HMV 2638- Another 960

It’s a nice bright contrasty picture.

There appears to be some slight ringing on the vertical edges and the white vertical lines are slightly displaced where the meet the horizontal ones. I mention it because that is not there on a previous photo, could be a different set.

Is it an artefact of the photo and digital manipulation on the web site or is the effect on the TV?

It would never be noticed when watching a programme, it’s an excellent result.

Posted by: MurphyV310

Brilliant results.

A couple of questions...... What colour is the LOPTY in the 960 and is it a single diode or multiplier?

Hi Trevor,

The set uses the same red Jellypot as fitted to later 900 and 950 chassis sets with a 3- stick half wave rectifier tray.

Apart from the power supply in both variants of the 960 the set is otherwise electrically identical to the standard 950 chassis.

Cheers

Brian

With regards to Thorn "Jelly-pots", members might also find my blog article of some use, see here

Posted by: Nuvistor

It’s a nice bright contrasty picture.

There appears to be some slight ringing on the vertical edges and the white vertical lines are slightly displaced where the meet the horizontal ones. I mention it because that is not there on a previous photo, could be a different set.

Is it an artefact of the photo and digital manipulation on the web site or is the effect on the TV?

It would never be noticed when watching a programme, it’s an excellent result.

Indeed there is. I would never have spotted it if you didn't mention it. The tuning could have something to do with it too of course.

Chris- That BRC Jellypot guide is most interesting. I guess 13kv for the 1400 is a misprint though?

With a 1400 Lopt fitted to the other 960 set the EHT is notably lower than with the original red type despite modifying the tray to a doubler. 

Cheers

Brian

The original Red jelly pot was alleged to be more unreliable due to having the 17kv overwind, these were then modified to a grey type with a lower output over wind and multiplier. Actually the red ones are better for using as a substitute in older sets like the Bush TV22 & other small screen sets.

Anyone know why the Jelly pot LOPTX, were more reliable than other types. The RBM pitch type in the TV161 range and later were poor, they changed the type to a different encapsulation but those were not a lot better. The Pye sets had started to use Philips LOPTX by the second half of the 1960’s and they were quite reliable, I replaced very few but comments on various forums suggest the Jelly pot just carried in working.

What were the construction difference that gave them the reliability and did Line output circuit itself have any major bearing on the transformer performance. I presume it was not just the potting agent used, perhaps the way it was wound.

We were not Thorn agents so sold/rented few of there sets or saw many for repair. 

Hi.

The jelly was a brilliant insulator, it also was an effective seal from moisture and of course the use of an overwind that didn't deliver high EHT was less likely to break down. The construction of the core was also highly efficient and very reliable, a tour de force by Thorn.

Posted by: MurphyV310

The original Red jelly pot was alleged to be more unreliable due to having the 17kv overwind, these were then modified to a grey type with a lower output over wind and multiplier. Actually the red ones are better for using as a substitute in older sets like the Bush TV22 & other small screen sets.

I've heard that too. This could explain why the resultant EHT on the 2633 set is lower with the 1400 Lopt and doubler fitted. It's also much quieter in 405 timebase whistle wise.

I wasn't aware the later grey ones were modified and would require a multipler. Not seen this documented in any servicing articles which would appear to suggest the original red type didn't fail that often.

John Wakeley said failure of the red Jellypot was quite common however.

I repaired most of the all valve and hybrid Thorn BW sets except perhaps the very early ones but not in large numbers, just the occasional one. They were always easy to repair even without knowing their stock faults. 

Thanks for the information on the Jelly pots, can’t remember much about Thorn LOPTX’s, don’t remember changing any.

They were certainly service friendly in the main.

Worth pointing out the earlier 900-950 series had component numbers on the component side of the PCB only. The 950mkII had them on the solder side too, the tradition continuing with the 1400 chassis onwards.

Hi.

One of the things I remember was the red lopt would fail if the EHT rose due to a feedback fault in the line output stage. The later ones with multiplier were not damaged by this problem. John Wakely is right they did fail but no where near to the extent of other manufacturers line output transformers.

Hi Trevor,

There appeared to be some insulation breakdown in the original red Lopt when the set was switched to 405-lines. Everything seemed fine when viewing 625 as long as you didn't change standards while the set was working. This was puzzling as the boost voltage is higher on 625!

Hi.

Thanks for the info. I wonder if the system switching could have been the problem? When switching between standards it is possible to have fast transients that could cause a flashover, again it was a sensible idea to de-rate the overwind and let a multiplier supply the CRT with its final anode voltage. So most likely these LOPT would have been OK in a single standard set of either 405 or 625 lines.

Hi Trevor,

The set had been operating for a couple of hours on 405-lines before the raster collapsed initially to a 6" square with much fizzing and spitting. The usual suspect components were checked to no avail. Further investigation revealed the spitting came from the Lopt. The sparking could be seen coming from the EHT tray connector point with the tray removed. 

I switched to 625, switched the set on with the camera set to video any arcing from the Lopt. All seemed quiet, hmmm... Fit the EHT tray and the set worked perfectly on 625-lines! No problems until switching back to 405 when the line collapsed with arcing from the Lopt as before.

Definitely a fault with the Lopt though as fitting a 1400 one solved the problem.

But it has left me wondering if the set was used on 625-lines only would the problem have showed up? 

I didn't throw it out incidentally. You're welcome to have it if any use.

Cheers

Brian

Hi Brian.

Thanks for the info and yes I'd take the lopt from you if that is OK. I could press it into service on one of my projects that run the LOPT in starvation mode. I was actually thinking of one in my MW6-2 project which only needs 6-7Kv

I'll keep in touch ?