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Old 15th Jun 2021, 9:09 am   #381
FERNSEH
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Default Re: Baird T5 restoration project.

Measured in jars no doubt
Hi Andy,
One Jar = 1100pF?

A capacitive unit of a Jar was featured in an episode of MacGyver who is known to be very clever.

https://www.wired.com/2017/01/the-ph...f-leyden-jars/

DFWB.
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Old 17th Jun 2021, 10:24 am   #382
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Default Re: Baird T5 restoration project.

More capacitors are receiving the re-stuffing treatment. The intention is to restore the original appearance of both timebase units, T5 and T23.
The dual capacitor pack came out of the T23 and is the original 1938 component. The 16mfd 200volt section is in the frame oscillator circuit and has the negative going sawtooth waveform across it. The 8mfd 450volt section is the line timebase HT supply decoupler.

DFWB.
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Old 18th Jun 2021, 7:56 pm   #383
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Default Re: Baird T5 restoration project.

Good progress today. All the 450 volt electrolytic capacitors replaced.
Three more electrolytic capacitors with more critical working voltages need to be fitted. The 16 microfarad frame timebase sawtooth forming capacitor was a post-war replacement, rated at 450 volts this too high for it's application in this part of the circuit. Correct voltage rating is 200 volts. It's likely the waveform across this capacitor is -100 to -125 volts P - P.
See first attachment, the capacitor is the huge component at the bottom of the picture.
Second attachment shows the progress made today. The two parallel connected 8 microfarad capacitors have been replaced by a single 16mfd unit.
The 25microfarad scan coil coupling capacitor will need replacing, as will the 2mfd frame oscillator valve cathode bypass capacitor. Voltage rating for this capacitor should be 200V.

DFWB.
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Old 19th Jun 2021, 7:03 pm   #384
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Default Re: Baird T5 restoration project.

Two more components fitted today. Both are now in the correct original pre-war positions.
The 50 microfarad line scan coil coupling capacitor is useless. This is a component that was fitted in 1947. I think on this occasion I'll eschew the desire to maintain the original appearance and use a modern capacitor fitted on a tag strip.
Compare the size of the 1947 component with a Philips axial lead capacitor and the Far East made capacitor that was fitted in the T23.
Reckon the Philips capacitor is the best choice of component for both timebase units.

DFWB.
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Old 21st Jun 2021, 9:03 pm   #385
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Default Re: Baird T5 restoration project.

The T5 timebase unit is ready for testing in the T23. Will report my findings tomorrow.
Meanwhile the T23 timebase is being tidied up, getting rid of those components fitted in 1993.

DFWB.
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Old 24th Jun 2021, 9:36 am   #386
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Default Re: Baird T5 restoration project.

Yesterday, I carried out the first tests on the T5 timebase unit.
Shortly after the HT rectifiers warmed up problems, the insulation of the cables between timebase and and power supply broke down so it was back to the bench again to replace the burnt out cables. Cables replaced the timebase was refitted in the T23, no smoke and sparks this time and within 30 seconds a defocussed raster. On the T23 the focus control is located on the timebase control panel and on the T5 it is fitted on the upper control panel alongside the other user controls, volume, brightness etc.
But there's lots more work to be done on the timebase unit. The width and height controls have little effect and the frequency range of the line hold control is limited. Likely more resistors will need replacing.
The Cossor 41MP line output valve was seen to be flashing over. This valve is operating near to or beyond it's ratings in a timebase with an inductive anode load. Never 'scoped the anode waveform but it's safe to say it'll be close to the 1KV figure.

DFWB.
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Old 24th Jun 2021, 10:09 am   #387
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Default Re: Baird T5 restoration project.

Hi David,
my experience with the T5 was that flashing over of either the line or frame output valves would happen if the height/width were advanced too high, if the frequency was way off, or if an noise got through the sync separator. It seemed to calm down once everything was properly warmed up.

In my Garrick, which is a develoment of the T5 design, PEN46 valves replace the 41MP valves. The PEN46 was designed for line output use.

Cheers
Andy
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Old 24th Jun 2021, 10:43 am   #388
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Default Re: Baird T5 restoration project.

Beery wrote: "In my Garrick, which is a development of the T5 design, PEN46 valves replace the 41MP valves. The PEN46 was designed for line output use."
Hi Andy,
for the sake of reliability I might try a PEN46 in the T5 and T23. The beam tetrode PEN45 has a higher anode impedance ra compared with the triode 41MP. In this application the valve operates as a switch rather than a linear amplifier.
Something to consider in the future, in the meantime let's get the T5 timebase working first.

DFWB.
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Old 24th Jun 2021, 5:44 pm   #389
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Default Re: Baird T5 restoration project.

Checked the values of the resistors and most are within the 10% tolerance range. Like the T23 the T5 timebase has hidden width and height pre-set controls. Connected the focus control to the timebase unit. Still unable to achieve full focus because the circuit has to be modified to suit the characteristics of the Cossor CRT.
The raster is deflected downwards because the 75,000 ohm standing current cancelling resistor has gone high value, it's connected to the third winding on the frame deflection unit.
The resistor will be replaced with two 150Kohm 2watt resistors.

DFWB.
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Old 25th Jun 2021, 8:44 am   #390
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Default Re: Baird T5 restoration project.

Is that you testing the T5 timebase in your T23? CRT is very nice and free from ion burn. Just a tiny spot.

Peter
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Old 25th Jun 2021, 9:02 am   #391
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Default Re: Baird T5 restoration project.

Hi Peter,
I'm using the T23 as a rig to test the T5 modules.
The Cossor CRT in in the T23 is quite good which is just as well because all the other 65K tubes I have are in poor condition.
The T5 EHT power supply was tested in the T23 and later this week the main power supply unit will receive attention. It works OK but the electrolytic capacitors need attending to.

DFWB.
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Old 26th Jun 2021, 11:01 am   #392
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Default Re: Baird T5 restoration project.

Concurrent with the work I'm doing on the Baird T5 I'm also working on the T23 in an attempt to return the units to their original appearance.
The card block electrolytic capacitors have been rebuilt and fitted along with many other components replaced which were used to just get the timebase working.
However, two components have been found which seem to have no function in the circuit. One is a 2microfarard electrolytic capacitor and two series connected resistors across the HT supply. See the attachment.

DFWB.
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Old 26th Jun 2021, 1:20 pm   #393
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Default Re: Baird T5 restoration project.

Looks nice but I hope you'll not leave those nasty yellow arrows in place.

Peter
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Old 26th Jun 2021, 10:36 pm   #394
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Default Re: Baird T5 restoration project.

Hi Peter,
the yellow arrows are now gone, no need for them.
The mystery 2 microfarad capacitor turned out to be a decoupler in the anode load circuit of the frame sync separator. The capacitor is now inside the can of an old style component.
Tested the unit in the T23 this evening. Everything was going well then the line output stage went haywire.

DFWB.
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Old 28th Jun 2021, 7:37 pm   #395
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Default Re: Baird T5 restoration project.

The T23 timebase is working properly again.
Most likely suspect component was the 47uF coupling capacitor between the line transformer secondary and scanning coils. It's function is important because changes the negative going sawtooth waveform to AC, that is, symmetrical about zero. Without that component the line scanning would be one sided.
The other suspect component was the 0.25mfd line TB timing capacitor. It's OK.
Now that both timebase units are working it's time to turn my attention to the power supply units in the T5 and T23.

DFWB.
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Old 29th Jun 2021, 7:06 pm   #396
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Default Re: Baird T5 restoration project.

Although both power supply units are working OK some tidying up work is called for.
The T5 unit has a fair amount of surface rust. It's an easy job to remove all the components and give it a good clean up.
The 16 + 24mfd capacitor block in the T23 unit will be rebuilt in order to retain the original appearance of this made in 1938 unit.
HT rectifier valves are the Mullard IW4-350. Indirectly heated cathodes.

DFWB.
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Old 2nd Jul 2021, 1:07 am   #397
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Default Re: Baird T5 restoration project.

Hi David,
that's looking good.
The original resevoir for the HT was a wet electrolytic of the bolt through type.
See the picture of the one found in the T5 that I restored many years ago, I was able to restuff it. Though it was a pain to open up as basically it is like a giant finned heatsink inside.
The HT smoothing caps were of the dry type, in a green cardbaord box, as in your T23.
Of course the T5 that you are restoring had been overhauled just after the war, so many of the original caps have been replaced. Those post-war replacements are still very much part of the set's history though.

Cheers
Andy
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Old 5th Jul 2021, 9:42 pm   #398
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Default Re: Baird T5 restoration project.

The T5 power supply unit was tested on the work bench today. All the original TMC electrolytic capacitors have been refitted so the unit has the appearance as how it was in 1947 when the set was overhauled by the service department of the successor of Baird Television Ltd.

The Variac was used to bring the mains supply up in a cautious manner which is just as well as the HT voltage off load is almost 450 volts, the same as the maximum working voltage of the new capacitors.

HKS has informed that TMC stands for the Telephone Manufacturing Company.

Tomorrow, the power supply unit will be tested in the T23.

DFWB.
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Old 6th Jul 2021, 7:11 pm   #399
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Default Re: Baird T5 restoration project.

"Tomorrow, the power supply unit will be tested in the T23."

The T5 power has been on test in the T23 all afternoon and is working perfectly.
No picture on the screen of the T23 because the Cossor CRT has a four volt heater and the heater winding for the Cathodevisor tube supplies only 2.5volts.

The T23 power unit is also receiving attention. The 16mfd HT reservoir capacitor and the 16 + 24mfd cardboard block capacitor now have new electrolytic capacitors inside the cases and are ready to be refitted.

DFWB.
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Old 8th Jul 2021, 9:35 am   #400
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Default Re: Baird T5 restoration project.

The T23 power unit will be reinstalled in the set today. The attachment shows the original dual block capacitor has been been refurbished and the single screw 16mfd reservoir capacitor has also been given the same treatment. Except for the 50ohm surge resistor the unit now looks almost exactly how it was in 1938. Like the EHT unit it would seem the power supply unit was made in 1936.
The cardboard block capacitor is dated 10-36.

DFWB.
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