towards the problem being connected with this contact. The switch itself is fine. 

I cannot account for the time delay. Neither do I understand the reason for S23 and S24 with the associated capacitor C10 in the grid circuit of the IF amp valve.

An suggestions please?

[boater sam]

I have had several of these sets over the years, this is the first one that has this peculiarity. Apart from the odd valve lineup they are good radios.

This one had a totally seized tuning pointer slider, old grease had fixed it immovably to the slide rail and it took a lot of IPA and cleaning to get it moving again. 
Other than replacing the coupling capacitor on the output valve this was the only fault except for this odd time delay and volume drop with the tone control and its driving me mad!

[Refugee]

I would check the tone control capacitors for leakage or just change them.
It will put bias on the grid of the voltage amplifier valve and make the gain change.

[boater sam]

Checked the capacitors, always a good place to start.

C10 and C32 showed definite signs of leakage, the silver mica ones on the 3 switch contacts were, as expected, fine,

Replacing these 2 wax paper ones cured the odd drop in volume on the 4 top cut tone positions.
Still cannot account for why there was the time delay before this occurred.