I last posted on this set a year ago here:
https://www.vintage-radio.net/forum/...d.php?t=160941
I left off work at the time because the LOPT died. It went off to Mike Barker for rewind, but because of Covid it maybe sometime before I see it again so I thought I would do some more work and experiment with using a LOPT from a KB VC2 which Peter Sanders kindly gave me. In the meantime a mate has also done an amazing job of reassembling the cabinet which had basically fallen apart.
Where I left off last year was the set hard wired to UHF/625 and displaying a good quality 625 picture with no sound. There were no results on 405, because as I say the set had been hard wired to 625/UHF and also the extra bit of system switch needed to make it 405/625 convertible was not fitted. The UHF part of the set comes in the form of a plinth which screws on underneath the set and is a whole additional receiver up to and including video output valve and sound detector.
Fortunately last year Dave (Dazzlevision) kindly provided me with that piece of system switch NOS, so I set about fitting it. In the original conversion the heaters between the VHF/405 and UHF/625 signal valves are switched, resulting in a delay on switching systems while the relevant heaters warm up. Murphy later published a mod which kept all heaters in circuit so I incorporated that while I was at it.
Having completed the conversion mod, I disconnected the screen feed to the to LOP valve and hooked up a 60W bulb across the HT line to take up the load of the missing LOP, attached a speaker and a scope an switched on. My previous good results on a UHF picture were there on the scope, but nothing on 405, and no sound on UHF. The lack of UHF sound was due to not one, but both sound discriminator diodes being o/c. Lack of VHF picture was due to the PCB track to the on board 405 video output having been cut as part of the previous hard wiring process. restoring that brought up a lovely 405 trace at the connection to tube cathode. I have to turn up the contrast control though to video overload levels to get any 405 line sound. I am not sure what that is about- I will leave it until I can get a picture up.
So I have now turned my attention to the KB LOPT which looked in a pretty sorry state -by the looks of it having been out in the cold and damp for may years. Lashing it to the scan coils via an S correction cap and using the set's valves and boost cap in-situ gave me initially 650 volts boost and 16KV EHT via a silicon rectifier on 625. Almost inevitably both voltages began to drop as damp transformer syndrome showed up. I have since given the LOPT Many hours of the DC heating process and that has improved the situation a lot but it is still not completely stable. Holding a screwdriver anywhere near the windings produces fizzings and arcs so I guess it could die at any moment, I will try putting some solid setting silicon on it and press on. One of the challenges is that the set needs a 150-0-150 v pulse from the LOPT for the flywheel sync. I will have to experiment with winding that on if the LOPT survives