I knew it was a mistake putting the lid on properly.
Having restored the functionality of the frequency display I decided to run the radio for a few hours over the weekend to evaluate performance further. Still no tx, of course or any PLL locking on 28MHz but I knew that already ...
I was listening to a QSO on 7MHz this morning between a very strong Belgian station and a string of UK stations, most of whom I could hear reasonably well when I realised that the frequency seemed to be locked on to 7.150.0MHz. Tuning either side by a kHz or two made no difference to the display. I had a funny feeling about this, so I tuned up and down the band to find that lo and behold that the display was only displaying frequency to a 10kHz resolution!
To see whether this was a heat related issue I switched the radio off and let it cool down for half an hour or so, but on re-powering the fault was still present, so I have concluded that I have ANOTHER dead chip! More than likely a duff 7490 in the counter chain, and that's TTL not CMOS, so another theory goes down the pan.
It is now only sheer stubbornness which is sustaining my interest in this project. In all the years of building and repairing equipment, I don't think I've ever come across a project like this. Trying to fix up a radio with numerous faults is one thing (and what I thought I was dealing with here), but fixing one which keeps breaking seemingly randomly whilst under repair is quite something else. It's a case of either "nolum illigitimus carborundum", or chuck the bloody thing in the (recycling) bin! I don't give up easily though. What kind of trauma has this radio suffered in the past? It's anybody's guess.