So, it's gone from severe motorboating to less severe motorboating.

It is NOT atmospherics. The fault is still there, you have only succeeded in masking it with increased filtering and/or reduced signal.

Good way to find out---hit it with a powerful signal (good antenna) and see what happens.

With a good signal such as the ABC there is no noise whatsoever, I usually listen to 5CS, a small regional AM (of course) station in Port Pirie which is over 100km away, so on some days there are varying levels of interference in the way of soft or loud white noise, sometimes varying volume, other days perfect clarity. This old set has no AVC and it's sounding good now. Eventually I'll replace all of the valves, but It's behaving as good as it ever has now and honestly all of the replaceable components are brand new except for some wire wound resistors whose value is not stated and others stated but in spec. Some capacitors are 450v ones but only need to be 45v, but I've been told this doesn't matter, in any case it's pleasing to the ear now and not suffering anymore from any diabolical terminal meltdown!

One of the things that can get you into the poo is actually changing the caps. If you increase the 1st cap, that will tend to increase the voltage out. The 1st cap will have around 300V on it if "B" voltage on the "B" rail after the choke / field is 250V. What gets the caps is that modern caps do not quote a surge value. #80 will produce close to double "B" working voltage, on startup, until the heater tubes sometimes aided by a voltage divider, conduct & drag it down. Do make sure the the second cap has a decoupling cap across it. often that is at the opposite end of B+. That is to decouple RF & often "standing waves", besides that Electrolytic caps are hopeless at decoupling RF: 0.047μF to 0.25μF common.

Why I suggest the Chopstick, for prodding is that it is not inductive & non conductive. It is always handy to lull the other half (if applicable) into a false sense of sentiment; When the real reason for eating Chinese is not brownie points, it's to knock off the chopsticks.

Notwithstanding that 6D6 / 6U7 is the worst tube, that I have dealt with for going berserk when not shielded, the 2nd detector first audio is a horror when it comes to destabilisation due to induction & radiation. Changing & moving caps in physical position can be one cause the second is caused by the modern cap not having "Outside foil"; That makes it more prone to induction from badly shielded wires and that can be exacerbated by poor lead dress. Ideally wires should be as close to the chassis as they can be.

If prodding around the set with a chopstick produces a change in frequency then it's some form of induction in that area. The signal strength could be implicated if its a strong station and there is a wrong part causing overload. I find it unusual when there is a #75, to not have AGC.

Other thing that helps is add 100pf from 1st audio anode to ground. Stops residual IF from being amplified by the output stage and getting radiated.

Good information Marc and Robert, thank you! Yes the 6D6 on this one has goat shields and I've sanded it where it contacts the clips to make a good contact, but today it's sounding great, no noise at all now!

Beware of changing the working valves. It is the old ones that are still going that are the good ones: Can you guarantee the replacement is going to be as good?

No not really, usually I replace in a process of elimination, using NOS if possible.

New parts mean nothing, if its an inherent fault.

The second question if it is a tsh, tsh, noise rather tan a slow sine, or square wave and is it affected by the volume control?

Why that line of questioning? It is not common but I have had a couple of sets "squegging" and that happens in the oscillator circuit. Make sure its all there and the components are right? If it is that there may be a need for an anti-resonance resistor to be added. Also make sure that you wired the filter caps correctly & if electrolytics were used, that the last one has a cap of 0.047mfd to 0.22mfd (in older sets) to decouple RF riding on B+.

#80 may be like 5Y3. Turn it sideways at your peril.

Yes they don't like being turned sideways. There are two bundles of electrolytic caps parallel with resistors and polyesters (I changed from the old papers, I used correct μF, but 450V where the originals were 45V, but have been told exceeding the voltage rating doesn't matter?

450V is a bit much to replace 45V. There could be issues in it maintaining form with electrolytic types, albeit the new are not as bad as the old. If "B" is meant to be around 250VDC on load, I will not use 450V types: Some of them are crap anyway.

Whilst I can check older valves, if they are working / test satisfactorily, they stay.

Just on the B+ decoupling cap, often that is found on the RF section end of the wire: There can be two at each end and I have seen it where each screen has one even that they are on a common wire. It's more common in transmitters, but is to get rid of "standing waves". Whilst it may look silly its not: Do not change, to save caps.

Standing waves are those that can develop on a long wire, or one that can become a resonant antenna & pick up signal, which it then uses to destabilise the whole show, like a 6U7 without a shield.

Ok, that is good to know, I have noticed that, caps at both ends of the same wire and wondered why!