[Radio1950]

I have carried out some more investigations, and especially on the second receiver.
To recap, both NRD515 receivers have some FM on demodulated SSB signals, due to 50 Hz effects mainly on the 24 MHz PLL VCO.
It is not noticeable in normal reception operation.
Receiver 1 was affected worse than receiver 2.

The cause of this effect is leakage 50 Hz from the mains transformer assembly conducted into the earth system.

I have minimised the effect on both receivers to a very large degree.

The effect is repeatable.

It is related to small 50 Hz currents flowing from the transformer assembly via the mount screws into the chassis area.
The voltage is very low and is difficult to measure with certainty.

The effect can be completely eliminated by electrically insulating the transformer from the chassis, eg by just using a plastic plate and temporary cable ties for trial.

A permanent fix probably involves using an additional insulating plate under the transformer, enlarging the screw holes, and using bushes or non-metallic fasteners.

I didn’t want to do this on a heritage receiver.

Nylon screws could be used, but I don’t trust them to hold such a large mass long term, and I don’t want a transformer loose in the receiver if they break.

A very acceptable fix therefore is to relocate any cable lugs, and have tight mount screws.

Two possible original cable lugs under the rear mount screws may be found in receivers.
One is the DC PS earth lead, and the other is the shield from the AC feed to the S Meter lamp.
I did notice that these lugs seem to introduce a little extra FM, probably via the very slightly increased magnetic or electric path due to the lug itself.

Also, I noticed that the effect is increased if the mount screws were only loosely tightened.

The head of one of the transformer mount screws interferes with the bottom screen of the 70 MHz oscillator.
A rather disappointing find.
Receiver 2 FM was markedly affected by this, and more than Receiver 1.

Only one screw is implicated, the others are clear.

Fix this by removing the bottom PC board, remove the screw, and remove the captive spring washer from the screw. Reassemble with modified screw.
Photos assist location of this screw.


Steps to take to minimise this FM effect

Remove all earth cables and lugs from any of the mains transformer mount screws and relocate to eg the back panel.
Tighten the four transformer mount screws quite firmly. Use the original threaded plate, not new individual M4 nuts.
Check that all PCB mount screws are firm, including the Power Supply.
Fix the mains transformer mount screw touching the 70 MHz oscillator bottom screen.

Thanks to everyone who assisted.