Thanks Chris, and I hope it will help others in a similar position.
Cheers Frank, and the CRT is amazing....it is so bright and sharp, not obvious in my photo.
Posted by: PYE625
That would be great Mike, and I'm sure Chris would like a scan for the library ?
That might be difficult at the moment - I don't have access to my scanner. I might try and see if I can get one of the MFPs at work to do it. For the record, whilst there are circuits in there much of the alignment and other information directs you to the TV 125 manual.
As far as I can tell, the "L" means it came with (or could be fitted with) legs...
It is not all plain sailing as there was an additional problem with the overwind. I noticed a very slight fizzing noise and could see the surface of the fabric tape of the overwind, adjacent to the entry point of the start of the winding, was warm. Here at the point of entry where the black sleeving is....
After removal of the LOPT once again and the overwind, I unwound the outer layers of the fabric tape from the doughnut. I could not really see anything wrong other than rather close proximity of the soldered joint of the input terminal to the final outer winding on the coil going to the top cap of the EHT rectifier. I removed the remainder of the fabric tape, remade and insulated the input connection, then wrapped the overwind up in polythene tape in a similar fashion to the original fabric tape. This has now cured the additional leakage, that I presume was occurring from the start to the finish of the actual winding. The conclusion for me is that the tar-soaked fabric tape is a very poor insulator long term.
Here is the overwind doughnut with new polythene insulating tape re-fitted to the LOPT....
The conclusion for me is that the tar-soaked fabric tape is a very poor insulator long term.
I think it's fair to say that the boys and girls in Power Road didn't expect their tellies to last over 50 years...!
That's very true Mike ?
At work there is a section of the bitumen-paper and rubber insulated underground armoured three-phase cable that would occasionally blow the fuses in the feed to one area. That too was about fifty years old. It has been replaced!
Bitumen or tar must have a water content, hence the steam released when heated. Or it absorbs moisture over time.
It makes you wonder how many of these transformers were scrapped due to leakage rather than shorted turns.....
Quite a few I bet. Plus the fact this set has historical melting of the overwind, so it must have failed some years ago and then put away and forgotten.
Well things are now looking better with the set after a few adjustments, however contrast is uncontrollable plus sound and vision cannot be obtained simultaneously. Quite a few things to look at. (Not to mention vertical linearity ! )
(Camera black band, not the set.)
I would not have attempted a repair of this type, the LOPTX’s were freely available from Bush or other suppliers at a reasonable price, couldn’t risk a call back or even worse damage.
Now with no replacements available Andrews method is a fantastic way of keeping them working.
I don’t remember the 105/115/125/135 sets giving me many if any LOPTX problems, the later 141/161 etc did though, again replacement kept on the van.
The tuning wand in the VHF Tuner breaks, probably not the cause of the reception problems but worth keeping in mind.
Bitumen or tar must have a water content, hence the steam released when heated. Or it absorbs moisture over time.
Both of these statements are true, but it's the second one which causes all the problems.
Natural bitumen does have a residual water content, however melting the bitumen to the point where it is liquid enough to be poured easily necessarily involves heating it to above the boiling-point of water: hence, if the bitumen is kept hot long enough, the residual moisture is boiled off. In the days of bitumen-filled joint boxes in power cables, you could tell the conscientious crews by the length of time the cup was left on the burner before the bitumen was poured into the box. Much the same is true, by the way, of roofers - a tip there for anyone with a felted roof.
The story doesn't end there, though. Like most natural products, bitumen absorbs water slowly over time, to the detriment of its insulating properties. Thinking about our line-output transformer, with the set switched on, the transformer gets warm, and in most TV sets it also runs in the warm air from the line-output valve. Obviously, this helps to keep the transformer dry if the set is used regularly. After twenty or thirty years out of service... ?
With anything in the ground, the story is much worse. If you put the ducts too shallow, every man with a Kango or a JCB comes along and digs them up for you - not a scenario that usually ends all that well. Put them deep enough to avoid this sort of trouble and, in some locations, they will be below the water table for all or part of the year. The ducts inevitably fill with water under these conditions, with the end results as described by Cathovisor - and by the way, the problem is both worse and more spectacular at 11 or 33kV. It is possible to overcome the problem by pressurising the cable with dry nitrogen, but this imposes a maintenance overhead in that someone has to come along and change the nitrogen bottle every so often. Since the early 70s, cables have had synthetic sheaths which are moisture repellant. However, there is a lot of older stuff still under the ground.
I'm puzzled by the L.
The U simply means it is fitted with a UHF tuner. I only ever saw the TV125 but never a TV135 - presumably they were only sold in areas of the country that were still waiting for BBC2 to start. Our transmitter was in the Crystal Palace area so I doubt if any sets without the UHF tuner were sold.
However, I only remember the TV135U and the TV135RU - the 135U was the hybrid model as previously stated and the 135RU was like yours - the R stood for the 'Rental' but had a cheaper cabinet as well IIRC. Your's has the normal TV135 cabinet, by the look of it.
The L is puzzling, though because Bush had used the L suffix during the TV115 series run but cannot have the same meaning here.
The TV115 had a dual standard timebase but single standard IF strip. Towards the end of the run, these sets started to be shipped as TV115L and also had a dual standard IF strip. The TV125 series followed but as the were all fully dual standard with or without the UHF tuner, the L suffix was not used. (They were originally shipped without the UHF tuner but were later shipped as the TV125U as the launch date for BBC2 approached with an eight guinea price increase - the same price as the upgrade kit to add the UHF tuner to the TV125.)
The TV135RU was a different chassis to the TV135LU.
The TV135RU used all valves except for some later ones with a transistor UHF tuner, the IF strip and Frame/line osc/sound output were on PCB’s.
The TV135LU was essentially a TV125u with a Panarama CRT and the cabinet of the TV135U hybrid chassis.
What the L version means in jest I said the last of the A336 chassis (Used in the TV125), with you comment about the TV115L I wonder if it was RBM labelling for intermediate sets that used a mixture of the last version and the next version to build a TV. Perhaps to use up inventory or the rest of the new build not quite ready for shipping but they had to get sets out of the factory.
Probably never know.
This afternoon I removed the chassis and both tuners for a clean and service. Looking at the CRT, it has a bonded plastic mesh type of covering on the rear. You can just see the edges around the scan coils and EHT connector.
A47-18W, it’s one of the earlier Panarama CRTs , Mullard Data released 1963, the 11W was 1965 and the 26W 1967. The 11W and 18W required a mask to cover the metal reinforcement, the 26W was a push through, the reinforcement was reduced to not cover the edge of the screen.
I don’t remember seeing that yellow mesh on the two later types, info from.
https://frank.pocnet.net/sheetsA.html
Here is a better view of the LOPT overwind (I bet the Tungsram PL36 is a replacement, all others are Mullard), the VHF and UHF tuners, plus some more pictures of the chassis....
I wish now I had used black tape around the overwind, but I'm not pulling it all apart again just for that.
The TV135RU was a different chassis to the TV135LU.
The TV135RU used all valves except for some later ones with a transistor UHF tuner, the IF strip and Frame/line osc/sound output were on PCB’s.
The TV135LU was essentially a TV125u with a Panarama CRT and the cabinet of the TV135U hybrid chassis.
[...]
Probably never know.
Well, as suggested earlier - it may well indicate that it could be used with legs. I can see no other explanation peering through numerous manuals! As you rightly said, the 135LU is essentially a 125U - as denoted by the note appended to the manual I had, a short interim run before the TV 135 proper came out.
Bush have form for this: the much earlier "M" models from the round CRT days usually had a newer chassis in them that formed the prototype for the newer models - a TV12AM has the timebase chassis of a TV22, for example.
Fab work! Glad to see it went to a good home, It was given to me by a friend of mine who purely rescued it because someone threw it out. It sat in his shed for years, until I acquired it, It then sat in my workshop for ages...
Good to see a reasonably healthy picture, be good to see the finished results.
Cheers,
Jamie.
