I'm not sure how to interpret that, but I have a couple of sets which still have their selenium rectifiers in situ, and they would seem to have gained some extra resistance over the years, or at least, their output voltage has fallen.
Inserting a silicon+R in lieu restores the HT to normal, so it looks like the selenium rectifiers are failing.
The EHT rectifier sticks (including triplers) used in the 70’s were usually made of selenium, the problems I had with them was normally complete failure in one way or another.
My thought was if the TV18 was selenium, it’s forward resistance could have risen with age and cause the poor EHT regulation on the ITT.
In the clearer light of day I have had another thought about the problem. Is the TV18 made for a smaller screen CRT (portable set) and cannot deliver the current required of larger versions of CRT’s.
I am assuming that both the TV20 and TV18 are 3mA, and of course PIV of 20KV and 18KV, but I could be wrong.
The TV18 in the set was marked TV18-S and the two TV20 I bought were TV20-S. Some general images of them on the internet are marked TV18-03, TV20-03 etc, so not entirely sure if they are all rated for the same current.
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Frank - Yep, if I'd read it a bit more carefully............ ??
My mind was a bit numb after reading something else, somewhere else. I hadn't cottoned on to the fact you were talking about EHT sticks - I was stuck in selenium HT rectifier mode! ?
Seeing the really superb picture Andrew's lovely ITT set produces makes my poor old BRC1500 look a bit second rate by comparison. The difference between a Mullard and a Mazda crt of similar vintage laid bare. Oh dear!
Maybe BRC should have fitted these with convergence controls!! ?
The later CRT’s used in the 1500 were not that good from new with poor focus, the phosphors were very coarse when looked at with the set switch off. It was a matter of fit what was available I presume, they were imports.
Whether that’s true of your Mazda CRT I don’t know though. I always found the Mullard CRT‘s much better for longevity than Mazda but that’s from a small number in the scheme of things.
Hi Frank, yes I've seen those very late crts- the phospor has been accurately compared to blotting paper! I think they were from Poland, presumably all UK souces for replacement tubes having been exhausted by that time. A lot of these single standard hybrid sets were already getting a bit long in the tooth by the early to mid 1980s, but were passable as cheap large-screen monitors for home computers like the conveniently black & white ZX81 or for cash-strapped pensioners who weren't bothered about colour.
Presumably there was some weakness in Mazda's manufacturing process that led to tubes like mine with poor focus.
You answered your own question; they were made by Mazda.
I don't know what it was about the boys and girls at Brimsdown but they were bloody useless at making CRTs, even valves. I give you from the 1930s, the AC/TP frequency changer with the self-detaching cathode - and many years later, the number of professional engineers who sang "Mazda tubes stay softer longer" to the tune of the jingle for their lightbulbs...
Oh yes, they did - I'd excised that from my mind! Having previously been the owner of a Decca TV/radiogram and currently an Ekco TV with FM radio, both indeed did have detached cathodes.
Like your set Andrew, mine - or at least the particular model (Alba T1520) - has a family connection in that it was our main tv until it was relegated to 'second set' status in favour of a rented Thorn 9000 series Ferguson Colourstar in about 1978. For all its imperfections I thoroughly enjoy using it. Even leaning over the back and getting that waft of not inconsiderable heat off the back cover, seeing the valves glowing in the depths and hearing that noisy frame output transformer all tickle the nostalgia buds. It's not a bad old thing really and definitely a keeper!
I wonder what the cause is of that problem of bad focus towards the edges... I have seen it before on several set's in the past. The emission of the cathode must still be ok as focus is good in the centre with reasonable brightness.
I did think it might be gas content in the CRT as the beam has to travel further at the edges, and any gas might defocus the beam of electrons. Could be total twaddle, but I can't think of anything else.
To understand the black art of electronics is to understand witchcraft. Andrew.
