GEC BT302 LOPT transplant.

[davidh1041]

This GEC has been standing around for years in a damp shed, the base of the cabinet is full of fresh worm, however they didn't like the taste for the sides and top which are in very good condition. As normal with my luck lately the Lopt would run for two hours, then the width would come in and the primary starts to melt. I really like its bow fronted look, so decided it would be worth spending some time on it.
I decided to replace the base of the cabinet using 9mm MDF using the original as a template. I fitted a thorn Jelly pot from a scrap 980 chassis. I had to use the Thorn scan coils, which luckily matched the frame transformer. The set runs very happily now, I just need to mount the transformer properly, waiting to be reunited with its now worm free cabinet.
Will update when the cabinet is finished.

David

[McMurdo]

did you take the picture in a mirror, or have you got the line coils reversed?

[Philips210]

Hi.

That's a good result David.

I have the later BT322 with the same type of cabinet and that has suffered the dreaded woodworm mainly in the lower panel as in your cabinet. I treated the plywood panel by laboriously injecting pure turpentine (out in the fresh air!) in the holes and then filling with plastic wood. So far it seems to be OK. I did replace the small curved ply panel at the front of the lower panel as this was beyond treating.

It was also good to transplant the BRC LOPTx in your GEC, and it would be good to see the circuit details. I wonder if a BRC 1400 could also be employed as an alternative. So far my LOPTx is fine and I think they're generally above average reliability wise if kept dry.

Look forward to seeing more pics.

Regards
Symon.

[FERNSEH]

An excellent modification, I love this kind of work, another 405 line TV set saved.
Might be worth checking the value of C148 (0.5mfd). It may need replacing with a different value to suit the BRC line output transformer and scan coil combination.
The design of the BT302 line output stage doesn't differ all that much from the later 980 series.
Using a BRC 1400 LOPTx: Viable project but it should be remembered the 1400 operates with a feedback stabilising circuit employing a VDR. Lots of spare power resulting in too much width and EHT without stabilising.

DFWB.

[davidh1041]

A quick progress report, i have started to reassemble the cabinet with a new base made from 9mm ply with the help from a friend with a router. The picture was taken in the bench mirror, hence the reversed testcard. I tried experimenting with the S correction capacitor, but found that the original value of .5uf gave the best results.
David

[Focus Diode]

Excellent restoration job, I can't fault the linearity at all.
Could you please provide details on connections etc to the Thorn Jellypot? This will be useful to others wishing to undertake a similar transplant.

Cheers,
Brian

[Tazman1966]

Performing very nicely indeed.

[davidh1041]

Here are the circuits for the two sets. As can be seen, the GEC uses very few winding's. I connected the boost and hot side of the scan coils to tag A on the jelly pot, the "S" correction capacitor to tag J. Left the 3rd harmonic tuning capacitor connected as in the Thorn circuit.
If you look carefully at one of the first pictures of the set running, you can just see a really annoying flyback line about 1" down on the right hand side of the screen. I reduced the value of R107 in the cathode of the frame multi-vibrator valve from 1k to 470 ohm to speed up the flyback. This has completely removed the line. Seems this mod has worked out well, i have now recaped the rest of the set, the original black Hunts were, as usual going leaky.
I will post a picture next time of the set back in its nicely styled woodworm free cabinet.
David