Hi Trevor,
To be honest I'm not happy with the appearance of those new side panels and now I have to decide to either veneer them or make new ones from plywood having better surface veneers.
Meanwhile, I've turned my attention to the cabinet mirror lid.
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.
Till Eulenspiegel.
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. Note the dark stain which is most likely to be oil.
Till Eulenspiegel.
Looks good, last time I did any French polishing was in school.
Frank
French polish is the ultimate finish, but I never was able to master the art of doing it.
To understand the black art of electronics is to understand witchcraft. Andrew.
I'm going to do all I can to save this panel. I know the owner of the set would like to retain as much of the original set as possible. Looking at the cabinet lid today, it is as Frank says it not at all that bad really. Have managed to remove much of that black stain, it's still there but now it blends in with the wood grain.
Till Eulenspiegel.
More work done to the cabinet lid. It's looking a lot better now. Some corrective work done to the damaged veneer at the front left side.
Picture of one I did earlier, sometime in the late eighties. The set is a very early production HMV 900. The 240/25 and 405/50 switch has been removed and I'm considering refitting the switch so that 240 line pictures can be demonstrated.
Till Eulenspiegel.
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.
Till Eulenspiegel.
I was looking at the circuit thinking how do they get enough scanning power then realising that the deflection angle of these early CRT’s was very small.
Frank
Hi Frank, the timebase power requirements for the Baird T5 are quite modest compared with modern CRT TVs. The CRT has a narrow deflection angle, say 45 degrees, and the final anode is supplied with "only" 5KV.
The self oscillating line timebase employs a Cossor 41MP triode, this valve is actually a tetrode with the screen grid internally connected inside the glass bulb. Early production 41MPs were true triodes with a balloon shaped bulb. https://www.radiomuseum.org/tubes/tube_41mp.html
Operation of the timebase. My idea how the circuit works is to assume that the forward stroke of the scanning waveform is created by a steady increase in current through the valve until a saturation point is reached. A high voltage pulse is developed across the valve which is fed back to the grid via the feedback winding, thus the flyback stroke is initiated. This is a function of -V = L . di/dt
Repairing the line output transformer. 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.
Till Eulenspiegel.
Hi Peter,
Don't know how the Ekco A22 picture got in there. Totally irrelevant to the Baird T5 restoration.
Work continues on the time-base 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.
Till Eulenspiegel.
Is the selenium rectifier the one with the nut and stud connections, colour brown?
Frank
Hi Frank,
it is indeed the type with the nut and stud. It's possible a similar type of rectifier exists in the EHT unit, if fitted the diode will function as a video DC restorer. Such a device is fitted in my T23.
In the T5 and T23 the video signal is AC coupled to the grid of the CRT resulting in total loss of the DC component. It is said the reason why the Baird TV system had such deep sync pulses was because the video DC component was not so well maintained as the M-EMI system was.
Till Eulenspiegel.
It is said the reason why the Baird TV system had such deep sync pulses was because the video DC component was not so well maintained as the M-EMI system was.
Till Eulenspiegel.
What an interesting thought!
Peter ?
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.
Till Eulenspiegel
At last after lots of interruptions including the recent Freeview TV channel reallocations I've finally made a start on the re-veneering of the newly made cabinet panels, starting with the CRT screen panel.
First task, lay on veneers around the chamfered CRT screen aperture.
That done, two pieces of wood veneer have been cut to size and will be glued into position.
The attachment shows the symmetrical wood grain pattern and this will be continued on the other two panels.
Since this picture was taken the veneers have been fixed into position using the toughest glue on Planet Earth.
Till Eulenspiegel.
A press to fix the veneers to the panel was set up last night. It consisted of a Formica faced board, an 88 pound (40Kg) Marconi VT50 and two heavy radios.
The contraption has done it's job so now the veneer edges have to be trimmed.
Till Eulenspiegel.
"Coming up next, the control panel."
And here it is, the control panel is prepared for applying the veneer.
Till Eulenspiegel.
The veneer you are using looks really good. Last time I veneered anything was 60 years ago at school. Animal glue in a pot to stick it on then French polished.
Frank
