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CRT substitution?

 
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CRT substitution?

Post by Cathovisor » Tue Jan 19, 2016 2:40 pm

A quick one: is it possible to replace a Mazda CRM172 with a Mullard MW43-69? The Ekco T311 I bought last week has without a doubt the most spectacularly dead tube I have seen short of one that's had a brick through it. Initial thoughts are that a 21Ω resistor needs to go in series with the tube heater, the external coating would need earthing and a rewire of the CRT base would be needed.

Typical Brimsdown cr*p :qq1

Anything else, or do I just scrap the set? It has a little worm, but is technically quite interesting.

 
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Re: CRT substitution?

Post by sideband » Tue Jan 19, 2016 7:53 pm

They work extremely well as long as the LOPT hasn't disintegrated. It's one of the few sets I had that would even resolve the dark grey dot in the middle of the black square of the greyscale bars (testcard).

 
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Re: CRT substitution?

Post by Cathovisor » Tue Jan 19, 2016 8:21 pm

The LOPT area looks like this:
Ekco_T311_LOPT.jpg

Now, this set fails the Cathovisor Criteria on several fronts, viz:

  • It's not a Bush;
  • It has a rectangular tube;
  • It has ITV.
- but it is technically interesting. The manual almost seems to imply gated AGC, it uses a cathode follower to drive the CRT and it has Band II radio coverage, feeding a large 8" x 5" speaker at the front of the set. So, whilst it may be some time before it finds itself coming under the soldering iron, is it worth doing - considering I have a similar set in the shape of a Bush TV77?

Interestingly, the doomed Ultra VT8-15 I bought (doomed because of the munchies) has a 6K25 lurking inside. Surely this must be one of the very last outings for the thyratron in TV sets?

 
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Re: CRT substitution?

Post by Till Eulenspiegel » Tue Jan 19, 2016 8:57 pm

Only one problem substituting the Mazda CRM171/2 with a Mullard MW43-69 is that the 17" Mazda tube has a 35mm neck whereas the Mullard has a 38mm neck. Although in most cases it is possible to squeeze the coils on to the Mullard CRT neck. Some compensation for 6.3volt difference between the two tubes in the heater chain. An additional 21 ohm resistor in the heater chain will do the trick.

Till Eulenspiegel.

 
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Re: CRT substitution?

Post by Cathovisor » Tue Jan 19, 2016 9:11 pm

I think there'll also be a bit of tube base rewiring needed, plus the coating of the Mullard tube would need earthing.

Hmmm. Will this be joining the Ultra on the bonfire...? :aaq

 
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Re: CRT substitution?

Post by marc » Tue Jan 19, 2016 9:23 pm

Cathovisor wrote:Hmmm. Will this be joining the Ultra on the bonfire...? :aaq

Hi Catho,

That LOPT looks better than most Ekco's I've come across. :aaq
Go on give it a go, might be interesting to see how a foreign interloper compares with your shrubbery. :qq1

Marc.

 
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Re: CRT substitution?

Post by Terrykc » Tue Jan 19, 2016 9:34 pm

Till Eulenspiegel wrote: Some compensation for 6.3volt difference between the two tubes in the heater chain. An additional 21 ohm resistor in the heater chain will do the trick.
Agreed in principal but is it really necessary?

Mains voltage is only adjustable in 20V increments which begs two questions:

- what is the actual mains voltage for a current of precisely 300mA through the heater chain?

- what is the mains voltage actually being fed to the set, on average?

From this we can pose a third question: would the heater current actually be closer to the design figure without the 21Ω resistor ...?

 
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Re: CRT substitution?

Post by sideband » Tue Jan 19, 2016 9:55 pm

Cathovisor wrote:Hmmm. Will this be joining the Ultra on the bonfire...? :aaq


...letting the side down there!

Seriously if you do decide to scrap it, at least rescue the LOPT (that looks in exceedingly good condition) and the frame transformer.Someone somewhere is probably looking for one or both of these.

 
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Re: CRT substitution?

Post by Till Eulenspiegel » Tue Jan 19, 2016 9:59 pm

Fit the Mullard tube and measure the AC volts across pins 1 and 12 and if it is close to 6.3 volts don't bother with the series resistor.
The EHT smoothing capacitor can be discarded. The earthed aquadag coating on the Mullard tube will do the EHT smoothing .
21" models: Most early Ekco large screen sets were fitted with a Mazda CRM211 CRT which has the 12.6volt heater.
Due to supply difficulties some sets were fitted with a Telefunken MW53-20 tube which has the 6.3volt heater. Can't remember if they added the extra resistor into the heater chain.

Till Eulenspiegel.

 
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Re: CRT substitution?

Post by Katie Bush » Wed Jan 20, 2016 12:02 am

From that little snippet of a pic, the chassis looks very, very, similar to my Dynatron TV34.. That's obviously a bit posher than than the EKCO models, but the chassis is a good performer, and well worth 'doing' in my opinion. :aad

That said.... I must see about getting mine 'under the iron'... :aal

Marion

 
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Re: CRT substitution?

Post by Cathovisor » Wed Jan 20, 2016 1:03 am

Terrykc wrote:
Till Eulenspiegel wrote: Some compensation for 6.3volt difference between the two tubes in the heater chain. An additional 21 ohm resistor in the heater chain will do the trick.
Agreed in principal but is it really necessary?

Mains voltage is only adjustable in 20V increments which begs two questions:

Or even, raises two questions :qq1 ('begging' and 'raising' a question aren't the same)

Terrykc wrote: - what is the actual mains voltage for a current of precisely 300mA through the heater chain?

- what is the mains voltage actually being fed to the set, on average?

From this we can pose a third question: would the heater current actually be closer to the design figure without the 21Ω resistor ...?

It's a question I often ask myself when people get (IMO) excessively fussy about tolerances in circuit values, be it voltage, current, resistance or capacity!

The total heater voltage is 193.5 volts, having gone through my copy of the Television Engineers' Pocket Book (third edition). On the 240V setting there is an additional 132Ω in series, creating an extra 39.6V drop @ 300mA. So the theoretical correct mains voltage would be 233.1V for 300mA on the 240V setting. I'm not taking into account the resistance of the various chokes in the heater circuit, although these will be small. Round here the mains can get pretty enthusiastic, so on balance I'd be running it on the 250V tap! So to your third question, it would actually make matters worse if we omitted the 21Ω resistor.

What I noticed in the photo I took is that the EHT transformer doesn't sport a U25 EHT rectifier, which the manual says it should: it's something with a B9A base. Could the reason the LOPT appears to be in such good order is that a newer type has been fitted as a replacement?

I think I've been convinced to spare this set from the skip/flames/whatever. I just need to source a CRT in the future...

 
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Re: CRT substitution?

Post by marc » Wed Jan 20, 2016 1:30 am

Cathovisor wrote:
What I noticed in the photo I took is that the EHT transformer doesn't sport a U25 EHT rectifier, which the manual says it should: it's something with a B9A base. Could the reason the LOPT appears to be in such good order is that a newer type has been fitted as a replacement?

I would guess that the LOPT has been replaced with Ekco's later version using the U26 EHT rectifier valve (possibly from the TMB272) and judging by the fact the LOPT is ever so slightly cleaner than the rest of the surrounding housing and valves (no by much though).

 
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Re: CRT substitution?

Post by Cathovisor » Wed Jan 20, 2016 6:21 pm

As this is now becoming more about the set than tube substitution, I've split the thread and will now lock this one.


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