Hi Alan,
Yes, an addenda from me - just for the record. You never know when a report on a piece of kit (such as this) might be of value to someone else one day.
The CRT base socket is a 12-pin affair (might be a B12H) and the solder lugs on the socket are fairly large. The trouble is that those solder tags are bent at about 45° to the plane of the socket - so there isn't much clearance between adjacent tags. Now add in the many (relatively large) components (and thick wiring) which are soldered to those tags and not only is it difficult to see the individual tags, but also difficult to see - and thus check - that various tags are not in contact with their immediate neighbours - which was the cause of the problem. The CRT cathode gets about -1200 v., fed to it from that EHT pcb. But the CRT socket pin adjacent to that has a wire with about +250 v. fed to it - and from a low impedance source, too. Since those two tags were touching, the -1200 v. point was being pulled up to + 250v. - and the EHT pcb, not liking that, (understandably), naturally got all upset & confused.
The route to the successful diagnosis was one of 'going back to basics' - since not all the voltages (normal operating conditions) are stated on the cct. diagram. I knew that the final anode should be at about 8.5 Kv. Now this 'scope has a 5 inch tube - so I would expect a final EHT of about 10 Kv. So where do I find the 'missing' 1.5 Kv? That must be at the cathode, producing the required 10 Kv. across the CRT. But on measurement, the cathode wire on the EHT board was at +250 v. Huh?
That can't be right! When that cathode feed wire was disconnected from the EHT pcb, the now unconnected pin was at -1200 v. Ah-Ha! 
And you know the rest.
Final comments.
It may be possible to obtain a replacement socket whereby the tags are perpendicular to the plane of the socket. Alternatively, adding heat-shrink sleeving to some tags would help.
It's times and events like this that establish the value of 'networking' - and that is only possible through the Internet and forums such as ours - the G.V.R.
All said.
Al.
Yes, an addenda from me - just for the record. You never know when a report on a piece of kit (such as this) might be of value to someone else one day.
The CRT base socket is a 12-pin affair (might be a B12H) and the solder lugs on the socket are fairly large. The trouble is that those solder tags are bent at about 45° to the plane of the socket - so there isn't much clearance between adjacent tags. Now add in the many (relatively large) components (and thick wiring) which are soldered to those tags and not only is it difficult to see the individual tags, but also difficult to see - and thus check - that various tags are not in contact with their immediate neighbours - which was the cause of the problem. The CRT cathode gets about -1200 v., fed to it from that EHT pcb. But the CRT socket pin adjacent to that has a wire with about +250 v. fed to it - and from a low impedance source, too. Since those two tags were touching, the -1200 v. point was being pulled up to + 250v. - and the EHT pcb, not liking that, (understandably), naturally got all upset & confused.
The route to the successful diagnosis was one of 'going back to basics' - since not all the voltages (normal operating conditions) are stated on the cct. diagram. I knew that the final anode should be at about 8.5 Kv. Now this 'scope has a 5 inch tube - so I would expect a final EHT of about 10 Kv. So where do I find the 'missing' 1.5 Kv? That must be at the cathode, producing the required 10 Kv. across the CRT. But on measurement, the cathode wire on the EHT board was at +250 v. Huh?
That can't be right! When that cathode feed wire was disconnected from the EHT pcb, the now unconnected pin was at -1200 v. Ah-Ha! And you know the rest.
Final comments.
It may be possible to obtain a replacement socket whereby the tags are perpendicular to the plane of the socket. Alternatively, adding heat-shrink sleeving to some tags would help.
It's times and events like this that establish the value of 'networking' - and that is only possible through the Internet and forums such as ours - the G.V.R.
All said.
Al.
