Well the restoration of this set trundles slowly onwards!! It has become something of a test bed for new ideas in recent times.
Firstly the LOPT and EHT issue.
De waxing & recoating the windings in EHT varnish did nothing to improve the performance or the EHT. The only positive result is that it now looks nicer
The EHT still sits around the 10KV mark and the transformer gets too hot after half an hour.
I've had the issue of LOPT's getting too hot before and have ended up fitting small CPU cooling fans. Very non original, but it does get round the problem and prevents failures.
Despite my best efforts, I can't get to the bottom of this so I've cheated!! By increasing the HT voltage from 195V to 210V, The EHT is 14KV when the transformer is cold. By fan cooling it, it stays at that level and the transformer stays almost cold even after several hours of use.
I think that will have to be the permanent solution as the picture is much better with the higher EHT.
This set uses total AC coupling of the video signal to the CRT cathode via a capacitor. Consequently, the black level performance is poor. On a dark scene, the background goes light grey and on a bright scene, goes jet black.
With the help of Fernseh and Argus25, I've added a sync pulse clamping circuit which keeps the black level very constant.
The improvement in the picture is outstanding:- now on a scene change it fades to black instead of light grey!
With the increased EHT and black level clamp, the picture on this 21" set is really rather good and is great to watch. The interlace is good as it does have an interlace filter which is important on a big screen.
Adding the black level clamp has now shown up shortcomings with the mean level AGC set up, so a different design of AGC circuit is being investigated.
It can be argued that a set should stay as it was designed with all it's inherent shortcomings. I can't argue with that, but I enjoy getting the best picture I can out of it so making 'improvements' are part of the fun: at least they are for me!
All the best
Nick