Bush TV24 picture fade

[Ian - G4JQT]

I'm at the limits of my knowledge...

I have a Bush TV24 MK1 that I completely rebuilt about five years ago, all new capacitors, resistors, valves and wiring. It has a new old stock tube that tested 'very good'. The EHT transformer was rewound by Mike Barker. For a few years it gave a crackingly good picture. It only gets about an hour or two's use a week, and sometimes none at all for a month or so.

During the past year or two the picture sometimes gradually dims and de-focusses slightly over the period of about an hour. The height also increases somewhat too. What is frustrating is that this does not happen regularly. As often as not it will be still fine after a couple of hours viewing.

When the set has warmed up after five minutes or so, the EHT measures about 7.3 kV with the brightness turned right down. When it's misbehaving the EHT drops to about 5.8 to 6kV, again measured with the brightness right down.

I've cleaned the tube around the EHT cap (it wasn't particularly dusty), and changed the PL38 and PZ30. The new PZ30 might have made some difference, but it's hard to tell without another long soak test, but it certainly didn't come back up to the 7.3kV when I first switched it on this evening.

Maybe a new EY51 might be worth trying? I'm assuming the problem is in the top deck and nothing to do with the RF/IF sections...?

Do these slow symptoms ring any bells amongst the much more experienced here? What else should I be testing?

Thanks.

Ian

[German Dalek]

Hi,
You are on the right trip!
If you have a magnifying picture when moving the brightness control up, loosing sharpness, it will be the HV-rectifier.
Do you have a chance, to test the PL/PZ valves with another set?

Regards,
German Dalek

[Ian - G4JQT]

Thanks for the reassurance GD.

Yes, I swapped the PL38, not much difference, and the PZ30 and EHT back up. But it comes up after it's been switched off for an hour or so with the 'bad' PZ30 anyway. Not given a good soak test yet with replacement PZ30. But the set doesn't even get daily use!

I think I read somewhere that the PZ30 does get hammered in these sets, but does a bad one 'recover' for a while when off and then slowly loose emission after some use?

I've tapped and bumped both chassis, wobbled all the valves and the picture and sound are completely stable, so it's unlikely to be a degrading connection somewhere.

Regards,

Ian

[Panrock]

The height increasing points to a general decrease in efficiency of the line output transformer with heating. All windings are affected. This in turn would seem to point suspicion at the condition of the core, which may have deteriorated with lack of use. The variability you noticed may have been caused by mains voltage changes.

But I am really guessing. I seem to recall Helder Crespo has done some rigorous research on this question.

Steve

[Ian - G4JQT]

Thanks Steve.

I think Mike cleaned and worked on the laminations during the rewind about 5 years ago, so I hope the LOPT is OK, although that was one of my fears.

I have investigated the effects of mains fluctuations by connecting the set to a variac, and adjusting the mains voltage did have some effect, but it wasn't significant, certainly not at the range of mains variations we get here.

I'm clinging on to the PZ30 being the culprit...

[Helder Crespo]

Originally Posted by Panrock

The height increasing points to a general decrease in efficiency of the line output transformer with heating. All windings are affected. This in turn would seem to point suspicion at the condition of the core, which may have deteriorated with lack of use. The variability you noticed may have been caused by mains voltage changes.

But I am really guessing. I seem to recall Helder Crespo has done some rigorous research on this question.

Steve

Hi Ian and Panrock
I did study this issue with the goal of finding (at least in my case) whether the overwind insulation or the laminations were to blame, although I didn't yet publish anything on the forum. From my experience with two different TV22 LOPTs that have been submitted to two different treatments, the main conclusion is that the problem is clearly in the EHT overwind insulation. Its effect is different from the usual ballooning from a low emission EY51 rectifier, and it can be solved by heating the overwind from within by passing a DC current of around 50mA through it for several days. This process not only helps remove the water/damp that invariably accumulates deep within the wavewound overwind over time, especially in sets that don't get frequent use, but it can also fix issues with overwinds that have been fully stripped of their pitch (using, e.g., the white spirit bath method advanced by Dr. Hugo Holden, aka Argus25) and re-pitched or varnished but where the varnish hasn't fully dried in the inner layers, as was my case. My varnished LOPT suffered from this issue for three+ years after treatment, simply because there was still undried varnish below the dried surface. The conductivity of undried varnish is high enough to cause significant losses similar to a shorted turn. Furthermore, if you use urethane varnish, the varnish curing process that takes place after solvent evaporation (i.e., drying) actually uses up residual water that may still be present. But this can only happen if the varnish dries in the first place. Anyway, the good news is that the DC heating method effectively solved this issue. I must emphasise that these findings totally agree with those of Heathercathodeshort in his post devoted to LOPT saturation in a Ferguson TV, although his heating method was different (intense heat from a heat gun applied to the outside of the overwind).

So, in short, in a set like yours (fully recapped and with good emission valves) I totally recommend that you pass a DC current from a bench PS through the overwind for 2-5 days. You should be able to notice the improvement in behaviour in 1-2 days already. The heating can be done easily on the TV22/24 without having to remove the chassis or the LOPT - just get the screening can lid and its back cover out and attach the PS leads to the EY51 anode and the PL38 top cap. I have successfully used 50mA, which dissipate around 2.7W in my overwinds.

In my case I could not be happier. This simple procedure fixed the EHT decreasing over time as well as its excessive dependence on brightness and contrast in both of my LOPTs, which again have been subjected to quite different treatments: one has been rewound (primary/secondary only, as usual) and its overwind has been repitched; the other has been fully stripped of pitch (white spirit bath) and then varnished, but all windings are original. Despite the two very different treatments, both LOPTs had exactly the same EHT problems, which has intrigued me for quite a while until I finally decided to use the DC current heating method, even on what I believed was a permanently sealed varnished LOPT. Noticing again the smell of solvent emanating from the warm overwind (even though some three years had passed since I varnished it) was also a clear indication that further drying was finally taking place.

if you decide to try this simple but very effective method it would be great to know how this has worked out for you.

Kind regards
- Helder

[Helder Crespo]

Just a quick note on the current heating method: this has been reported on the forum several times, usually as part of the waking up procedure for a set prior to connecting it to the mains or a variac, and some people would not even think of powering up a set without passing current through the overwind for some time first. This is normally done on the original LOPT with its usually cracked (hence compromised) pitch insulation. I am not aware of other reports of this method being applied to repitched or varnished LOPTs (I assume that Ian's LOPT has been repitched) but I found out that it effectively solves EHT issues in treated LOPTs too.

[Ian - G4JQT]

Thank you for your detailed reply Helder.

Yes, my LOPT was repitched after the rewind. It gets very hot in that enclosure and the pitch does sag so I have kept but removed the lid.

If the new PZ30 does not fix the problem I will certainly do the 50mA trick as you suggest and report back. But if it works, is it a permanent fix? Presumably if there is moisture ingress the process needs repeating every so often, particularly if the set isn't in frequent use?

Regards,

Ian

[Helder Crespo]

Hi Ian

I manage the heat issue with a reversible fan mod (the fan is extracting hot air from around the can area). You can see it here:
https://www.vintage-radio.net/forum/...7&postcount=41
This is of course optional and some people prefer to keep their sets fully original (as I do to, but again this is fully reversible and beneficial, in my opinion): with the fan in place I don't have to open the top of the can (this can stress the bakelite case) or remove one of its sides, which is great because in my sets the reduced screening negatively affects the bottom of the image (this has also been observed by other members).

Very good point regarding how long the fix lasts. The fix is not permanent unless you then seal the overwind using e.g. anticorona spray from RS. This approach (current heating + spray sealing for a more permanent fix) has been reported a few times in the forum, namely here:
https://www.vintage-radio.net/forum/...9&postcount=39
and here:
https://www.vintage-radio.net/forum/...2&postcount=13

I haven't yet sealed mine as I first want to make sure I got rid of as much solvent and moisture as possible (e.g., I see no further improvements from heating) prior to this. So my plan is to leave the LOPT with the current on for ~5 days (I haven't done more than 1 consecutive day as I don't want to leave it unattended) and then quickly spray it while hot to prevent moisture ingress in between. Right now, I've had to repeat the heating process after leaving the set off for a few weeks (summer break). But it's also good to know that it can be repeated if necessary, and it seems to give similar results each time.

Kind regards
- Helder

[Helder Crespo]

Originally Posted by Ian - G4JQT

If the new PZ30 does not fix the problem

Hi Ian
I too have tested and replaced all valves in the main deck in my previous attempts to solve the EHT problem, including trying a few NOS PZ30s. I ended up with a nice stock of NOS PL38, PZ30 and ECL80s as I tried 2-3 of each, which did not solve the problem. Hopefully these will be useful for two other sets I still haven't tackled, and as backups/replacements for the present set.
- Helder

[Helder Crespo]

Originally Posted by Ian - G4JQT

During the past year or two the picture sometimes gradually dims and de-focusses slightly over the period of about an hour. The height also increases somewhat too. What is frustrating is that this does not happen regularly. As often as not it will be still fine after a couple of hours viewing.

Hi again Ian
Just to clarify, I was experiencing these very same symptoms (decay in ETH after one hour or so every other operation cycle) together with excessive dependence of EHT on contrast and brightness, which was always there regardless of the decay occurring or not. This got me thinking that perhaps fluctuations in EHT together with a degraded insulation could be producing some temporary/reversible breakdown that could take the LOPT in the wrong direction from time to time. Like in your set, this would not occur every time and seemed to be reversible whenever powering up the set from cold. I had also tried another original metrosil as well as an adequate VDR replacement, to no avail. I had also replaced the EY51 with yet another NOS one and tacked a HV diode across it just in case. Nothing. The DC current heating was the only thing that actually made a difference here.
- Helder

[Ian - G4JQT]

Thanks for your replies and suggestions Helder. But curiouser and curiouser...

I have had the set on and off for an hour or two at a time today and the EHT has done this (with brightness at minimum) :

Switch on 7kV. 20 minutes later 6.4 kV. 20 minutes later 7 kV. watch an entire episode of Randal and Hopkirk and it's now 7.4kV. Picture holding up too so I don't think these voltage measurements are artefacts of the testing equipment.

Maybe using it has a similar effect to running 50mA through the LOPT! I'll have to see how things go.

Thanks again,

Ian

[Helder Crespo]

Hi Ian
Yes, that totally happens and I've seen it with my repitched LOPT, especially with the cooling fan off, but not so much with the varnished one since the latter is more sealed (even though not perfectly so) and it's harder to drive out moisture and/or solvent by heating it from the outside alone. The temperature inside the screening can can get quite high (I've measured from 60 to 65º outside the can) and this will of course increase the temperature of all internal components, LOPT included. In this situation, the overwind is invariably heated from the outside (resistive heating is negligible in the overwind under normal operation, and most of the heat is in fact generated by the neighbouring PZ30 and PL38, as well as within the iron core). However, this process takes too long and is not nearly as effective as heating the overwind from within. Not to mention that you're submitting the two most expensive valves in the set to unnecessary stress and wear just to warm up the LOPT.
- Helder

[Helder Crespo]

Originally Posted by Ian - G4JQT

Switch on 7kV. 20 minutes later 6.4 kV. 20 minutes later 7 kV. watch an entire episode of Randal and Hopkirk and it's now 7.4kV. Picture holding up too so I don't think these voltage measurements are artefacts of the testing equipment.

Hi Ian
I'm curious about one thing: how does your set behave, even when the EHT is stable at 7.4kV, as you increase the brightness?
- Helder

[Ian - G4JQT]

OK. Switched it on this morning. After five-minute warm up with brightness right down, EHT 7.8kV! With test card C set to good brightness and contrast, EHT falls to 6.8kV. Will leave it on while I'm in the house for the next few hours and will report back...

[Ian - G4JQT]

Incidentally, while leaving the set switched on for relatively long periods I always have the brightness to minimum. Is this the best setting for these tests?

[Helder Crespo]

Originally Posted by Ian - G4JQT

Incidentally, while leaving the set switched on for relatively long periods I always have the brightness to minimum. Is this the best setting for these tests?

Hi Ian
I don't know whether that's better for tests or not. I usually set the brightness for normal viewing (e.g., using TCC) or a slightly higher level and just leave the set like that all day. I also like to see how the various image parameters evolve over time (for example, I've done a few time-lapse videos of the screen that also show the measured mains voltage and where you can clearly see effect of mains fluctuations on the image). The larger beam current (compared to having the brightness at a minimum) results in a higher load to the LOPT but that's supposed to be part of normal operation anyway.
Looking forward to the results of your tests.
- Helder

[Helder Crespo]

Originally Posted by Helder Crespo

I usually set the brightness for normal viewing (e.g., using TCC) or a slightly higher level and just leave the set like that all day.

Quick note: I use the TTC to adjust normal brightness and contrast. I then switch to a variable programme for long run times (my main source is a set top box) to avoid screen burn-in.
- Helder

[Ian - G4JQT]

After a few days of more use that the TV24 usually gets, the EHT seems to have settled down. With brightness right down and after five minutes it's hovering around 8kV, then slowly falls to about 7.7kV. Normal viewing brightness brings it down to about 6.5 to 6.8kV.

All very odd to me, but maybe it makes more sense to you Helder!

I've put the back on now and will hope for the best, but I've been here before...

[murphyv310]

Hi.
Surely the hotter the LOPT gets the resistance of the wire, especially the overwind will increase and the EHT drops accordingly, even on a LOPT that has no moisture ingress? I eventually gave up trying to get a stable EHT with even rewound LOPTs and fitted a Thorn Jellypot, which I stabilised at as little under 9kv which stays there no matter what the brightness is set too.