Pye B16T Tube Problem?

[Mikey405]

Hi All.

I have a problem with my Pye B16T - The picture starts off fine but then gets brighter and brighter, the only way to get the brightness down (apart from decreasing the signal into the set) is by turning the brightness control up to maximum and then decreasing it suddenly back to zero. Eventually the picture is so bright that it is unwatchable even with the brightness at minimum.

The voltage on the control grid varies nicely between 0 and 85-ish volts depending on brightness control setting, and on the screen grid it's about 310 volts-ish. When I adjust the brightness control, the DC voltage on the cathode starts off at about 65-ish volts with the brightness control at minimum and creeps up to about 85 volts on its own. (The AC hardly changes.) If I turn up the brightness control to maximum and then back to minimum, the picture (and voltage on the cathode) goes back to normal again before creeping up by itself again.

I can't see any obvious problems with the video amp and the input to the video amp is static. It may just be my lack of valve / tube knowledge but should the voltage on the cathode be so much influenced by the voltage on the control grid, or could there be leakage between the two electrodes in the tube? (I.e. a duff tube...)

Thanks all.

Kind regards.

From Mike.

[Mikey405]

Hmm. I just realised that with the brightness set to zero (i.e. the control grid earthed) the brightness still goes up and up, so perhaps my theory may be a bit dodgy... I'll check the DC level on the cathode when the brightness is way too high with the control grid at zero. (But just now I'm off to bed.)

[Heatercathodeshort]

It's very unusual for an MW22-7 to develop shorts/leaks but of course there is always one! I would go for the video amp. Monitoring the cathode voltage will be interesting. Possibly an H/K short but I would have thought this would have destroyed the picture definition. J.

[Danny]

Hi if you decrease the signal to the set and the brightness level changes then its not likely to be the tube. but ive seen this type of fault before and its always been a resistor ever in the brightness network or in the supply to the video amp. ive not got the diagram to hand so cant name any likely culprits but take care with this set. you may not find the fault by checking cold resistances as the resistore may be failing under load. Danny

[Mikey405]

Thanks John and Danny. I'll certainly do some checking when I get back from work this evening. I'm a little puzzled though as to why, when I turn the brightness right up and then back down again quickly, the picture goes back to normal again, especially as the brightness control is such a simple circuit with a potential divider between HT and earth feeding the control grid and not involving much else at all.

I guess my first port of call will be R25 to R28 on the top side of the chassis. I guess if the signal is just floating up and down (at perhaps the whim of one of those resistors) then it's going to behave a little bit unpredictably.

[Heatercathodeshort]

I think the term 'floating' is very near the possible cause of this problem. I have two D16T's one of which has not been turned on for a long time. I tried it last night and low and behold the brightness level rose over a period and could not be turned down! It became very bright before switch off and I noticed a hot wax smell coming from the top deck. Too busy to investigate at the moment but the two set faults could be connected. Turning the brightness control does not have the short term correcting effect as your receiver. Due to the infinite impedance of a CRT, even slight charges on the electrodes will sustain operation for a period. I have experienced some very obscure faults over the years with simple answers! J.

[dominicbeesley]

I had a similar problem recently and only tracked it down after spotting that the flyback blanking caps were going short when they got warm and had been soldered in at the wrong end of a resistor...just a thought

Dom

[Mikey405]

Thanks John and Dom again.

Well, the plot thickens... After doing a lot of checking and relevant-resistor replacing and getting nowhere at all with the problem I spoke to my mate Tone (a member of the forum) and after quite a few voltage measurements (with a test card being displayed), we decided to check voltages with the video amp valve (V6) removed and a 22K resistor between cathode and deck. On switch-on (and with the brightness control at zero [zero volts on G1]) a fairly dull blank raster appeared. After a few minutes the brightness started creeping up, and up, and up... (The voltage on the G2 remaining completely static at 310V and the voltage on G1 remaining static at 0V). But... with the picture getting brighter, the DC voltage on the cathode (with the video output valve removed and a 22K resistor shunting the cathode to deck) started going UP. The only way to get the voltage back down was to blast the brightness control to maximum and then set it back to zero again. After that the picture would normally start drifting up again (although inexplicably it would sometimes drift back down again very slowly).

We then, as an experiment, connected up the tube as a triode, so the G2 lead was disconnected, the output that went to G1 was disconnected and reconnected to G2, and then G1 and the cathode were strapped together. This is how the set is working now (with the video output valve back in place) and the picture is not varying in brightness at all, although it's an awful lot duller.

In my mind this still points to a duff tube, perhaps rubbish between the G1 and G2 or G1 and the cathode? Or perhaps the tube is gassy and acting like a thyratron, perhaps conduction is starting between G1 and G2 or the cathode and G2 and then because of the gas it is self-perpetuating and that is why it gets brighter and brighter. I'm not sure why then raising the volts on G1 and then suddenly reducing it is extinguishing the conduction, but then I'm not 100% sure of my theory either.

My question though is, why does the picture get brighter when the cathode is strapped to deck (well, via a 22K resistor), the G1 grounded, and the G2 steady at 310V with the cathode voltage RISING to 120+ volts above G1 with still a very bright picture?

Thanks everyone (especially Tone for his insight).

Kind regards.

From Mike.

[Boom]

Is it possible the tubes grid 1 is o/c?

[Mikey405]

Hi Dave.

Well the G1 control does work as expected - it's connected to the brightness control and that does work okay and the brightness varies with the voltage on the electrode, so I'm assuming that it is okay, but you never know - I'm not ruling anything out just now...

Thanks Dave.

Kind regards.

From Mike.

[PJL]

120V across 22K is 5.5mA...what is on the heater wrt chassis?

[murphyv310]

Hi.
Try and remove the CRT base and give the leads a good scrape, re-tin and refit the base, I think you may get a cure!

Give everything inside the base a good clean with solvent too and also around the end of the CRT where the wires enter the glass.
If that doesn't help you must have an internal leak in the CRT.

[brianc]

Hi Trevor
The MW22/7 tube has a B8G base which has pins fused into the glass so it isn't possible to remove the base.

[murphyv310]

Hi Brian.
Ah now I know what CRT I have in the loft!!! You can have it if you want.

[Mikey405]

Hi Trevor and PJL and Brian.

I have changed the tube base (socket) already, but I will certainly clean around the pins of the tube - I guess it could well be tracking from pin to pin. As soon as I get back to Solihull I'll give it a shot. I'll also check what's on the heater pins, although one side is earthed so I guess I'm not going to find anything too sinister.

I think that, if the pins are nice and clean and there is no tracking, I'm going to have to find a new tube, or run this one as a triode, but it's pretty dim like that.

Thanks Trevor and PJL and Brian.

From Mike.

[brianc]

Originally Posted by murphyv310

Hi Brian.
Ah now I know what CRT I have in the loft!!! You can have it if you want.

That would be lovely Trevor. I have 5 sets which use that tube! However, I think that transport could be a problem.

[murphyv310]

Hi Brian
PM sent

[Mikey405]

Well, fitting the tube that I had ready for my D16T kind-of proved that the first tube was a dud, but it also proved that my D16T tube was also a dud. The picture (which started off as okay, if not particularly bright) faded away, getting darker and darker before my very eyes and now looks like it has zero emission. But for the 10 minutes-ish that it was working the brightness didn't vary at all.

As an aside, something else weird happens to this set when I turn it on. Sometimes at turn-on I can see a brief flash occuring in the UU8 and / or the HVR2 EHT rectifier valve. Usually that is it, but very occasionally there will be a loud hum and the fuse will blow. I'll do some checking but it could conceivably be anything shorting the HT in the set, the UU8 itself, or perhaps the transformer insulation. Last night though there were fireworks like you've never seen (or heard). The most fantastic arcing noise was heard and the HVR2 lit up like a Christmas tree before the fuse blew. I could not see where the "audible" arcing was coming from but it happened again at 2 seemingly random intervals, although I managed to turn the set off quickly each time.

The ferocity of the arcing just goes to show how absurdly dangerous that EHT connector would be to anyone touching it...

[Heatercathodeshort]

The transformer used in the D16T is incredibly reliable but always looks as if it has shorted and bubbled out most of it's wax. The arcing could be due to a shorting EHT smoothing can, the .1uf 7kv Visconol. The Dubilier with the Bakelite top is much more reliable. The HVR2 valve holder is another possibility but it is mounted on stilts and rarely breaks down. The EHT lead may have failed with a pin hole or touched something hot melting the Polythene insulation. Finally the heater winding on the mains transformer, HVR2 may have broken down but if this is the case it will be a first for me.
This scene brought back happy memories for me. As a lad I obtained a D16T from the scout jumble sale for 1/- [5p] and proudly carried/wheeled it home. [Not another telly...Well I suppose it's a little one. The dust was yet to settle on the 15" HMV brought home the previous Saturday..] I plugged it in and got a blue flash from the GX32used in my version blowing the house fuses. I think I replaced it with the only octal rectifier I had probably a 5Z4G. The set worked! Very well in fact but the screen was dirty and I decided to dismantle the set and give it a clean...During the process unnoticed by me some sort of spring clip had fallen from the cabinet front and jammed in the HVR2 valve holder. On applying the mains there was a sinister hum followed by the loudest arcing I have ever heard even to this day! The house fuses blew again to the horror of my parents that were watching the Ferguson 236T in the living room.
It took me a while to sort out what had happened and remove the offending object. The set then worked brilliantly even with the straining 5Z4G but it taught me a lesson of observation! I must have been 12 or 13 at the time but even at that age knew the dangers of mains EHT having read all the articles in Practical Television magazines until the pages fell apart.
Note! The EHT on the B/D16T is around 6kv and LETHAL. It has tremendous power and just blows anything in it's path! The transformer appears to survive horrendous overloads. The UU8 H.T. rectifier being a Mazda does tend to suffer from internal shorts and flash over but nothing like as bad as the AC/DC version the U801..
Hope you get it sorted, Regards, John.

[Mikey405]

Hi John.

Well, I've been through and made sure any HV leads (including heaters for the HVR2) are away from anything metal and I've cleaned up everything HV I could see. I'm still a little unsure of where the arcing was coming from but it seems to be behaving itself now, and none the worse for it's little hissy-fits over the last few days.

Apart from this tube problem the set is working fantastically well, and even looks resplendent with its new front slider controls that you provided some time back, and which I'm still over the moon with. And Trevor has very kindly offered a tube, so it might even be completely tip-top sometime soon. All I need are the rear sliders which somebody has not-very-subtley drilled out and splintered all the bakelite.

The arc you got from your HMV sounds pretty frightening, and similar to this monster - it nearly made me jump out of my skin the first time it happened. The only louder arcing I've ever heard is from a mate's monster Tesla coil, and that's deafening.

Thanks John.

From Mike.
PS. A B16T for 5p - Good grief.