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-   -   Baird T5 restoration project. (https://www.vintage-radio.net/forum/showthread.php?t=152005)

beltinge bore 3rd Apr 2019 2:36 pm

Re: Baird T5 restoration project.
 
Hello David

I think I might know which EKCO young Beery is referring to! It sits in my garage having had much new wood and plenty of woodworm treatment.

For some reason SHMBO won't let it in the house.

Regards Steve.

FERNSEH 15th Apr 2019 2:59 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
Time to apply wood dye on the side panels, but before any attempt to do this I'll test the various dyes and polishes on wood off-cuts. The attached picture shows that the finish needs to darkened in order to resemble the wood surfaces of the original panels.

DFWB.

FERNSEH 15th Apr 2019 3:07 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
After several applications of wood dye on the test piece I'm sure the side panels can be stained and polished.
But the wood dye should be applied outdoors and not in the workshop, nasty fumes given off during the application process.
As the attached picture shows the panel surface colour is still too light and needs more wood dye in order that it resembles the original dark finish.

DFWB.

FERNSEH 3rd May 2019 8:59 pm

Re: Baird T5 restoration project.
 
3 Attachment(s)
It has been established that the two power supply units and the sound and vision receivers are serviceable. The loudspeaker is OK, it just needs the frame repainting. Now the timebase unit must receive attention. In order that it can be tested my Baird T23 will need to be repaired. Another scary job because the CRT must be removed to fix that intermittent connection on the tube base.

Pictures of the T5 timebase unit. This is the early version without the interlace diode.

DFWB.

peter_scott 4th May 2019 7:43 am

Re: Baird T5 restoration project.
 
2 Attachment(s)
In the third photo there are three pots sitting on angled metalwork. What do they control and how do you access them for adjustment. They don't appear to be shown on the drawing below and there appears to be just a couple of blanking plugs corresponding with that location on the front panel.

Peter

FERNSEH 4th May 2019 10:31 am

Re: Baird T5 restoration project.
 
2 Attachment(s)
Hi Peter,
Those controls are presets for the line and frame timebase frequencies and picture height adjustment.
It's possible that very early production timebase units would have a 240/405 switch and that's the reason for those blanking plugs on the control panel.
The first attachment shows the control panel of the Baird T5. The circuit diagram is of the frame timebase in the T23. A Mazda AC/HL is used as the interlace diode.

DFWB.

peter_scott 4th May 2019 11:36 am

Re: Baird T5 restoration project.
 
Thanks David. Do you just push finger and thumb up inside the access to adjust them? Do you think this set has had the 240 line circuitry?

Peter.

FERNSEH 4th May 2019 12:00 pm

Re: Baird T5 restoration project.
 
Hi Peter,
In my Baird T23 timebase chassis there is evidence of wiring that might well have been used for the 240/25 and 405/50 systems switching, although the wires definitely haven't been connected to anything. Something similar can be seen in the timebases of first generation EMI sets where the dual standard cableforms were still fitted in later 405 only receivers. In very early T5 models the preset controls would have been used to permit easy switching between the two transmission standards.
The preset controls are only accessible when the back cover is removed.

DFWB.

FERNSEH 13th May 2019 9:44 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
The mirror lid escaped the attention of the woodworms and can be refurbished. It doesn't look all that pretty for sure but there is no doubt it can be saved.
A few pieces of veneer will have to be inserted to correct slight damage. The surface silvered mirror is in excellent condition.
A new laminated safety glass has been made. The original had become delaminated and was discoloured anyway.

The line output transformer has open circuit windings and that's holding up the timebase restoration.

DFWB.

beery 14th May 2019 6:26 am

Re: Baird T5 restoration project.
 
Hi Fernseh,
the one I restored which had electrolytics dated September 36 had tell tale shaddows of where the extra parts had been. Unfortunately the circuit diagrams I have, have had the dual standard parts deleted. Note that there was no extra valve like in the EMI sets.

I would be interested in the winding details of the lopt as it might give me some clues for the one I need to make for the Garrick which uses very similar timebases.

Cheers
Andy

peter_scott 14th May 2019 7:12 am

Re: Baird T5 restoration project.
 
I wonder if the Baird line timebase was more stable than EMI's or just didn't rely on having line syncs or whether they just perceived a little disturbance at the top of the picture as a minor problem relative to the types of issue raised by Cossors concerning the Baird transmitted signal.

Did the Cossor line timebase make any special arrangement to maintain line speed in the absence of line syncs?

Peter

FERNSEH 14th May 2019 9:10 am

Re: Baird T5 restoration project.
 
The Baird 240/25 system sent only a single broad frame sync pulse so the possibility of hooking at the top of the picture was a real possibility if the line hold control was slightly off frequency. But there was no circuits to compensate for this.
The Cossor engineers also noted that the deep sync pulses limited the picture contrast. Baird sync pulses were 50 per cent of the video waveform, the reason for this was allow for the poor black level referencing which resulted in the picture information being present in the sync pulses.
It was a hopeless TV system with no potential for improvement.

And yet the Baird TV receivers were excellent.

DFWB.

FERNSEH 21st May 2019 6:52 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
The old lacquer on the cabinet mirror lid surface has been removed to expose the condition of the veneers. Some minor corrective work is required but the veneer pattern isn't very nice. The book matching isn't perfect, but that's how it was made over eighty years ago.
Later today I'll rub on some French polish and then we'll see how the veneer really looks.

DFWB.

FERNSEH 22nd May 2019 9:07 am

Re: Baird T5 restoration project.
 
1 Attachment(s)
Here's how the cabinet lid looks after French polish was dabbed on yesterday evening. It doesn't look too good. I'd suggest that the old veneer should be removed and replaced with something better.

DFWB.

beery 27th May 2019 5:34 pm

Re: Baird T5 restoration project.
 
Hi David,
if the veneer is not damaged in any way, but just not very nice, then surely it would be best to keep it as it is the only original panel.
Mind you, if I was the original customer, paying 85 Guineas in 1937, I might be a little annoyed at the poor veneer matching, but then again, who would really look at the top of the lid...

Cheers
Andy

Panrock 28th May 2019 7:42 pm

Re: Baird T5 restoration project.
 
It would look better if the dark marks were first bleached out. And toner applied to the non-veneered sections.

Steve

Panrock 28th May 2019 8:25 pm

Re: Baird T5 restoration project.
 
Also (sorry to be a pain) but it looks like patches of the old lacquer remain and more preparation is required.

Steve

FERNSEH 26th Jun 2019 10:01 am

Re: Baird T5 restoration project.
 
1 Attachment(s)
It's evident some wee beastie has chewed the top of the line output transformer but I don't think all is lost because the damage is only the outer turns on what turns out to be the sync injection winding. See the attached circuit diagram.
The transformer has been removed from the chassis for inspection.
The primary is the inner winding, the secondary serves as the oscillator feedback winding and supplied the scanning coils.
The capacitors have been tested and all are useless. All the electrolytic capacitors are of TMC make and were fitted in 1947.

All the other chassis assemblies are working and just need the capacitors replacing or in the case of the waxies - restuffing. Most of the resistors are close to the indicated values.

DFWB.

beery 27th Jun 2019 11:51 pm

Re: Baird T5 restoration project.
 
Hi David,
Was it attacked by a woodworm that had lost its way?

Any info you might work out in terms of turn ratio etc could be useful for the Garrick restoration.

Cheers
Andy

FERNSEH 3rd Jul 2019 8:52 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
Hi Andy, as the damaged outer winding is only for injection of sync pulses and it's possible that the loss of a few turns shouldn't have much effect on the line hold.
The receiver uses a fair quantity of 0.5mfd 400V B. I. capacitors. It goes without saying every capacitor is leaky as one would expect in an eighty year old set. New Hi-Viz capacitors have been ordered from RS components and these will be used to restuff the original capacitors. It is essential that the original appearance of all the chassis assemblies is maintained.

DFWB.

FERNSEH 9th Jul 2019 9:38 am

Re: Baird T5 restoration project.
 
2 Attachment(s)
Work continues on the timebase chassis. Last night I discovered a selenium rectifier which I assume to be an interlace diode. It's most likely this component was fitted in 1947 along with all the other components that were replaced to return the set to full working order.
My Baird T23 also has an interlace diode. The component used for the purpose is a diode connected AC/HL triode or in my set a Mazda V914 double-diode.
Note the very vintage radio valve holder, looks like something from the 1920s.

DFWB.

FERNSEH 10th Aug 2019 10:37 am

Re: Baird T5 restoration project.
 
1 Attachment(s)
Two timebase units. See attachment. The unit on the left is from the T5 and the other is from my T23.
The latter I have to confess wasn't restored in a sympathetic manner, simply made to go only. That one will receive attention, but only after the T5 timebase is fully restored to working order.
The BICC 0.5mfd capacitors have been restuffed and now the TMC electrolytic capacitors will be attended to.
The two timebase units are fully interchangeable which means the T5 timebase can be tested in the T23. The same goes for all the other T5 chassis units.
The Cathodevisor 15MW4 can't be tested in the Baird T23 because the CRT mounting arrangements were modified in order that a Cossor 3265 CRT can be employed.
The cabinet restoration is held up because I'm still looking for walnut veneered 9mm plywood boards for the side panels. Been touring about Newcastle and Gateshead visiting junk shops and secondhand furniture stores. No luck so far.

DFWB.

davyrocket2 10th Aug 2019 10:45 pm

Re: Baird T5 restoration project.
 
Hello David is there no other source of 9mm ply say from a wood supplier or alternative source, is this not used on boats like marine ply or is just that the walnut version hard to find regards David

FERNSEH 10th Aug 2019 11:31 pm

Re: Baird T5 restoration project.
 
Hi David,
The problem is that the original side panels are in such a bad state that there is no way of saving them.
Next week I will visit a timber firm in Gateshead and take a look at what's available. And it's only those side panels which are holding up the job. I've considered applying veneers on the newly made panels but past experiences of doing this have proven disappointing.
So in the meantime I'll concentrate on the electronics. The timebase chassis employs a large number of electrolytic capacitors and all are unserviceable. It was the same with the T23.

DFWB.

FERNSEH 27th Aug 2019 10:41 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
The plan was to employ the Baird T23 to test the chassis units from the T5.
But first the Cossor CRT had to be repaired, it will be recalled there was a bad grid connection on the base. That's been put right.
The T23 still doesn't display a picture. A bright raster appears on the screen but the brightness control doesn't have any effect. In these early Baird sets the tube cathode is at chassis potential and the positive going video is supplied to the grid. The brightness control supplies a 0 to 100V negative bias to the tube grid, that's missing.
Pull out the grid CRT plug and connect up the oscilloscope, the video returns. Refit the plug and the video disappears again. Faulty CRT? Hope not, it's more likely the missing grid bias is causing the tube to go into grid current and bypassing the video signal to ground.
So I've got to get the CRT grid bias working before the T23 can be used to test the other set's modules.
The attachment shows the 15" Cossor CRT installed in the T23.

DFWB.

beery 28th Aug 2019 1:31 pm

Re: Baird T5 restoration project.
 
3 Attachment(s)
Hi David,
how does the brightness circuit work?

In the T5 the negative end of the EHT was used to develop a negative voltage across the brightness control. The grid was connected to the wiper of the brightness control via a 500K resistor that enabled the grid to modulated from the output of the vision receiver (via a coupling capacitor). The CRT cathode is connected to one side of the heater and that is then connected to ground via a low resistance pot in the HT PSU, which is used to set the heater current. See the attached circuit.

Your T23 is a T5/T20 hybrid. I've attached the circuit of the T20's TRF unit. If the grid bias is adjusted in the same way as the T5 then R18 in the TRF circuit would have to be deleted (point 'M' in the TRF circuit is connected to the CRT cathode).
However, does your T23 control the brightness via the cathode as in the T20?
I've attached the circuit of the T20 PSU, note the CRTs heater/cathode is connected to brightness control VR4 which is connected to HT via another R18.
Also your set has a DC restoration diode as well...

All this might well mean that you have to test the T5 EHT, HT PSU and vision receiver all together rather than mixing them with bits of T23...

Cheers
Andy

Heatercathodeshort 28th Aug 2019 4:28 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
Hi David,
I was looking through some pictures on Utube yesterday and discovered this capture of a Baird T5 operating in Waterloo Station in 1936. It looks genuine by the light reflection in the mirror. I have read about the Waterloo demonstrations but this is the first time I have actually seen a picture.
John.

FERNSEH 28th Aug 2019 5:31 pm

Re: Baird T5 restoration project.
 
Hi John,
Although the cabinet the Baird T5 is not as well made as the contemporary EMI models the T5's electrical performance is as good or even better than the HMV models. The T5 cabinet is a bit flimsy compared with the solid and heavy EMI sets. And the Baird T5 has that big fifteen inch CRT.

Meanwhile my T23 is still giving problems. Glasnost: it turned out I'd connected the video lead to the wrong connector on the tube base. That's been put right and the video signal is now supplied to the CRT grid connector..
So there is now a picture on the screen, but the brightness control still doesn't work!
The oscilloscope proves that the tube grid voltage can be varied by the brightness control. However, it is not the sync tips that are setting the black level reference, the negative brightness control voltage is referencing to the peak whites of the video waveform. I'd say the DC restorer diode needs reversing so that it conducts on the sync tips.
I reckon the CRT will have to come out again so that the base connections can be checked again.

DFWB.

FERNSEH 28th Aug 2019 7:01 pm

Re: Baird T5 restoration project.
 
Beery wrote: "Your T23 is a T5/T20 hybrid. I've attached the circuit of the T20's TRF unit. If the grid bias is adjusted in the same way as the T5 then R18 in the TRF circuit would have to be deleted (point 'M' in the TRF circuit is connected to the CRT cathode).
However, does your T23 control the brightness via the cathode as in the T20?"

Hi Andy,
I haven't checked to see if R18 is present in the RF chassis but it's safe to say that it has been removed by the manufacturer. The 15MW1 CRT in the T5 has a directly heated cathode whereas the Cossor 3265 tube has an indirectly heated cathode. The cathode is connected to one of the heater connections so like the T5 it is at chassis potential. And like the T5 the brightness is controlled by varying the grid voltage which is derived from the negative return in the EHT supply. The resistor bleeder chain across the EHT supply provides the constant current for the voltage drop for the negative CR tube grid bias.

DFWB.

FERNSEH 29th Aug 2019 3:42 pm

Re: Baird T5 restoration project.
 
2 Attachment(s)
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.

If it is at all possible the brightness control circuit will be rearranged so that the CRT cathode voltage will be controlled in the same manner as the in the model T20. The video drive to the CRT grid will be DC restored with the reference level at a fixed potential.
See the third attachment in Andy's post No. 176.

The second attachment in this post shows the overly bright picture on the screen of the Cossor CRT.

DFWB.

FERNSEH 29th Aug 2019 5:12 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
The attachment shows the Westinghouse WX6 video DC restorer diode. The red band indicates the anode. Is that correct?

DFWB.

peter_scott 29th Aug 2019 6:02 pm

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

Peter

beery 29th Aug 2019 6:52 pm

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
Andy

julie_m 29th Aug 2019 7:45 pm

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 .....

FERNSEH 29th Aug 2019 9:58 pm

Re: Baird T5 restoration project.
 
2 Attachment(s)
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.

ronbryan 29th Aug 2019 10:52 pm

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.

Ron

Jac 30th Aug 2019 6:33 am

Re: Baird T5 restoration project.
 
1 Attachment(s)
Hello David,

The diode is named as in the attachment.

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

Jac

FERNSEH 30th Aug 2019 12:38 pm

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.

Jac 30th Aug 2019 1:36 pm

Re: Baird T5 restoration project.
 
That can be quite a set-back David.
Hopefully you find a solution for it.

Jac

FERNSEH 30th Aug 2019 3:41 pm

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.

peter_scott 30th Aug 2019 4:03 pm

Re: Baird T5 restoration project.
 
If it's shorted with a piece of cathode material would it not be possible flash it away?

Peter

Heatercathodeshort 30th Aug 2019 4:15 pm

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.

FERNSEH 30th Aug 2019 4:28 pm

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.

FERNSEH 30th Aug 2019 4:48 pm

Re: Baird T5 restoration project.
 
2 Attachment(s)
Quote:

Originally Posted by Heatercathodeshort (Post 1172299)
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.

beery 30th Aug 2019 7:48 pm

Re: Baird T5 restoration project.
 
1 Attachment(s)
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

FERNSEH 30th Aug 2019 9:35 pm

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.

beery 30th Aug 2019 10:37 pm

Re: Baird T5 restoration project.
 
Quote:

Originally Posted by FERNSEH (Post 1172249)
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

Argus25 31st Aug 2019 12:11 am

Re: Baird T5 restoration project.
 
Quote:

Originally Posted by FERNSEH (Post 1172061)
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.

FERNSEH 31st Aug 2019 12:29 pm

Re: Baird T5 restoration project.
 
3 Attachment(s)
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.

Heatercathodeshort 31st Aug 2019 4:18 pm

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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