Decca VHF FM radion

[Mike Watterson]

Thanks Mike, With a strong signal there is only a few volts on the ratio detector capacitor and the audio is very low too.
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The magic eye doesn't move much either.

Do check that ratio detector cap and that the + goes to chassis. People often replace them the wrong way round. The Magic Eye is driven from there, so I'd not expect it to change!
You won't get much audio either.

You can eliminate the Mixer-osc as the problem by injecting IF (10.7 MHz or 6.5 MHz?) via 22 pf & 100 K from the signal generator. Doesn't need modulation. The ratio detector cap should be -15V to -25V with signal from genny and zero without it.

I doubt the EB91 or EF41 are a problem as they'd rarely wear out, but check screen grid volts on the EF41s (in case decoupling paper caps never changed or resistors gone high). You'll get almost no IF gain at all if the g2 (screen) volts is too low.
With radio off, also unplug the front end triode (ECC? What) and the two EF41s and check continuity of IFTs from anode on valve socket to HT. Check the grid drive side on the IFT bases. I've often seen a VHF IFT with a snapped wire, especially the Philips skinny ones. One half of the VHF dual triode is simply an amp with almost no gain to stop the LO leaking to aerial. The other half is a self oscillating additive mixer, like how the LO/Mixer works on an AM transistor radio. If you get anything at all, then that's probably working as the oscillator doesn't run at all if emission is too low. So examine IF and ratio detector as suggested.

The EB91 is unlikely to be a problem, but if you don't have a known good one a pair of 1N4148 will work, observe orientation, poked into the valve socket! They only used EB91 because it was cheaper! The 1953 to 1959 battery valve sets with VHF all used semiconductor diodes for FM detector.

Don't worry about magic eye or audio till you are seeing at least -10 to -20V at the ratio detector cap with an off air signal.