1936 Baird T5 TV Receiver restoration
Look what I've volunteered to restore for a customer, a woodworm infested Baird T5 TV receiver. For all intent and purpose a new cabinet will have to be made for the set. The easy part of the restoration will be the electronics.
In some respects these pre-war Baird TV receivers were better than the sets made by EMI. The T5 employed a superhet vision receiver which has four stages of IF amplification using TSP4 pentodes. No RF amplifier, the signals from the aerial were supplied direct to the mixer valve which is another Mullard TSP4 pentode. The local oscillator is a triode valve of the AC/HL or MH4 type. As the vision intermediate frequency is low, in order to avoid a visible dot pattern a full wave vision detector is employed using a Mullard 2D4A double-diode. Another Mullard TSP4 performs the task of video amplifier. The positive going video is supplied to the CRT grid.
Sound signals are processed by a separate receiver unit. Four valves: TSP4 IF amplifier. FC4 octode regenerative detector. 354V AF amplifier. Mullard PENA4 output.
The first attachment shows the woodworm damaged control panel. The second the equally damaged cabinet floor and third picture shows that receiver and timebase shelf will need replacing.
It appears that the electronics need to be removed very soon or the cabinet will collapse.
Interesting that you mention about the full wave vision detector, I was reading the booklet in the attached link. The TV also uses a low IF and full wave detector and I didn’t know why. Question now answered and makes full sense.
It is an RCA document from 1939, an excellent read, I wish it would have been available or something similar when I was in my teens.
Full wave vision detectors were employed in Rediffusion cable receivers. Because the carrier frequencies were low, for 405 lines BBC1 4.95Mc/s and 8.45Mc/s for ITV there was a real possibility of a carrier dot pattern if a simple half wave video detector was used. I'd imagine that the British Relay Wireless cable TV receivers employed the full wave vision detector.
Bush made a TV receiver for Rediffusion and Top Rank cable TV systems. Model TM100?
In the GEC BT8161 pre-war TV set the vision IF frequency was only 4Mc/s and sound IF 500Kc/s so it was desirable to employ a full vision detector.
Yet we had the dot pattern with CTV, PAL being worse in this respect than NTSC due to the different offset.
Some sets did have a 4.43MHz rejector in the video circuit but that took a lump out of the video. Not that many viewers noticed or cared, it’s surprising what vision faults viewers will ignore.
Nice project David. Regarding the FW vision detector the TFE & STFE front ends in British Relay viewing units did employ that way of detection, earlier sets with valve front ends didn't so the sets like the M17/3 & M19/11 didn't.
The chassis assemblies look awful but in fact it's only surface rust and all the components are in good condition.
Worthy of discussion, perhaps elsewhere in the Forum is the regenerative sound demodulator employed in the later production model T5. The FC4 octode operates in a mixer circuit, rather like the frequency changer in a radio receiver. The sound IF signal is supplied to signal grid 4 and grids 1 and 2 function as a 500Kc/s oscillator The audio output present at the anode requires filtering to remove the IF and 500Kc/s components before it is supplied to the audio amplifier.
The Baird T23 is a TRF receiver. Like the T5 model we are discussing the sound demodulator is very similar, except the signal grid of the valve receives the 41.5Mc/s signal. The oscillator operates at 500Kc/s as in the later T5. However, the Mullard FC4 will not work in the T23. Instead, a Marconi-Osram MX40 pentagrid valve is used. Unlike the FC4 the MX40 does not have a suppressor grid.
The attachment shows the position of the three chassis on the slide-out baseboard.
Meanwhile the set is disintegrating nicely, see the attachment.
Coming up soon, the scary business of removing that huge CRT.
When I saw the first photos I was thinking is there enough wood left to support the electronics.
the big problem is finding the correct walnut veneered plywood to do the cabinet rebuild. I'm phoning and emailing all the known secondhand furniture shops and wood recycling businesses around Newcastle and Gateshead.
The CRT is most likely a post-war replacement. The original tube was the Cathodevisor 15MW1 and the tube fiited in this set is a 15MW4. Never come across this tube before. So how do I remove the tube?
One idea is to place the set on it's side and withdraw the tube horizontally. Or, in the manner as the tube was removed from the Baird T23, just lift it out vertically. Whatever method, extraordinary care will have to be exercised handling this tube.
The general consensus is that the CRT should be lifted out vertically. In order this can be done in a safe manner I've made a special cradle to keep the tube safe while it is out of the cabinet.
The plywood used to make the CRT cradle came from that Decca projection TV cabinet which many will remember was painted green. I still have the cabinet but that horrible green paint has been stripped off to expose a very nice walnut veneered surfaces.
It will be safe in its frame, the parrots will keep it company,
The infestation looks as bad if not worse than a certain HMV I had for a short while. David had done a fair bit to it including a base and radio panel. All was ok for a while then it all kicked off again. At that point it was passed on, I believe it may still be with Eric in Bishopbriggs. I don't relish handling that tube but the state of that cabinet means it must be seriously weakened. Let's hope that the tube is good and the whole job isn't a waste of time.
Three 2400mm X 1200mm sheets (8 x 4) sheets of plywood have been ordered, delivery today.
The 9mm sheet is top quality grade A and is to be used in the construction of the front and side panels. The 12mm and 15mm sheets are BB grade for the construction of the cabinet floor and chassis shelf.
But before any work can be done to the cabinet that CRT has to be lifted out.
Are you veneering the cabinet yourself?
Hi Frank, the 9mm plywood is furniture grade A and has a nice veneer pattern for the construction of the cabinet sides. The veneers for the front panel will be book matched in the same manner as the original. There's also a lot of detailing to attend to on the front panel. The plywood sheets were delivered this morning. These will be cut into more convenient 1200 X 1200mm panels for ease of handling.
Restoration work has restarted. A new shelf for the receiver and timebase chassis assemblies has been made. 15mm plywood is the material used to make this part. The first picture shows the panel marked out for various holes and the centre section which will be cut out for the CRT neck components. Before anymore work can be done to the cabinet the CRT will have to come and that's one part of the restoration I'm not looking forward to. The second picture shows the terrible condition of the original shelf.
But before the CRT can be lifted out the screen aperture panel must be removed. This has been done and now I find that the panel is not in good condition because those nasty woodworms have been feasting on it.
So that's another part to be replicated.
There is so little good wood in that cabinet it’s surprising it’s not fallen apart with the weight of the electronics, must have been some fat woodworms.
Only a few parts of the cabinet can be reused, the mirror lid can be saved but in reality this going to be a new cabinet. The tube is ready to be taken out, lifted out vertically and placed on the special stand and then stored in a safe place well away from the workshop.