B&W TV Thorn all solid state large screen B&W chassis

Service manager where I worked, Rory Moore, used to say "If it lights up and moves, they'll be happy"!

There are plenty of examples of features in TVs that went un-noticed. One of my favourites was Philips' "Power Focus" in the early 60's. It modulated the focus voltage so that the CRT beam was in focus right across the screen. But unless you were technically minded and knew what you were looking for, you'd never see a difference. Philips dropped that idea.

The competition, AWA, touted "Deep Image". This marketing slogan was very effective, despite there being absolutely no technical basis for the claim. This is an AWA P1 portable from 1963

@irob2345 Well the image is “Deep” inside the mask. Ok I will get my coat and leave quietly. 😊

Ah Frank, you could have worked for AWA's advertising agency!

Many years ago, one of the oil companies, trying to differentiate their product, had this catchy jingle with the words:

"It's got Butane, Iso-Pentane, and Toluol too!"

Not an invalid claim, but not a unique one either!

Another company countered with:

"No Foolium, no Bullium. Just pure clean petrol"

I can't see much in the way of Thorn brushstrokes in that circuit at all apart from the drawing of the diagram. The frame osc looks to be a much simpler version of the type used in a lot of Thorn chassis over here around the time this circuit diagram was drawn, 18/6/1974. The PSU is unusual for having pass elements in both +ve and -ve. I see what you mean about the lack of boot strapping, very minimalist and also there is no frame linearity adjustment. That saved a pot too🤣.

The no lin pot is interesting, with current feedback you really don't need it. The Oz HMV F series (very far from a minimalist design!) from the 50's had current feedback and no lin pot.

Frame osc is Oz Pye. They used a BRY56 in place of the 2 transistors. And no lin pot.

Talking about circuit design "brushstrokes", there is another Pye one in this circuit and that is the technique of connecting the collector load resistor of the complementary output stage drivers to a higher voltage so as to avoid the need for bootstrapping. Perfectly legitimate and it saves an electro and a resistor! Pye here did that a lot.

I had forgotten that when I made the No Bootstrapping comment.

The reason for the upside-down voltage regulator is to allow a cheap and readily available 2N3055 NPN power transistor to be used as the pass device, with its collector (case) mounted directly to the heatsink. This was another widely-copied Pye technique.

Well I haven't brought the chassis up yet - I need a selenium stick rectifier. The old one failed and melted its insulator a long time ago.

But I thought I'd try my luck with the Made In England Ediswan A44-120W/R so I hooked it up to my HMV Y2 because it was convenient.

I wasn't holding out much hope, it looks like a very high hours TV, not unexpected from an ex-rental..

But YES! A nice bright picture! Focus tap could use a tweak but it looks good!

So I think I'll save this set.

A previous respondent commented on the substantial screening around the LOPT. Apart from keeping stray wires away from the 20kV, remember we had Band 1 VHF channels here (Melbourne and Brisbane once had Ch0 on about 42 MHz) and reception from the very nearby rabbit ears was expected. The screening helped to reduce radiation of the scan harmonics.

Disappointingly, firing up the HMV after a 6 week hiatus its 25" CRT looked sad and tired. It got a lot better after a 10 minute run, but I might look around for a better CRT for this rare and valuable TV. Pete in Albury reckons he has a good one in the basement.

I've got a magnetic cartridge on the way for the vinyl player in this unit. The existing preamp just needs a simple mod to suit a mag cart, it was an option when new. The existing Sonotone 9TA ceramic has the usual rubber bridge disease and sounds horrible.

Looks like my selenium stick rectifier is actually made of unobtainium!

So I think I'll solder 20 x SMD 1.2kV 1A fast recovery diodes in series and put them in a glass tube.

Anyone got any better ideas for a diode that can safely handle 20kV?

Or I might go old school and fit a 1S2 and a loop around the LOPT core.

Posted by: @irob2345

↑

Anyone got any better ideas for a diode that can safely handle 20kV

BY8420

Thanks for that, I didn't know that part existed.

Still not easy to source, but I'll find some!

2CL77?

Thanks, much easier to source!

I have some on the way.

Thanks fellas, diodes have arrived.

The day after I tested the CRT I stripped the TV down and gave everything a good bath (bar the speaker).

It's had time to dry out on the patio table, with some 30 degree days here.

I soaked the A&R mains transformer in polyester lacquer because when they were new and under warranty these transformers would start buzzing after a couple of weeks in service when the winding's lacquer failed.

It was pointless fitting a new one, same thing would happen. So I used to carry a spray can of Instant Estapol in the van kit. I'd give the tranny a good soak with it and advise the owner not to switch it back on for a couple of hours. This technique always resulted in a permanent fix.

So this weekend it goes back together and we'll see what happens.

That will be the newest TV in my collection, my Kriesler colour set is older!

Anyone used these diodes before?

Is the jagged stripe the cathode?

Everything was badly corroded - I had to cut and join the wire from the LOPT.

I put 2 diodes in series and potted them in the original insulator with hotmelt glue.

Let's see how that goes.

@irob2345 I have not seen them before but that would be my thought. Soon find out.

Yep, stripey end is the cathode end. I hear your query, though- some time back, I was enlightened the hard way that many LEDs have cathode to package flat, some have anode to package flat... At least that only incurred a disappointing lack of lumens, as opposed to a destroyed transformer or detonated capacitor.

I keep various of the 2CL69 onwards series in the spares box for 'scope maintenance and similar high voltage, low current tasks, there's a table of the various different cathode squiggles to identify the PIV and series numbering.

https://www.cdiweb.com/datasheets/edi/2cl69-77.pdf

I expect that I'd be teaching you to suck eggs by saying this, but for the elightenment of others note that many rectifiers of above about 1500V PIV or so actually have several junctions in series under the encapsulant. With particularly high voltage types, the number of silicon junctions means that they resist testing with low-voltage ohm-meters and component testers. Hence the high forward voltages quoted in the linked table.

Well a couple of weeks back I assembled this TV after its bath. 1.5 amp fuse input to the bridge was blown. The 33 volt rail measured a few hundred ohms each way - I had to leave it at this point.

Then today I fired it up on the dim bulb and got a noisy raster and no sound.

Open circuit speaker, unusual size and rated 68 ohms.Drove down to Jaycar and picked up a smaller speaker, being the biggest they had that would fit in the space. Made up an adaptor from clear perspex.

68 ohm speakers are made of unobtainium, I was stuck with 8 ohms. A look at the amp circuit showed that it should be OK to drive the lower impedance if I increased the coupling cap from 47uF to 220uF. The transistors have 10 ohm 1 watt emitter resistors, pretty safe.

Since the bulb glowed a dull red, I bypassed the bulb, connected a signal to the antenna and got a picture. Not a great one, but a good start.

Looks like someone has tweaked all the pots - RF AGC adjustment made a huge improvement.

So I let it run while I prepared the week's meals. After a while, some line pairing and loss of height. Then it came good by itself. Trying the obvious first, I tapped the Height pot. And that was it, just a noisy pot. After running it up and down a few times, the pot no longer showed any disturbance when tapped.

Next, I set the 33.5V rail correctly (it was set at 37 volts - no wonder the EHT rectifier had fried!) and set the video level correctly. Nice bright pictures now!

A buzz in the sound was fixed by giving L9 a tiny squish with a plastic alignment tool - see the picture - did you guys ever see this cheapskate idea in the UK? They were used first by Astor I think, to implement the rigorous adjacent channel trapping needed in Oz TVs.

I loomed up the messy wiring, swung the chassis back in and screwed in into place - and got no picture. Fault tracked to the CRT cathode wire on the base PCB, a bad crimp in one of those wire to board pins.

This very simple design actually works very well - a lot better than the 1500-based hybrid design that it replaced. It runs cool, gives a sharp picture and it holds black level correctly.

This is the only Thorn T series chassis I've ever seen - or am likely to see again!

@irob2345 Is L9 that flat black coil at the top of the pcb next to the polystyrene cap and green cap? If so I have not seen that before, very cheap skate.

The set gives an excellent picture.

My opinion of the 1500 chassis was cheap to make and not the best performing set, however it was rented by the 1000’s and most customers were happy with them. Well liked in the trade as they were very easy to fix.

I was wondering if that squish-coil idea was used anywhere else. Yes it is that black coil. Nothing technically wrong with it - I certainly had no trouble tweaking out the buzz in the sound.

I'm told that this chassis would never had made it into production unless it was cheaper to make than the 1500-based R2 chassis. Looking through that circuit, you'd be hard pressed to suggest further simplifications.

One slightly annoying aspect when watching moving pictures on it is the vertical linearity squash at the extreme top and bottom of the raster. It looks like the S correction is over-done. That would have to do with C57 and C58. The Philips C1 has a more complex network in the feedback path.

I soaked the little mains transformer in polyester lacquer for a week because buzzing transformers were an issue with this (cheap) brand of transformer in other TVs when the poor quality impregnation lacquer gave way after a few weeks operation. It worked, it's completely silent.

Today I let it run on a test card while I made a steak and kidney pie (domesticated aren't I? There's just me here now).

A once - a - second "flick" started happening at the top of the screen. Putting a finger on the aquadag stopped it. "Aha" I thought. "A bad dag earth!"

Turned out to be a crack in the CRT socket board, on the track linking the dag earth back to the chassis.

Duly bridged. Fixed.

Focus on the Ediswan CRT is reasonable, but adjusting the focus pot (yes it has one!) makes no perceptible difference despite the voltage at the CRT pin going from 0 to 440 volts. Never seen that before.

We didn't see many Ediswan B&W CRTs here, this is the first one I've seen. By the time this TV was made all the local CRT makers (Anodeon, AWV, Philips and Thomas) had shut down or were in the process of doing so.

I think it's time to clean up the cardboard back and re-install it. Another rare and unusual TV restored!