KB KF40 picture ballooning/faulty LOPT ?

[RT 1006]

I’m in the process of restoring a 1953 KB KF40 console TV set which I acquired recently from a local auction house.

The set has clearly been stored in very damp conditions for a very long time as the outside of the cabinet is scruffy and the chassis is not much better, but for me that is all part of the challenge and the satisfaction that will be gained from (hopefully) returning it to fully working condition with a completely restored cabinet.

I have replaced all wax paper caps, all Hunts mould seals, replaced the mains smoothers and reformed the remainder of the other electrolytics. A bucket load of out of tolerance resistors have also been replaced.

I now have a very good picture via my Aurora with good height, width and linearity. Unfortunately if I increase the brightness the picture balloons severely. Also, soon after switch on the picture begins to balloon and will eventually fade away after approx 20mins. All classic symptoms of falling EHT and this is indeed the case as I can watch it fall steadily on the meter from around 6kv to below 2kv as the raster disappears. Line pulses to the line output valve have been checked and remain at constant amplitude.

Having read a number of threads I understand this could be a result of a faulty LOPT, however after 20 minutes and after the raster has faded the EHT overwind feels only just above ambient temperature. If it was faulty and had shorted turns would it not be much hotter? Also, after 20 minutes use the DC resistance of the overwind increases from 585Ώ to 630Ώ, I’m not sure if this is abnormal or if it is just down to the slight increase in temperature of the windings rather than shorted turns as I would expect shorted turns to reduce the resistance?

I also understand that the fault could be as a result of damp in the overwind and given that the set has probably spent many years lying in somewhere like a damp cellar or shed this may not have helped. Can anyone confirm if damp would cause these symptoms? If this is the case is removing the LOPT from the set and sitting it on a radiator for a few days likely to cure things?

I have spent a good few hours on this set and would be most grateful for any thoughts or suggestions anyone can offer that would help see it fully operational. Many thanks in advance.

[Boater Sam]

Being optimistic I would think the transformer is probably OK if it has worked for 20 minutes OK. I would expect it to show signs of distress.
More likely it could be the eht rectifier losing emision or heater volts, or the screen or cathode supplies to the tube. I am not familiar with the circuit but would explore other possibilities than a hasty decision on the transformer.
Chech the tube outer earthing too.

[mark pirate]

I have had some success with removing the LOPT and giving it an hour in the oven at the lowest setting, I am pretty sure that your LOPT is showing the classic symptoms of trapped moisture.

Another method is to feed DC voltage from a bench PSU through the windings.
Good luck with it.

Mark

[Peter.N.]

If the LOPT has got damp which I suspect it has, it will need a long period of gently drying out, in an airing cupboard or similar. As the set warms up any moisture in the EHT winding will evaporate and permeate the whole winding.

Peter

[FERNSEH]

Why not try passing a small current, say 20mA though the overwind? This has been suggested in other topics in the Forum about reviving old line output transformers.

DFWB.

[Top Cap]

It certainly looks like a classic low emission EHT rectifier fault, not sure where you would get a R12 from but maybe the more common EY51 can be found. As pointed out in an earlier reply, check the glow of the R12's heater. Does it reduce in brilliance as the blooming occurs? If not then I would suspect the R12 otherwise it would point to losses in the over-wind and a period of damp removal may cure it. There is an over-wind feed resistor R113 (15k) that might be worth a check. CAUTION: Don't forget to discharge the EHT from the tube final anode before bringing the soldering iron near to the R12 connections. In days of old, forgetting to do this would blow up the otherwise trusty Henley Solon soldering Iron, not sure what it may do to the more modern irons. When making the solder connections to the R12, leave nice rounded solder caps with no solder spikes as this may encourage arcing or corona.

[Mikey405]

Hi RT.

I think I'd be inclined to temporarily solder some high voltage diode like a 2CL77 (available virtually anywhere - especially eBay) from the overwind to the tube final anode cap. This will at least rule out the EHT rectifier valve.

Thanks RT.

Kind regards.

From Mike.
PS. If that works then I think I'd just solder the diode in neatly and keep it there. Not for the purists of course...

[RT 1006]

Thank you to everyone above for all your responses, these are all very much appreciated.

The R12 EHT rectifier was my first suspicion, I replaced it with a spare EY51 I have but the symptoms remained the same i.e the picture gradually increases in size and fades away as the EHT decreases, at the same time the rectifier heater steadily fades away. There's no saying the replacement EY51 is good so I like Mikey 405's suggestion of replacing it with a HV silicone diode and will get one ordered up.

The EHT also seems low at around 6kv as measured with my HV probe and displayed on a DMM, the service info for the set and data sheet for the tube both specify a final anode voltage of 12kv, there's no saying my probe is measuring accurately though.

Thank you to Top Cap for mentioning the EHT overwind feed resistor (R113 15K), I replaced the original resistor earlier on in the resto as it had gone high in value but I only replaced it with a standard half watt carbon film resistor. I'm just wondering if it should be replaced with a high voltage component given the high peak pulses it will be subjected to it may be breaking down under load?

I replaced the 6CD6G line output valve V18 with a NOS one but the symptoms are the same. I'm wondering whether to source a replacement 6U4GT reclaim rectifier V21 just to rule it out but not sure if this valve would be responsible for these symptoms.

I've also been looking at the tube earthing as suggested by Boater Sam. I had cleaned up both spring clips from chassis to tube as they were quite rusty but I've also bridged a wire across onto the tube outer surface just for good measure.

Thank you again for all the replies. If I can figure out how to do so I'll get some pics of the set in question posted up.

[Heatercathodeshort]

Did you say you were only getting 6kv from cold? If so this is about half of what it should be.
Check the main HT rail. The metal rectifier could be going high resistance at it warms up.
John.

[RT 1006]

Yes 6kv from cold, that's a good point John, I'm going to go out and check the HT rail now. Thank you, Kevin

[RT 1006]

A quick update, the main HT rail remains steady at around 265v which is pretty much in line with the service data.

I've just ordered up some 2CL77 HV diodes to rule in/out the EHT rectifier valve.

I've also ordered a replacement 6U4GT to rule in/out the reclaim rectifier as this means all 3 valves within the line out stage will have been exchanged.

Depending on the outcome of the above and if symptoms remain the same I will then investigate the low EHT and then take steps to remedy moisture in the LOPT.

Still hoping to get some pics up when I get a chance and will post updates on the above.

[RT 1006]

An update to progress over the holidays so far,

I soldered in a 2CL77 in place of the R12/EY51 EHT rectifier and that has solved the picture blooming and EHT fade out to a certain degree so it looks like the original R12 and my spare EY51 have both been low emission.

EHT now remains relatively stable but it only sits around 6.5kv which is about half of what it should be. I'm confident my EHT probe is reading accurately as I have checked it against my Philips 19TG142A and get a good 14kv reading. I've replaced the 6CD6G horizontal O/P valve and the 6U4GT reclaim rectifier but symptoms remain the same. I have no way of testing the two valves so can't be entirely sure they too are good but their overall boxed condition suggests they are probably OK.

The set displays a good albeit reduced size picture, but after 5 - 10 minutes there is a lot of picture jitter followed about 5 mins later by loss of line sync. adjusting the line hold control will lock the picture before it starts to drift off again.

My plan is to concentrate on remedying the EHT before looking at the sync problem. All the caps and a number of resistors in the line out stage have been replaced so I think I now need to look at the line pulses coming from the horizontal generator unless anyone can suggest something I have overlooked? All suggestions or comments gratefully received.

I'm now going to try and upload some pics.

[Top Cap]

RT, your EHT valve rectifiers are probably OK. If the EHT is down then the heater over-wind will not be giving enough heater voltage. When you get the EHT up to its proper level you may find the valve rectifiers are working OK.
Les

[RT 1006]

Thanks for that Les, that's a good point.

I've been checking around the line output stage again, in particular I've been looking at the LOPTx. The primary winding measures 105Ώ and the secondary/EHT overwind measures around 585Ώ. Though I have no service info for this specific model service data for other very similar KB chassis states LOPTx resistance of 50Ώ and 300Ώ respectively, which is roughly half of my readings. I’m thinking this would tie in with my low EHT which is half of what it should be. What I don’t understand is how the LOPTx would go high resistance if faulty as I’d have thought it more likely to go low resistance with shorted turn etc? There are no inter-winding shorts or shorts to chassis. It could be that the service data is wrong but does anyone else have any other ideas or explanations that could help me get to the bottom of this?

[Top Cap]

Well it could be damp in the EHT winding which has already been suggested, a shorted turn would I think make the EHT drop to a much lower figure. Check out the boost diode 6U4GT as this could make a low EHT if insufficient energy is being reclaimed. Likewise check the capacitors around that circuit like C81 (0.08uF) and C79 (0.06uF), probably around 750V to 1kV DC working.
Others I would check are the decouplers C72 (16uF), C74 (0.1uF) and the line coupler C68 (0.05uF). Apart from the obvious cathode resistor like R107 (390 Ohms) I always look for divider chains like that feeding the screen grid of the 6CD6, in this case R111 (5k) and R08 (15k). Anyone who worked on the GEC mono chassis with the EH90 will know exactly what I mean

[G8KBG Tony]

Weak line drive could be the problem. Have you changed the 12AU7/ECC82 line oscillator valve?

Tony.

[Hunts smoothing bomb]

In post #1 it was stated that line drive remains constant.

My money is still on damp in the lopt.

Remember, it has been said that the EHT steadily falls off over 20 mins and also it has been observed that the heater of the EHT rectifier follows suit and steadily extinguishes.

Moisture in the lopt will dampen the EHT output rather like a falling resistance connected across it as it warms up, this will then also kill off the volts to the heater of the EHT rectifier as it is fed too from a winding on the lopt.

Remove the lopt and stick it in the airing cupboard or boiler cupbard for a couple of weeks and then refit and see what happens.

Cheers

[G8KBG Tony]

Hi Lee,

Yes, I agree that damp is the most likely culprit, but I just wondered if the oscillator valve had been changed. It's a quick and easy check to make before condemning the LOPT.

RT 1006 stated that the drive had been checked and was constant, but what did he check it against? There are no waveforms in any of the service details I have and as such, the drive could be 'constantly' low.

Best wishes,

Tony

[John Caswell]

The fact that the picture is small all round would point low HT, and I see that there is a selenium rectifier fitted. These were notorious for failing.
Might be worth looking there.

John

[Top Cap]

I think we are all saying that other checks are worth doing before lopt. In the old days the cost of a new lopt would condemn the set as the man in the street regarded it better to spend the money towards a new TV. As a consequence we engineers of the day would spend every effort to make sure there were no other factors and HT rectifiers, line drive etc would all be verified before the dreaded dead lopt was announced to the customer. Saying that, I would expect the selenium rectifier to be stinking like a cat mess☺.
So check everything out that you can and then if nothing else is found then a lopt dry out would be the next try.