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Bush TV86C Possible LOPT fault

 
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Bush TV86C Possible LOPT fault

Post by channel405 » Wed May 04, 2016 12:34 am

I would like to tap into the experience and expertise here before I condemn my Bush TV86C LOPT.

I was watching the other night when the picture went dim and faded away. At the same time I noticed the noise from the line output stage changed- line whistle was still present and same pitch but the gentle rustling noise from the LOPT area disappeared with the picture.

On further investigation I noticed that the EHT rectifier heater winding around the transformer was glowing slightly and I could hear a slight arching. I did a temporary repair to the break (I know the entire length should and will be replaced). The EHT output to the final anode also came came off during this repair so soldered it back to where I thought it should be. On trying the set I got no picture despite the heater glowing on the EY86.

Thinking I must have soldered the EHT output on the wrong pin I checked the diagram which indicates it connected to one of the heater pins. Still no picture. So I tried the other heater pin on the EY86 base, still nothing. There was arching from the EY86 top cap to my screwdriver so seemed to be life coming from the LOPT overwind. By now I was starting to suspect some other fault.

On further looking I noticed R73 on the top of the LOPT was getting hot and discolouring. Checks on the CRT base revealed the first anode and focus was down from 360v to less than 200v.

I've checked caps and resistors (set has been recapped and many resistors replaced) and disconnected the frame stage (R104) but no difference. However, when I disconnect the EY86 top cap, R73 runs cool, volts are back to 380v on CRT base and I get much bigger/healthier arcs from the line output valves. Something is dragging the LO stage down. Would a wrongly connected EHT connector do this? I'm wondering if the EHT rectifier heater winding issue was a coincident with the main fault of shorted turns in the LOPT. It's always been a bit noisy as in rustling, maybe this was an indicator but I did have it rewound a year ago and it's worked up to now.

So opinions please!

 
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Re: Bush TV86C Possible LOPT fault

Post by nuvistor » Wed May 04, 2016 8:41 am

So with the EY86 valve TC removed the rest of the stage seems to run ok.
Some possibilities,
EY86
Heater winding still faulty
S/C or leak on the DC side of the rectifier, CRT, wiring etc including heater winding.

About all I can think of for now.
Frank

 
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Re: Bush TV86C Possible LOPT fault

Post by slidertogrid » Wed May 04, 2016 9:54 am

You could also try temporally fitting a EHT stick rectifier to the rectifier valve top cap and removing the valve base and heater winding . That would at least prove if the lopt is OK and like Frank says that the tube isn't short. I have never had a CRT go final anode short or leaky but I have heard it does happen. possibly final anode to A1 short?
I hope you sort it and it's not the LOPT....
:thumb
Rich.

 
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Re: Bush TV86C Possible LOPT fault

Post by Rebel Rafter » Wed May 04, 2016 1:07 pm

Hi from RR. I would guess that if you got a good arc from your line o/p top caps and also from your EHT rectifier's top cap connector then your line o/p transformer should be fine, it's usually the case that if the transformer has shorted turns, which is the most common line o/p transformer failure, then there wouldn't be any signs of life at all in the line o/p stage. It's usually the case that when a line o/p transformer fails then there's no sign of arcing at all and the line o/p valve's anode overheats like crazy because of the extra load on it. It seems to me that your transformer should be fine especially as it is still powering the EY86 heater. One thing you can try, and be VERY careful and make sure it's THOROUGHLY insulated before switching back on is to disconnect the EHT cavity connector from your tube but leave everything else connected and then see what happens, you won't get a picture but you should get some sign of life at your line o/p stage, i.e. plenty of arcing to your screwdriver if it's ok. The down side of this of course is if the line o/p stage does then come to life is that there must be a short in your tube, although I've never had a fault of this kind in any tube I've dealt with, but then I haven't done much work on such an old set. The usual tube failures are things like low emission, heater- cathode shorts or leakage, shorted or partially shorted heater, or open circuit heater, or shorts or leakage between the other lower voltage electrodes, or loss of vacuum which is usually indicated by violent arcing within the tube. I would check to see if there is any high voltage capacitor connected across your EHT o/p which might've shorted. There is also the remote possibility of a short within the EY86. Also from what you've described it looks like your EY86 heater winding might've suffered insulation breakdown between it and another winding. These heater windings are usually made from EHT type cable and are usually fairly straightforward to replace unless it's encapsulated. You need to try and remove it from the transformer and check for any pinholes in the insulation. Also if you do this you need to count the turns. RR.

 
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Re: Bush TV86C Possible LOPT fault

Post by crustytv » Wed May 04, 2016 3:44 pm

Here's something to check.

If there's an overload on the D.C. side of the rectifier it might be possible with the lights out to see if there is a blue glow in the rectifier, is there?

If there is a blue glow this is due to one of two things, the heater winding is leaking to the core of the LOPT or the CRT could have an internal short. To eliminate the CRT remove the final anode, if the blue glow disappears your CRT is the fault. If the blue glow remains then you need to replace the Heater winding.

 
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Re: Bush TV86C Possible LOPT fault

Post by nuvistor » Wed May 04, 2016 8:07 pm

Slight possibility, I suppose that if the CRT is conducting very heavily due to wrong grid/cathode voltage it could pull the EHT down.
I would check other suggestions first though seeing that the fault started as arcing on the heater winding.

Frank

 
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Re: Bush TV86C Possible LOPT fault

Post by Katie Bush » Wed May 04, 2016 9:17 pm

Just a cheap cheerful input, but if there's plenty of unrectified EHT at the EY86 top cap, and nothing from the EHT output lead (to CRT) - has the rectifier gone to air? Or at least, gone soft?

 
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Re: Bush TV86C Possible LOPT fault

Post by Rebel Rafter » Wed May 04, 2016 10:28 pm

Hi, folks, RR here again. Surely if the EY86 had gone to air it's heater wouldn't glow, still it might glow slightly if it's got a slow puncture, a bit like those I get in my inflatable boat sometimes... But I still think it's the heater winding, as it's an Ey86 it's 6.3 volt so there'll be several turns unlike the Dy8x which is only 1.4 volt so usually only have one turn so I suppose with more turns of cable there's more of a risk of breakdown of insulation. Is there a strong smell of ozone? you usually get that with leaky EHT, the old GEC 2110's were good at that. RR.

 
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Re: Bush TV86C Possible LOPT fault

Post by channel405 » Wed May 04, 2016 11:33 pm

Thanks all for your suggestions.

I'm going to be away with work now for the next couple of days so please excuse the lack of updates, will report back with my findings once returned and looked at the set in view of your replies.

The EY86 has been swapped. I might well change the heater winding but the EY86 heater seems to be glowing both with and without the top cap connected. There's no sign of the heater winding flowing/arcing currently. I'll be back!

 
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Re: Bush TV86C Possible LOPT fault

Post by Katie Bush » Thu May 05, 2016 7:02 pm

Rebel Rafter wrote:..... Surely if the EY86 had gone to air it's heater wouldn't glow, still it might glow slightly if it's got a slow puncture .....


I'm not so sure.. As I recall, when I was a mere "valve popper" for my granddad, I seem to recall 'busting' a few valves, extracting the heaters from them (possibly 'P' series -PCL, PCF etc) and lighting them up.. We were looking for a heater to replace the one in a dud smoke generator in a Hornby OO gauge 'Brittania'.

If I remember it correctly, they still light up, and worked quite well for the job, they just weren't terrifically robust.

Anyone got a dud valve to pop and try lighting up the heater in air?

Marion

 
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Re: Bush TV86C Possible LOPT fault

Post by Lloyd » Thu May 05, 2016 8:02 pm

Hi,

I've tried lighting up air-filled valves before! I always find that if it does light, then it's very much dimmer than before, and the valve envelope gets very hot. If you upped the heater voltage a fair bit you could get them to light up to near normal brightness, but they usually popped very soon after.

I have had some valves with a slow leak to air, they usually lit up OK, or just a bit dimmer than normal, but the getter splodge in the top of the valve used to slowly shrink away, normally with little holes appearing in the splodge. Those valves still sort of worked, just not as well as they should. If left for long enough the splodge would turn white, then you definitely know its full of wind!

Now thinking about it, I have a certain Ekco TV which had some bother with it's EHT rectifier, it looked OK, and the heater lit up, but the picture was rubbish and would bloom with the brightness turned up, sometimes the heater would flash bright orange (at least I think it was the heater!) and when it did the picture would improve no end, then I think it packed up altogether. New EHT rectifier and all was well. That valve would even do it's 'thing' when powered from the bench PSU, it was like someone had lit a match behind it, never seen a valve do that before, or since! Blue flashes and suchlike, but never orange... :aaq

Regards,
Lloyd.

 
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Re: Bush TV86C Possible LOPT fault

Post by channel405 » Sat May 07, 2016 10:17 pm

Once again thanks all for your input and comments.

Quick update: I took the heater winding off and found four cracks in the insulation, one where it had been arching and started to burn up. So hopefully this is the culprit. Yet the heaters were definitely glowing? Anyway, when I get a chance I'll fit a new winding and report back.

I'm still a bit confused about which is the correct pin for the EHT output to the final anode- different diagrams say different pins, but will try where it is for now.

 
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Re: Bush TV86C Possible LOPT fault

Post by channel405 » Sat May 07, 2016 10:19 pm

Forgot to add to last reply- no blue glow in the EY86 and no smell of ozone etc.

 
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Re: Bush TV86C Possible LOPT fault

Post by Terrykc » Wed May 18, 2016 2:36 pm

channel405 wrote:I'm still a bit confused about which is the correct pin for the EHT output to the final anode- different diagrams say different pins, but will try where it is for now.

Take your pick - 1, 4, 6 or 9 ...
EY86.png
EY86.png (11.7 KiB) Viewed 1373 times

In my experience it was common to connect all four pins together - similarly the three pins for the other end of the heater.

As corrosion of the valve pins and sockets was common, this probably delayed the point at which the set finally stopped working.

One article I read suggested that this problem was caused by the high water vapour content produced by the radiant paraffin heaters common at the time these sets were around but, whenever I asked a customer afflicted with the problem if they had such a heater, the answer was always no!

 
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Re: Bush TV86C Possible LOPT fault

Post by Katie Bush » Wed May 18, 2016 2:46 pm

Terrykc wrote:
One article I read suggested that this problem was caused by the high water vapour content produced by the radiant paraffin heaters common at the time these sets were around but, whenever I asked a customer afflicted with the problem if they had such a heater, the answer was always no!


"We're far too modern for that - we have a butane gas fire instead!"
- Which are/were far worse at liberating water vapour into the air. :aa

Though to echo Terry's point about linking the pins, I do recall seeing this on several different makes and on numerous occasions in my granddad's workshop.. I seem to recall reading somewhere that keeping them "equipotential" somewhat helped to limit the corrosion.

Marion

 
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Re: Bush TV86C Possible LOPT fault

Post by Rebel Rafter » Wed May 18, 2016 9:59 pm

Hi, from RR. One little point about EHT wiring, a little point but important, I know most folk on here will know this anyway, but for those who might not, when soldering connections in the EHT wiring just be careful not to leave pointed bits sticking out anywhere, try and keep all EHT connections smooth and rounded, as pointed connections at EHT potential tend to cause corona discharge and arcing which can be at best a right pain in the rear and at worst can be a bit of a fire hazard, so now you know. RR.

 
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Re: Bush TV86C Possible LOPT fault

Post by Cathovisor » Wed May 18, 2016 10:38 pm

Interesting notes here regarding the difference between the EY86 and EY87:

http://frank.pocnet.net/sheets/030/e/EY86.pdf
http://frank.pocnet.net/sheets/030/e/EY87.pdf

 
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Re: Bush TV86C Possible LOPT fault

Post by turretslug » Wed May 18, 2016 11:04 pm

So, if you're taking your telly to 14,000 feet- make sure it's an EY87! Perhaps originally for the Bolivian and Nepalese markets?


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