CTV Ultra 3500

Certainly does look like one to remember, I just randomly pulled one of many line modules I have in stock and yes, same scenario. Although another couple don't have hunts buts those yellow Philips lozenge types.

In the days when we still used mono monitors in lighting galleries, one brand used to develop a solid vertical black line on the picture: quite narrow, but annoying. That was an electrolytic decoupling the line driver transformer primary feed, rated at 25v on a 24v rail. Sadly (for the electrolytic), the flyback pulse on the primary comfortably exceeded the working voltage. So they went o/c.

Did I mention the 24v rail also powered the video circuits...?

A snip of the circuit. C531 shown. Am I right in saying this capacitor is there to stop the transformer ringing as the transistor switches? It is still puzzling me as to how if affected the gain. When the fault was present I reduced the signal with a splitter and this set lost sync and the picture went flat and grey. the other side of the splitter fed my other 3500 on the bench upstairs which was completely unaffected.

C516 in the rc network damps the transformer ringing, the cap you changed protects the transistor against high voltage transients. 

@michael-dranfield Thanks Michael. I can understand how that would work. The set has now been running nearly three hours and is displaying a nice steady picture. I'm thinking of putting the back on tomorrow and letting it get warmer to see if it all stays that way.

@slidertogrid Think of the cap across the transistor as the condenser across the points in a car...

Open circuit caps can cause some strange symptoms and be hard-to-find

I recall a new Pye T30 that had "kinks" in side vertical lines that varied with scene brightness. It looked like an issue with the East-West modulator (110 degree CRT) but all the waveforms looked crazy.

What it turned out to be was a .56uF poly "greencap" that bypassed the +128 volt feed to the LOPT, right at the transformer. It was open circuit. No sign of physical damage.

I have certainly had a refresher on the 3500 chassis! I read the information from RR that Chris downloaded and I remember some of the faults and cures. Mike's analogy is a good one the electronic 'ping' you get when the current decays in a coil, that is what I was thinking as a ring but it's not a ring of course but a spikey Ping..  😀 I can see my old tech tutor rolling his eyes as I type that! 😏 

I’d best have a look and see if there’s one of them hidden in my set! I certainly don’t remember changing one of them in it…

@lloyd It seems to be a bit pot luck with this capacitor. My (well Mick's) spare working line panel, the scrap one I took the coil from and my other 3500 have all got yellow capacitors in that position. I haven't checked my 3000 but if it fails I will know what to look for! 

This set threw it's toys out of the pram again yesterday evening. Suddenly the picture went snowy, this time permanently. The gain had dropped and quickly come back a couple of times before. The gain had decreased when I had the line on the screen which I still can't work out why, but this was different, I still had snow when I removed the signal and this time I still had colour but the picture was noisy. 

I had a suspicion it may be the tuner but clouting it made no difference not even a flicker, I remember often a low tuner would come back if you gave it a clout. Then you could try resoldering the PCB inside carefully which would often cure it. If not it was off to MCES for a repair or exchange. 

In my case it was off to the shed for a look... Ah! Remember the KB featherlight that put up a fight and then went off with a LOPTY fart? I knew there was a reason to keep it! Tuner was soon replaced. I half expected there to be no change and to be perusing a bl00dy AGC fault, but no! My jumping to conclusions diagnostics was right this time!  All good! 

Now, a Jiffy bag, write the fault on a label and stick it to the tuner and post it off to Manchester.... Oh ! A slight flaw in my plan there...

This tuner is going to be one of those parts that I should bin but I know that I won't. It'll kick about on the bench Ad Infinitum.

 😉  

It’s always good to keep these things! The day you decide to bin it you’ll end up wanting it for some obscure part! My lock ups are full of stuff like that, well, one of them is, the other one is now nearly empty! One more trip and it’s done!! 

The set has produced another fault ! Running the set today the colour went off! Not surprising really as the decoder hasn't had any attention, two small black .47 uf Callins type capacitors were splitting and one had lost half of it's encapsulation. Replacing them made absolutely no difference of course. Tomorrow I'll override the colour killer and see what happens...

Overriding the colour killer made absolutely no difference. Luckily this time I wasn't faced with multiple faults like the previous 3500 decoder in which I had duff Transistors, capacitors and the Crystal faulty.

I put the board in the other set on the bench, insulated the bottom of the panel from shorts with a bit of cardboard hung it out of the back of the chassis and made a few voltage checks. I found no Emitter Volts on VT304 and it's a Lockfit! Always a good bet for a duff Transistor. I replaced it and away we go. Blimey! I had set aside three days! 😀 Returning the panel to the set and we still have colour. So on soak test again.  I told it - Any more of this and you go on the "untested" ex-rental pile!! 😉 

If VT304 was intermittent as they often were, a clue was the greyscale shifted very slightly green when the oscillator stopped.

@jayceebee  That is worth knowing.  I can't remember having many decoder faults back in the day.  I must have had a few, I can only remember power and line panel faults which is where most of the trouble arose I suppose.  It probably didn't help that the anode cap was at the bottom which meant it was very near the back of the decoder and RGB panels.

I was told Thorn / BRC put the tube in upside down on the 2K and 3K as they had no raster correction and decided pincushion distortion would be less noticeable at the bottom...  Is there any truth in this or is it an engineer's myth?

I remember seeing the odd 22" 3000 with the oddball not-quite-22" Mazda A55-120X that had more rounded corners like the 19" tubes. It seems a strange was to save a few quid on circuitry, to make a special tube? 

The anode cap at the bottom meant it was a prime target for any condensation that would run down the tube taking any muck and coal dust with it. I remember taking sets out of the van into the customers house and the screen being soaking wet with condensation. I kept a hair dryer in the van to dry them out before powering them up. It caused many 'funny' comments from the customer!

Then years later we had the same problem with Video recorders. How they would shred a tape if you didn't give them a waft! I honestly had a customer say to me once that the "Dew light" wasn't working on his VCR as there was dew on his lawn but the light was off... 🤣.

Very early 22 inch crt were A55-14X.

Now there's something I didn't know about: 55cm colour CRTs. I knew about 67cm ones as my parents' SABA had one (A67-200X).

EDAS 🙂 

Posted by: @slidertogrid

↑

I was told Thorn / BRC put the tube in upside down on the 2K and 3K as they had no raster correction and decided pincushion distortion would be less noticeable at the bottom...  Is there any truth in this or is it an engineer's myth?

That was my understanding, I was also told that Thorn were losing money on every 2000 they made and small savings such as the pincushion inductor and turning the CRT were partly to save money. Didn’t later 3000/3500 fit the inductor, I thought the PCB had the place for it but never fitted to earlier sets.

Theres no doubt the 2000 chassis was ground breaking in its design but the picture never impressed me compared to other hybrid dual standards available.

@nuvistor I'd heard that one about the CRT being mounted upside-down as well. Of course, we may all have read it in the same place...

Posted by: @nuvistor

↑

That was my understanding, I was also told that Thorn were losing money on every 2000 they made and small savings such as the pincushion inductor and turning the CRT were partly to save money.

I think that gets a mention in The Setmakers - that Jules Thorn reportedly said that "he was putting a ten pound note in every set".