CTV Philips G8 LOPT modification

Is there a way to tap into the line output before it gets to the winding on the LOPT? If so, then the microwave oven diode tripler I did on my Rediffusion Guildford could work. I tapped into the cap of the PL504 line output valve which gives out 7kV so it was just a matter of constructing a tripler to give 19kV (allowing for voltage drop of the diodes).

Original set-up:

Modified:

You'd have to add additional diodes and capacitors in order to match the requirements of the G8.

I navigated a similar path, with the G8's predecessor, namely the G6. My G6 came without a LOPT, I managed to track down a spare, which was likely a pull from a scrapped set. That LOPT ultimately ended up failing. Next I tried a PYE 697 LOPT, not only that but the superb and robust Weyrad version.

697 LOPT Sub

This LOPT performed beautifully and took a lot of abuse as I modified the cct, under the guidance of @jayceebee to get it to function. Almost got there but was thwarted by not being able to get a line lock, it was almost, but never quite fully locked. This turned out to be the 697 LOPT having too low -/+ 47V pulses, whereas the Philips LOPT was up in the hundreds.

CVC5 LOPT Sub

Another member, @ctc15, suggested the ITT CVC5 LOPT would be a perfect candidate, and it produced those all import high voltage pulses for lock. It just required mapping into the cct, which ctc15 supplied, coupled with some component modifications to make it work. I drew that all up on a diagram to ensure I made did not make any mistakes.

I managed to locate a CVC5 LOPT, which was a pull from a set that had not been stored in ideal conditions. This filled me with dread, but beggars can't be choosers, however, it did appear to clean up well.

Whilst in the process of adjusting a few resistors on test, to obtain a full height raster, there was a screech from the LOPT area. The isolation transformer started to hum, wanting to go into full Chernobyl, my Kilowatt meter, showed 426W and climbing, that LOPT also failed! I was not surprised considering it's state, nonetheless I was gutted. As you can see from the photo below, it was a working, the CVC5 was a perfect candidate for transplant. I was so close to success, and almost moving on to diagnosing the decoder faults.

At this point, when the LOPT failed, I had run out of spare LOPT's. Since then, I have managed to obtain, allegedly, a brand new Weyrad version of the CVC5 LOPT and a further two which came from sets that were stored well. All show as being good on the LOPT tester, one day soon, I hope I'll get back to the 48-page G6 thread and attempt to finish what I started.

I guess what I'm trying to say, and I hope I've conveyed with all this waffle (apologies), there's a lot of work involved. It's not just a case of finding a drop-in replacement. You're on a path that will test your perseverance, it will be a rollercoaster of a project with many frustrations along the way and no guarantee of success.

I can't offer any insight into your G8 substitution plans, other than I'm sure I read somewhere that the superb, reliable RBM A823 LOPT, might be a possible candidate. There are far better heads on here than mine, who may have some thoughts on that for you.

If you were ever to go the jellypot route then it would have to be the 8500/8800 or maybe even the DST from the 9800 although the latter was made by Philips and prone to failure, they run with a similar HT to the G8 of around 200V. it would be extremely difficult I would think to use the 3000 types due to the much lower HT of 60V.

Crustytv mentions the A823, I would also suggest the G.E.C. 2110 and PYE 725 may possibly be potential candidates but are these available in any greater numbers than the G8?. Because the G8 was known to be so unreliable there could be more of these NOS still around on shelves in collectors workshops than any others I've mentioned. I can't speak of the G.E.C. and PYE but the A823 and 8500 were never held in stock because they were so reliable and we only ordered them when needed. Just my thoughts and any substitution requires lots of trial and error, as can be seen in the G6 thread, some may turn out to be not feasible for may reasons.

@wayned 

I can't pretend to know how the G8 LOPT actually works in depth so would have to see what others think to that idea but seems like it could be the least wasteful option, using every last drop the LOPT has to offer!

@crustytv

I forgot about your G6 LOPT troubles! I followed the thread along as best as I could, was a real gut puncher every time a LOPT failed. Hopefully you'll get her up and going soon with one of your new LOPTs. I fear that the longer we're in this hobby the more common those types of occurrences will become unless we're careful. Luckily I don't have any real cause for concern at the moment with my LOPT but my recent scare made me think. I'll keep the A823's LOPT in mind too.. 

@jayceebee

I thought the same about more quantities of G8 LOPTs, but i was thinking of way into the future too when the last of the NOS G8s stocks are dwindling out, people hoarding onto them for themselves, will also keep those GEC and Pye LOPTs in mind too.. In an ideal world we'd never have to but thats all part of the fun.. 

Thanks all for the replies
'70