Earlier in this thread, John W (HCS) suggested you could hook up any LOPT to the top caps of the LOP valve and the boost diode and it would work well enough to light the heater of the EHT rectifier. Ever the sceptic, I wasn't convinced it would work without scan coils, boost cap etc. connected as well, so I have been waiting for the chance to try it for myself!
To be fair, John said it probably wouldn't work that well with the circuit arrangement in the Pilot. That's because feedback for the line oscillator comes via a cap from the LOPT so the oscillator doesn't work in isolation from the LOPT.
I have the LOPT removed from my Marconiphone VT 161, which I thought had died and which I was going to get re wound at some point. When I was doing my 'ringing' experiments last year to see how good and bad transformers behaved, I was very surprised to find that this transformer, which I was sure was dead, gave a good 'ring' trace on the scope, indicating that it might well be OK.
So I hooked it up to the Pilot to see what would happen!
The Pilot uses a 'de- saturated' LOPT (see post 113) whereby the HT current is passed through the winding that drives the scan coils via a width choke. This neutralises the DC component in the primary winding and results in greater efficiency.(as far as I understand it)
I had to modify this arrangement to make it more similar to the VT161, taking the HT direct to the boost diode cathode.
Switching on, very little happened (I'd already taken feedback for the line osc via a cap from the LOPT). I then added a boost cap in the same configuration as the VT161 circuit and the thing became much more lively.
I eventually had to replicate the circuit of the VT161 before the transformer would oscillate properly at the correct frequency. When it did, the EHT rectifier heater lit up nicely.
However, it point blank refused to drive the scan coils- I suppose the impedance was very different. Hardly surprising as the Pilot is 110 degree narrow neck tube and the VT161 is 90 degree fat neck tube.
I really wanted to measure the EHT though so I had to connect up to the CRT final anode so that the smoothing cap formed by the CRT was present. Obviously, I had no scanning capability so had to ensure that the CRT didn't produce an intense white dot!!
With the brightness at minimum, I switched on. Everything was OK, the screen was completely blanked and the EHT measured 15KV!! So I am pretty confident that the VT161 transformer is viable. The EHT winding is covered in melted wax though....
The problem came when I thought what would happen when I turned the TV off. With no beam current, the EHT would stay charged and as soon as the voltages on the CRT electrodes collapsed, there was every chance of a major intense white dot on the screen!! I really hadn't thought that one through
I had visions of having to leave the set on for ever because it would kill the tube if I turned it off!
Eventually I decided that the best method was to let the CRT heaters cool so there would be no emission and hence no dot. Pulling an IF valve broke the heater chain and let everything come gently back down to earth with no drama
So my conclusion from all that is that my VT161 transformer is probably fine and that the one in the Pilot is not as good as it should be.
All in all, an interesting experiment, (for me anyway!) so thanks John for putting the idea in my mind
So for better or worse, I am going to remove the wax and re- insulate the Pilot transformer and see what happens!
All the best
Nick