I am going to have a bit of a clear out. So here I have a Marconi VT73DA. I was given this set about 20 years ago. I had heard of these sets from older engineers years ago and how unreliable they where, not was expected from Marconi! It's main fault at the time was an open circuit frame oscillator transformer primary. This I cleaned in isopropanol fluid which then allowed me to get to the broken winding, removing a couple of turns and re-terminating cured the fault. I went through all the wax capacitors and eventually had the set up and running. All the problems as described for this set where then evident. You can have a focused screen but no brightness or a visible picture but out of focus.
The construction of this set looks like it is home made! Tags or twisted wires all over the place. If you compare it with the Bush TV12 which looks properly manufactured, as it is, there is no comparison.
After doing all this work the tube was found to be as flat as a pancake, only visible in a blacked out room.
So 20 years later its on the desk again. It ran up with a raster just visible and slight noise from the speaker when the volume control is moved but no actual picture and sound.
This is , I think, a Birmingham set so will be on Band 1 Channel 4. Anyway a few pictures to start with and see how I get on.
Looks like an interesting set - hopefully the tube might improve a bit with use.
Laurence
After doing all this work the tube was found to be as flat as a pancake, only visible in a blacked out room.
Then this tube is a prime candidate for the nondestructive tube regeneration process pionered at vrat over 12-years ago. This process has brought many a "flat" tube back from the brink.
The method and steps to be taken, are detailed in the thread linked below.
https://www.radios-tv.co.uk/community/postid/20627/
CrustyTV Television Shop: Take a virtual tour
Crusty's TV/VCR Collection: View my collection
Nice telly!
One of these was the very first vintage TV I ever encountered, I was probably only about 12 or 13, and was wandering around Weedon antiques centre, and there was one there, priced £45, and it was missing its chassis! There was no back on it, so I could be nosey, and look at the tube. I’d never seen inside an old TV, so was really surprised to see the tube was round! I didn’t get the set, but I have been looking out for one ever since, always missing out on the odd nice one at RetroTech, or on eBay. One day maybe!!
Good luck getting it going!
Hi Lloyd, its yours if you want it, I'm in Hertfordshire. Anyway I continued to have a look at it and archived 40 volts of drive on the cathode, picture just visible in a blacked out room. Had to apply the full output of the Aurora to get this. The set appears to be as def as a post. Crusty your comment has been noted. I am now digging out my r.f. generators to check the alignment.
Generators I'll cover in the appropriate section
I have had a look at the no audio problem and the deaf i.f. Some of the i.f. cores are below the retaining spring. I'm guessing someone in the passed has tried to change the channel from Northern possibly to London as the cores are screwed into the coils. I have just done a quick spot frequency alignment and it is coming up. Now have sound and a sort of locked dim raster.
As you can see the tube is somewhat soft. More checking on the i.f. to do. I'll tell you how bad this design is, the sound output pentode is a Z77, EF91 to the rest of us, who thought that was a good idea??
I'll tell you how bad this design is, the sound output pentode is a Z77, EF91 to the rest of us, who thought that was a good idea??
No worse than using an EF80 or as in one Derwent TV I saw an ECC82.
Or a PCF80 in the Thorn 980/981 chassis. ECC82 😱 , that does seem to be asking a lot of it.
Shame about the CRT, is it an EMIscope? They had a reputation as bad as Mazda at the time.
John.
EF50 as a frame output valve in the Pye B18T...
Hi Cathovisor, I remember all the new hybrid colour sets using the new PL508 frame output valve. More power required for the larger neck tube and to drive the convergence circuits. ITT did not do this with the CVC5-9 variants, sticking with the PCL805 which was used in monochrome hybrid sets. When I first saw this I thought that is not going to be reliable, somewhat overworked? Time soon showed us that it handled the job fine! which it did.
@crustytv I tried this but first measured the heater volts with a true RMS meter and it was just over 20V ac. The heater looks normal. Tried with a d.c. power supply to get 300mA and this was at 19V d.c. I then applied 145V to the grid via a 56k resistor and this went up to 1.1volts across it, negligible current. Looks like there is know were to go with this tube. The picture does go a beautiful negative as you bring the brightness up.
I'm afraid it time for the set to go.
You can’t win them all unfortunately and the chances of finding a replacement or equivalent are probably very low now. Just out of curiosity what could be used as a replacement I wonder. Is there a suitable tetrode type with a similar neck diameter? Obviously it would need a suitable supply for the first anode.
John.
Have a look at the US ETF site for a possible replacement, not that rare in the US.
https://www.earlytelevision.org/parts_available.html
Have you tried bopping the cathode? Nothing to lose really....
But what a HORRIBLE chassis! What WERE they thinking??
Is this the same chassis as the infamous HMV 1807 ? Matt Spanner did an excellent article in the BVWS Bulletin about that model which looks to be the same? Volume 45 Autumn 2020.
He modded the horrid focus arrangement to make it work with higher EHT and ended up with quite a good picture capable of daylight viewing. The original focus circuit adjusted the EHT which meant you could have a bright out of focus picture or a dim in focus one. He was lucky in that his set had a good tube. Apparently most of these tubes didn't last beyond a few years. Probably because of the EHT always being low?
It probably wouldn't stand reactivation as such but maybe if you get the EHT higher and modify the focus circuit as Matt did it may improve with a run?
These were truly horrible sets back in the day. It is surprising that any survive! Nearly every manufacturer had its "Edsel" (or in the UK it's Allegro...) it seems...
Well, I thought I'd need a new CRT in my EKCO until I replaced the selenium rectifiers with silicon and what looked like a tired 70 year old CRT came alive with the increased EHT.
But is that s**tbox worth restoring? Or would you do it for the challenge, like a friend of mine did recently with an HMV PL chassis series TV? Mod it to what it should have been?
The original CRT in that HMV PL was strong, probably because the chassis spent so much time out of service and in the shop that it never did the long hours!
@irob2345 If you look at the under chassis picture you will see I have a silicon diode and 10 ohm resistor added to pep up the h.t. rail to the correct level. This is not a set I have ever wanted, just herd about them and there appalling reputation. It was not nice to see one! Yes the HMV set is an immediate relative.
Its been hard work getting this going but it is a working chassis. The Ekco TMB272 uses a CRM92 tube which could be fitted with a few modifications. This as I have said is of no interest to me so I'll put on the usual site to see if there are any takers. Its free if anyone wants it here!
