Oh no - another V4!

[KeithsTV]

At work we have a small vacuum oven and a colleagues TV125 LOPT spent a week in there at 50C in a vaccum. On refitting no fade out of the picture.

The dessicator approach would undoubtedly work but you'd need to check the dessicant regularly as it will eventually saturate.

Keith

[murphyv310]

Hi PJL
Of course the dampness effects the inductance of the tranny, but what I dont really understand why it changes when warm, is the molecules of the water moving in sympathy with the line frequency and turning to vapour (steam) but are trapped in the pitch/varnish etc of the transformer, is the main problem in the overwind or the primary, snag is these faults were rare at the time the sets were in common use.
I've even sealed one of these tranny's in a bag with silica gel and the saturation remains so far heat only works for me, perhaps a dessicator would work perhaps someone could try it!

[dominicbeesley]

Thanks lads,

The thing that got me was that in the TV24 where I had this problem the EHT dropped but not the scan drive so the picture got bigger. Incidentally even after two years with the back over the radiator this isn't 100% cured it's now gone from about 20mins useable picture to about 2 hours but still fades out.

I was going to remove the LOPT anyway. The chassis is very rusty at the back so I was intending to clean this up and so will try out an oven trick on this. I just wanted to get the set as near to 100% reliable before I started removing bits.

I've tried all the R's that Steve P mentions above but not the low value caps. I'm still not sure how best to check these will a blast from a megger be a good test?

Dom

[dominicbeesley]

Sorry I crossed over with the dessicator bit. I have a friend who has a desicattor for making dried foods - no idea how it works I wonder if it would fit and whether she'd let me!

One thing I was musing on - I doubt it is this - but what would be the effect of a break down of the insulation in the scan coils - would it have a similar effect?

D

[PJL]

I don't think there is much going on in the scan coils. I would guess you can short them out if you don't mind watching a straight line, the line output would still function fine and generate EHT and boost.

I have no idea on the heat either, as said, it would increase the humidity of the trapped air. Not sure how this would impact winding capacitance or if we could be looking at localised corona discharge.

[dominicbeesley]

Quick Q - what *should* the line drive control do? What should I be measuring to check it is working?

Cheers

Dom

[Heatercathodeshort]

Hello Dom,
I safe way of adjusting the line drive on any old 405 t.v. is to screw the trimmer in until you get a 'kink' in the centre of the picture. Unscrew until it just disappears and you should be OK. Too little drive will damage the PL81 and may slightly over heat the LOPT. Hope this helps. J.

[dominicbeesley]

Ah this definitely looks to not work then - it doesn't do anything like that screwed in all the way or all the way out - will try and actually make it into the workshop this afternoon!

Dom

[Heatercathodeshort]

Interesting Dom,
I expect you have already checked/replaced it but R117 100k and R120 150K are worth a check. These are in the anode and screen supply to the line oscillator and may have gone high reducing the drive. [Pye V4 circuit] An O/C R125 470k [grid leak] can give some odd symptoms including a 'wasp waisted' effect on the picture shape. Just a thought. J.

[dominicbeesley]

Thanks John,

I've checked and rechecked all caps and resistors in this area C99 and C102 checked out ok on the component bridge and I couldn't measure any leakage at all when I connected them up in series witha 22M resistor and a 250vDC supply.

However C101 was still not doing anything so in desperation I replaced it and now I can get something to happen but not what you described earlier.

If I screw it right down the picture narrows and darkens - but no kink. If I screw it out I get a wider brighter picture but no kink. So not sure where to set it - I just left it at the point where waveform [16] is roughly 80v - that gives a fairly narrow picture though.

BTW What is the kink due to? Is it something to do with the core saturating because of too much standing current?

I'll have another look when I'm feeling a bit better - it looks like there might have been some bodging in this area as the line-hold preset is mounted on a funny little bracket due to it not having the correct thread on the back! So all might not be as it seems.

Dom

[PJL]

I think it limits the amplitude of the peak in the waveform on the grid and therefore the -ve grid bias on the PL81 created from rectification on the grid. Why not have a look on a scope?

Too little -ve bias and the PL81 might not turn off I guess? Too much and it might turn off prematurely?

[dominicbeesley]

Well on the scope the control just alters the size of the flyback spike at the grid by the looks of things - theres a sharp 400v negative going spike at the other end of the 27p capacitor.

My question however is more to do with the "kink" that John describes above I'd like to know what causes the kink - I doubt that a kink will show up on the grid waveform - but is more something to do with the tranny saturating or passing through a hysterysis point or one of the many things that I can never quite get my head round....however it's not doing it!

Dom

[PJL]

Hi Dom, Doesn't it change the bias?

[murphyv310]

Hi Dom.
Have you replaced the G1 coupling cap (0.01uf) to the PL81 and 470k grid leak? Normally you would get a "kink" or a vertical line about one third of the way in from the LHS of the screen when you adjust the trimmer, the other way is to set it up for minimum cathode current in the PL81.

[DangerMan]

The line drive trimmer adjusts the point in the scan when the line output valve turns on.
For example, if the line o/p valve turns on too late there will be a visible slowing down of the scan before it gets going again. This slow down causes the bright vertical line, where the scan is compressed; to the left of it the boost diode is conducting and to the right the line o/p valve is conducting.
Pete

[FRANK.C]

Hi Dom
For what it is worth, in the Ultra YA72 (I have still to finish it) that had a lot of trouble with LOPT saturation, is was not possible to get the kink. Eventually to cure the saturation problem I replaced the overwind and now the kink is easily obtained.

Frank

[PJL]

Thanks DangerMan, the PL81 is off, the current is ramping down into the boost cap, the line drive trimmer changes the slope of the leading edge of the line pulse so the PL81 is turned on at the exact time the current reaches 0 at mid-screen.

[dominicbeesley]

Hi all thanks for the replies

PJL - yes it does change the bias, well it changes the depth of the waveform at the grid and so the avarage bias changes.
Hi Trevor, yes, I've done all them and rechecked them just now plus tried a few different PL81s and ECL80s
DM - thanks - that makes sense I think though doesn't seem to be what is happening here
Frank - looks like I ought to tackle the LOPT first then.

Anyway here's some pics of where I've got to after a couple of panics when the EHT cable popped off and then the LOPT can didn't go back on properly with lots of arcing - and head scratching!

             In     Out
Boost        322     379
K            6.65    6.2
G            -9      -18
S            171     174

These done with a DMM as my AVO8 is playing up!

Screens show with line trimmer all the way out and all the way in, the traces show the PL81 screen current and PL81 grid drive. The picture at switch on is almost full width - very frustrating!

I'm now beginning to wonder if there is TOO MUCH capacitance at C101 (I replaced 101 because it measured too low). As an experiment, before dismantling the LOPT for cleaning up and drying out I'll try running with a 100p for C99?

Thanks again for the help will be back after the weekend!

Dom

[dominicbeesley]

Hello all,

Had a quick last look at the circuitry before removing the LOPT. With a 100pF cap for C99 (Tader C50) I still couldn't get a kink - but I think I did get higher EHT - well there was an arcing sound and sparkles on the picture with the trimmer all the way out!

Anyway I've removed the LOPT and made notes and pics of how to put it all back!

Questions arise though.

First, there's a perspex top to the LOPT holding the rings that go round the EY51 will that not distort if I cook it in the ovem

Second, Am I likely to completely knacker the LOPT like Chris if I cook this in the oven at say 57 degrees?

Third, how difficult is a rewind - does it require special equipment? - I'd be up for the challenge of a rebuild if all else fails!

Dom

[FRANK.C]

Hi Dom
I baked my transformer at over 100 C but there was no perspex in it.

The overwind is usually wave wound which is difficult to do. Normal layered winding wont do as the self capacity of the winding is too high. I got around that problem by winding two coils and connecting them with a diode. See this thread.

Frank