Bush TV12 dark contrast/low CRT?

[EF80TVVALVE]

Some further progress this evening. I decided to sub the tube for the one from my D18 just to see how things compare. The Original CRT was a MW22-14C, the one from the PYE was a MW22-14. There are some slight differences in size notably the 14C was a wider diameter as I found out when replacing THE mask and CRT strap, the neck was also slightly thicker than the MW22-14.

The results are as shown, it gives a more black and white picture which I prefer to the blue hue of the 14C but it has some ion burn. The picture appears to be brighter with better contrast though. There is more shadowing present to the right hand side that I can't completely cure but the EHT is still only at 4.9kv (it's actually gone up!) so I'll wait until everything is working perfectly before doing a final picture set up, an EY51 is on the way.

One further thing it has proved is that it isn't the CRT causing low heater voltage due to a short. This tube also reads just above 5v but tested fine in the PYE before removal. Next I'll be looking at the chain to see if I can identify where the fault is and rectify, I'm sure this will have a further improvement on the picture.

I may still re-fit the original CRT after it's all sorted just to see what it finally looks like, from there I'll choose which I think is best and the PYE can have the other though I think by the time I'm finished there won't be much in it between the two.

[beery]

Swapping tubes around like that often works best if you put the tubes with ion burn in the sets with the highest EHT.
Gerry Wells called this "The Soft Tube Shuffle".

Later Mullard CRTs had a separately moulded faceplate, so they have slightly different dimensions to the earlier ones. At some point the faceplate mould developed a fracture which you can see as a curved mark on the edge of the faceplate (which has to be rotated out of the viewable area). That curved mark looks like a crack in the glass, but it isn't, fortunately.

Cheers
Andy

[EF80TVVALVE]

Wise advice there Andy, I'm sure once I have the EHT up a little more and the heaters sorted then it will be much better, it certainly wasn't bad in the PYE, that said it's likely the tubes will end up being swapped back anyhow as this was merely a test to see how the set performed with a known good tube.

Funny you say about the faceplate mould, I'm sure I've seen this on a few 9" roundies in the past now that you mention it. Never thought much of it but it makes sense.

[mark pirate]

Don't expect too much EHT from the TV12, mine has the later yellow spot transformer with higher EHT, it just about makes 6kv.
Running side by side with a TV22, the later set does have a brighter picture.
I would expect a rise in EHT once a new EY51 has been fitted, but probably only 500v.



Mark

[Heatercathodeshort]

Most TV12 series operate at just on or just below the EHT requirement of the MW22-14/14C. The picture looks very good to me.
I would not recommend handling tubes unnecessarily. The scan coil insulation becomes fragile after so many years and the tube glass is stressed during the process.

[Not directly a subject of this thread but just about a year ago a Mullard MW22-7 was brought over for testing by a friend. It was repacked in it's original box and placed back in his car. We chatted for a while outside my garage and then about 11pm, some four hours later there was a terrific boom! The gun assembly was discovered under the passenger seat the following morning...This is just a warning. Wear goggles when handling old CRT's. Glass does change it's characteristics Over many years.]

[EF80TVVALVE]

Still waiting on the new EY51 but hopefully it'll arrive today or tomorrow, the post has been a little slow lately! The last time I had the set on I could hear a slight occasional tick from around the LOPT so I'll also have to investigate that and see if there's anything untoward going on around there. We'll see what the final voltage ends up at after the EY51 has been replaced, I'll also test the other valves on the AVO and see how good they are.

Thanks for the advice John, I never really mess around with CRT's but as both sets are very easy to remove them from quickly I thought it would be interesting to see what differences were present. I'm always very cautious with handling such tubes and never stress anything, eye protection is always a must too especially considering the glass of these tubes is now over 70 years old

The picture is okay, it could be better though as I know the little TV12 is capable of producing a sharper picture. It's the lack of uniform focus that I'd like to sort amongst anything else, the centre of the screen can be made very sharp but as it reaches the outer edges it generally becomes very poor. It's had to tell on photos taken of the set but it's very noticeable when watching.

[EF80TVVALVE]

Small update on the TV12 - the new EY51 has been fitted, the EHT voltage is still very much the same. I decided to measure all of the heater voltages on the top deck to see what's going on, a few are close to their rated operating voltage, one slightly higher and some very slightly lower, V3 (frame osc EF50) is the only one that sits above at 6.55v. The CRT still sits at approx 5.5v, the PL38 is also at 28v which is 2v down on its data, is this enough to cause the valve to under-run? V4 (sync sep) is also at 5.7v which seems a little on the low side too. The PZ30 is also a volt under on the heaters and the PY31 is also half a volt under. I presume the larger the rated heater voltage the less effect a few volts less will have on the performance of the valve.

The decoupling caps seem okay in the RF deck, they're mica types on this set, I don't know how much more reliable they are than waxies in this application but I may change them for reliability's sake, certainly none have gone short but I don't know if they're compeltley leak free.

That's about it for today, I am going to check the emission of the PL38 and see how strong or weak it is, I'll have a bit of a think as to my next course of action, I'm not too sure if these differences of voltages on the heaters should be investigated further or just accepted.

[mark pirate]

A good experiment would be to run the CRT heaters from a heater transformer or a power supply, check the heater current at the same time.
I have been amazed how much difference 1 volt can make to the picture brightness!

Cheers
Mark

[EF80TVVALVE]

Hi Mark, I think that would be a good idea, if I can find what current the heater is drawing then I could say conclusively if it is the CRT at fault or a problem with the heater chain. The CRT that's in it currently tested fine in the D18T but with that said the D18T heaters could be running higher as I haven't tested the rest, only the CRT before removal.

I do have a gut feeling something isn't quite 100% with the heater chain though as whilst it's not impossible, I'd be quite unlucky to have two CRT's with heater shorts, the other valves being a few volts down when many others are spot on is also an indication. I've only had one other Mullard tube with a heater short and that struggled at just a couple of volts when in the heater chain, I'd expect others with a short to be quite similar.

[Graham G3ZVT]

Originally Posted by EF80TVVALVE

Funny you say about the faceplate mould, I'm sure I've seen this on a few 9" roundies in the past now that you mention it. Never thought much of it but it makes sense.

This 10 inch Cossor 108K had what I hope isn't a crack.

[mark pirate]

I'd be quite unlucky to have two CRT's with heater shorts

I have yet to encounter a 9" Mullard with a heater short, and have had a few!
The spec sheets are online, so should be easy to check current draw once the correct voltage is applied.

I had a set that was running the CRT heaters at 5.3 volts, the difference to the
picture quality once running at 6.3v was truly amazing, even the ion burn was far less noticeable.


Mark

[murphyv310]

Hi.
To get a little more EHT consider an EHT silicon rectifier for two reasons.
The small but noticeable heater current drain of the EY51 does use a little energy from the line stage also the forward voltage drop difference will give a couple of hundred volts more EHT, to coin a phrase "every little helps".

[EF80TVVALVE]

Graham, you can usually tell if it's a crack by looking at the suspected line at different angles, if the inside surface of the seam has a sort of mirrored effect then it's likely to be a crack, when such lines are made from casts they will be clear all the way through.

I've checked heater current, it's virtually bang on 300mA at 6.3v so the tube isn't at fault. I wonder if the EHT will come up somewhat when the heaters of the PL38 are returned to 30v rather than 28, I know the CRT will definitely improve when back to 6.3v. The next job will be to fit new decouplers and see if that improves things.

Trevor, I have considered this already, as you say there would be some increase in the EHT. I'm going to wait until all of the heater chain is back to rights first before I decide on such modifications but if I'm still finding the EHT to be low and causing issues then it will definitely be something to try out.

[EF80TVVALVE]

Hi everyone,

Well some time has been spent on replacing the heater chain capacitors but unfortunately no change from before with the CRT heaters sitting at 5.4v. The only other thing I can think to do now will be to individually clean all valve pins and sockets in case any oxidisation has developed between contacts causing the resistance to increase. Other than that I'm about all out of ideas as to what else could cause this problem, it's starting to leave me stumped especially after confirming that the CRT heater is okay. Any ideas would be welcomed.

[EF80TVVALVE]

I've now spent some time cleaning the pins of valves and the sockets, I also had to replace one EF50 socket due to one damaged pin which was making poor contact. Despite all of the work there's still no improvement in heater voltage and it still sits at 5.4v with the other lower valves still measuring the same as before. I've had a thought regarding the thermistor, the original was damaged and I ordered a new old stock CZ1 replacement online, could it be possible that this isn't quite right or even the wrong type? I'd suspect all valves to be slightly low if this was the case but as stated before most sit at their correct voltages other than a couple and the CRT.