Peto Scott TV169 Restoration.

[LENINGRAD T2]

Hi Trevor,

The EHT unit in Peto Scott and in other projection televisions functions independently as a source of high voltage. Do you agree with me that it is possible to replace it with any other high voltage source for the time of e.g. tests and start-up of the TV? By using a regulated high voltage source, it is easy to estimate at what values of this voltage MW6-2 has an acceptable brightness level and at what values it can serve as a normal CRT. I mean your tests with MW6-2.

[Murphyv310]

Hi.
The MW6-2 certainly will function at lower EHT levels. Unfortunately with low EHT there would never be enough brightness to drive a screen at that level. In fact mostly you had to drive them flat out to get usable results for day to day viewing. Running them hard made the EHT dip to 20kv or even less.
There are other ways to generate 25kv that would be more stable but I wonder how long the tube would last at 25kv running flat out.

[Murphyv310]

Hi.
Not so good progress today. I temporarily refilled the can with oil. On advancing the HT the can started to flash over at 22kv. This was a big disappointment as all was OK with just the new diodes. I removed it again and tried a dry run. The sparking was from the lower EHT connection and also from one end of a new cap. In fact it has to be due to routing of the caps which is due to the fact its impossible to get replacements the same as the originals. Available caps are just physically different.
So I tried as a bit of a laugh a thorn 1500 jelly pot to my surprise it works, no EHT regulation admittedly, even a 3000 EHT transformer works. With a multiplier it was easy to get 25kv. In fact it might be an idea to use the original transformer and an external multiplier.
https://youtu.be/U67QMpx6mLA

[Jac Janssen]

Those jellypots seems to be quite versatile!
Unfortunately no equivalent in this country...

Jac

[Geordie McBoyne]

Of course the line output transformers were designed to operate at 15,625Hz whereas in the Philips EHT unit it is 1,000Hz. I believe the method of operation is that the transformer "rings" by the application of the pulse present at the anode of the EL38 and the duration of the oscillation is chosen so that it is sufficiently diminished before the next pulse comes along.
Sometime ago I removed the 25KV overwind from the line transformer of an Ekco CT104. This set is electrically identical to the Pye CT70 dual standard colour TV set. After removing the overwind it was found that the pulse waveform at the anode of the PL509 isn't sufficient with a tripler to produce the required 25KV.
Looking at a GEC 2040 single standard line transformer we find there is an EHT overwind consisting of 100 turns or thereabouts.
A few insulated turns were wound on the Ekco's transformer and it was found that the flyback pulse on 625 lines was about 8 volts per turn.

Geordie McBoyne.

[Jac Janssen]

Of course the line output transformers were designed to operate at 15,625Hz whereas in the Philips EHT unit it is 1,000Hz. I believe the method of operation is that the transformer "rings" by the application of the pulse present at the anode of the EL38 and the duration of the oscillation is chosen so that it is sufficiently diminished before the next pulse comes along.
Geordie McBoyne.

That's exactly the case David.

A 1:100 probe near the EL38 anode:

   

Jac

[Murphyv310]

Hi.
Fingers crossed that the can may be repaired. In the bright sunlight I could see a tail from one of the very thin wires. I wrapped it tighter around the connection and resoldered it. I afterwards made sure all joints were rounded. All seems well even out of the can. It's now all reassembled and waiting on the new oil.
Hoping this is now a goer.

[Murphyv310]

Hi.
Really good news. The oil came today and I have refilled the can after warming the oil and can. Filled it to maximum and let the oil stabilise, checked the level after cooling and have tested the transformer at full output with no issues. It's now going to run for a few hours and then I'll ensure the level is correct.

[Murphyv310]

I've done a short clip with the work completed.
https://youtu.be/lpkl2ksAcM0

[Jac Janssen]

Good result Trevor!

Jac