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Old 29th Aug 2019, 6:12 pm   #181
FERNSEH
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Default Re: Baird T5 restoration project.

The attachment shows the Westinghouse WX6 video DC restorer diode. The red band indicates the anode. Is that correct?

DFWB.
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Old 29th Aug 2019, 7:02 pm   #182
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Default Re: Baird T5 restoration project.

Looking at the Westector below it appears that red is the cathode.
https://www.radiomuseum.org/tubes/tube_wm26.html
http://www.radioman33.com/pages/docu...-la-diode.html

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Old 29th Aug 2019, 7:52 pm   #183
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Default Re: Baird T5 restoration project.

Yes, definitely the cathode. It is a common thing on early solid state diodes that the red terminal is the cathode. It seems designed to confuse people, but I suspect the thinking was that if it were a PSU, the cathode of the rectifier is connected to the positive connection of the smoothing capacitor...

I guess it is no more confusing than modern electrolytics having a stripe indicating negative whilst tantalums have a stripe for positive
cheers
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Old 29th Aug 2019, 8:45 pm   #184
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Default Re: Baird T5 restoration project.

They probably coloured the "business end" red because current flows out of it.

Although it would have made just as much sense to have coloured the "tail end" red because current flows into it .....
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Old 29th Aug 2019, 10:58 pm   #185
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Default Re: Baird T5 restoration project.

The wartime civilian radio receiver uses a WX6 as the second detector. The attachment shows the diode in an U20 chassis. The anode of the rectifier is connected via the secondary of the IF transformer to the bottom end of the volume control. So it follows the red end of the WX6 is the anode.
The WX6 in the T23 will be reversed and we'll see if the DC restorer works as the sync tip clamp.

DFWB.
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Old 29th Aug 2019, 11:52 pm   #186
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Default Re: Baird T5 restoration project.

The Wartime Civilian Radio has to employ a small forward bias on the WX6 to make it conduct in a more linear manner. The bias is obtained across the 180 ohm cathode resistor. The most positive voltage is applied to the black end via the 330k and 47k and the most negative voltage to the red end via the IFT secondary. In order to make the diode conduct under these conditions, the black end has to be the anode and the red end the cathode.

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Old 30th Aug 2019, 7:33 am   #187
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Default Re: Baird T5 restoration project.

Hello David,

The diode is named as in the attachment.

Very interesting looking diode, the WX6, and an early application as well.

Jac
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Old 30th Aug 2019, 1:38 pm   #188
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Default Re: Baird T5 restoration project.

Well now I know the reason why the brightness control doesn't work in the T23. The CRT has leakage between the anode and grid, possibly a piece of cathode material has got lodged between the electrodes.
When the brightness control plug is pulled out sparks are seen to jump across the grid and earth connections of the socket.

DFWB.
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Old 30th Aug 2019, 2:36 pm   #189
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Default Re: Baird T5 restoration project.

That can be quite a set-back David.
Hopefully you find a solution for it.

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Old 30th Aug 2019, 4:41 pm   #190
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Default Re: Baird T5 restoration project.

Hello Jac,
It's all too evident that there is an inter-electrode leak in the CRT.
However, I think I can do something about the matter without changing the tube. It's unlikely a another good Cossor 3265 or 65K will ever turn up again and there is absolutely no chance whatsoever of finding a Cathodevisor 15MW1.

A 0 - 30 volt bench power supply has been hooked up the grid and cathode of the tube. Result, almost complete visual cut off with -30volts applied to the grid. So it follows if the CRT gun can be supplied from low resistance sources it will be possible to continue to using the same tube. In the Baird T20 the brightness control function is implemented by varying the CRT cathode voltage. The positive going video is supplied to the grid as it is in the T5.
Unfortunately for me the tube cathode is connected at the base to one of the heater pins, so in order to remove the wire link the tube will have to come out again. Taking a tube out of a T5 or T23 is easy, just disconnect the wires to the base sockets, remove the EHT connector and just lift the tube out vertically and away from the cabinet. Simples.
Refitting the tube is bit more scary though, although in a sense it is the reversal of removing it.
Let's see if there are any alternative means of using this tube without too many modifications to the set.

DFWB.

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Old 30th Aug 2019, 5:03 pm   #191
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Default Re: Baird T5 restoration project.

If it's shorted with a piece of cathode material would it not be possible flash it away?

Peter
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Old 30th Aug 2019, 5:15 pm   #192
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Default Re: Baird T5 restoration project.

I have exactly the same problem with a 7" EMI triode type 3/2. Switch on, picture appears for a few seconds then the screen floods with light. Switch off for four seconds then switch back on and tube will be OK for hours. Sometimes a second switch off is necessary but it always comes back. Disconnecting the cathode lead under fault conditions still leaves the tube brilliantly illuminated with no picture of course.
I have tried every trick in the book to clear the short, all to no avail. At first I thought it may have been some conductive grot at the glass pinch where the lead out wires emerge from the gun. They are very close together, brittle and delicate. The EHT for the anode also enters at this point. All OK but the wires in the pinch are VERY close together, not really nice practice. At least it does give a picture.
It sounds like your Cossor tube has a similar problem. I believe these early EMI tubes were prone to shorts. Hope you get it sorted David. John.
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Old 30th Aug 2019, 5:28 pm   #193
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Default Re: Baird T5 restoration project.

"If it's shorted with a piece of cathode material would it not be possible flash it away?" Hi Peter, that's something to consider.
Many years ago I bought an English Electric 1550 TV set. It's been languishing in the workshop ever since. That set uses a 15" Cossor CRT, if it's a type 65K then it can go in the Baird.

DFWB.
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Old 30th Aug 2019, 5:48 pm   #194
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by Heatercathodeshort View Post
It sounds like your Cossor tube has a similar problem. I believe these early EMI tubes were prone to shorts. Hope you get it sorted David. John.
Hi John,
The first picture shows the Cossor CRT you kindly gave me to fit in the T23 when I was experiencing tube problems. Unfortunately because of it's bulb shape it can't used in the set. The other picture shows a Cossor type 65K. Many T23 sets were modified by the factory so that this tube can be used.

DFWB.
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Old 30th Aug 2019, 8:48 pm   #195
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Default Re: Baird T5 restoration project.

Hi David,
if you want to try the T20 method then you can disconnect the grounded end of the CRT heater current control. The main complication is that the brightness control plugs into the EHT unit, but you would need to get an HT feed from the main PSU to the control, and join the bottom end of the EHT supply to chassis.

Lets look at some values...
I don't know the value of the T5/T23 brigntness control.
From the T20 circuit I posted earlier, The brightness control (VR4) is 10K. The resistor that feeds is (R18) is 80K.
The other R18 in the TRF unit is 500K.

Cheers
Andy
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Old 30th Aug 2019, 10:35 pm   #196
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Default Re: Baird T5 restoration project.

Hi Andy,
The Cossor takes its 4 volt heater supply from the timebase heaters.
One advantage the Cossor 65K has over the Cathodevisor tube is that it has an indirectly heated cathode so there is no requirement for an isolated from earth heater supply. If all fails with the present tube I'll have work out how to fit John's CRT in the set. A new adaptor plate perhaps?

DFWB.
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Old 30th Aug 2019, 11:37 pm   #197
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
Well now I know the reason why the brightness control doesn't work in the T23. The CRT has leakage between the anode and grid, possibly a piece of cathode material has got lodged between the electrodes.
When the brightness control plug is pulled out sparks are seen to jump across the grid and earth connections of the socket.
Hi David,
I was just wondering if there is actually a spark going across the ends of the brightness control. What I mean is, what if the high potential across the brightness control socket comes from the 1M EHT return resistor being high... And if that is the case, what if then one end of the brightness control was o/c? Would that not also give you unajustable brightness?

It does seem odd that although there seems to be a high leakage in the tube, it can still be modulated easily by the video drive.

Maybe croc clipping a meter between the wiper connection of the pot and chassis (I would not want to dab a probe onto it live if there is any EHT around) will give you more of a clue. Then you could run the same test again with the grid and heater connections of the CRT removed to see if the voltage range of the pot wiper is the same.

Cheers
Andy
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Old 31st Aug 2019, 1:11 am   #198
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by FERNSEH View Post
The sketch shows the DC restorer in the T23. There is something not right about this circuit. The positive going video is supplied to the CRT grid. The function of the DC restorer diode is to conduct on the sync pulse tips to the DC reference level set by the brightness control. Normally the DC reference will be a positive voltage but in the Baird T23 it is negative and in all accounts this is the wrong polarity for the circuit to work properly.
The circuit in your post might look odd, but is good with the diode shown as the correct polarity for a DC restorer.

The easiest way to understand these circuits is to imaging the diode is a little leaky or has a 1M resistor in parallel with it, and for a moment is not a diode.

This causes the coupling capacitor from the plate of the output valve to charge to a potential equal to the sum of the plate voltage and the absolute value of the negative voltage on the brightness pot's slider.

When an AC signal comes along it attempts to swing the voltage of the coupling capacitor's output terminal above and below the DC level of the brightness pot's slider. If there is the diode present, when the voltage attempts to fall below this value, on sync tips, the diode conducts, altering the average charge on the coupling capacitor, which tends to stabilize the sync tip voltage at the potential of the pot's slider. So as the brightness pot is rotated the sync tips follow that voltage.

It is not necessary to actually have a leaky diode or a theoretical resistor in parallel with the diode (that is when enough signal is present to drive the diode into conduction). However with no signal present, especially with a valve diode, there should be a resistor in parallel with it to allow for this condition. No doubt with the vintage semiconductor diode, there is more than enough leakage to allow for the no signal condition and no resistor is required.

Its sad that the CRT turned out to have leakage issues, if that is the case. Andy's suggestion in the post above looks very good.

Since the leakage resistance of the diode is ill defined, you could also try tacking a 250k to 1M resistor across it and check if that gains increased brightness control.

Last edited by Argus25; 31st Aug 2019 at 1:19 am.
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Old 31st Aug 2019, 1:29 pm   #199
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Default Re: Baird T5 restoration project.

Hi Argus,
I've redrawn the circuit of the DC restorer to show the 500Kohm resistor in parallel with the WX6. Note also the 200Kohm resistor in series with the diode.
The third attachment show the underside of the T5 EHT unit. No DC restorer was fitted in this receiver.

DFWB.
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Old 31st Aug 2019, 5:18 pm   #200
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Default Re: Baird T5 restoration project.

Hello David, Way back in the 1940s I think at birth, we must have been fed with the same brand of baby food. Do you remember the cream coloured tins of 'National dried Milk'? You do! I knew I was right.
I say this because we both seem to have the same attraction to narrow angle massive autobomb 15" tubes.You seem to have a delight in removing and replacing that huge tube in the T5.
Thinking about it, this year I have played with 2X EMI 3/32 15" monsters, an EMI 15" pre war 3/6A, 2X EMI TA15s and the frightening Mazda CRM151. I think they must have added something extra to Farleys Rusks back then. John.
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