Bush TV22 LOPT problem

[Keith]

Argus25
Thanks for the the quick response. A previous "repairer" clearly got it wrong - I imagine the width and/or horizontal linearity would have left something to be desired!

[1100 man]

Originally Posted by Argus25

Well done. Of course before you power the transformer up in circuit to generate EHT it must be re-coated. If it is powered in a set with no coating at all, corona discharge is a big risk.

The first transformer was the Pilot one. I bought some 'insulating varnish' from RS which was specified at 60Kv/mm. As I couldn't get the core apart, I removed the wax with both bobbins still in situ. Rather than dipping, in this instance, I decided to brush it on. Over a couple of days I built up 10 coats sitting it near the woodburner to dry off between coats.
Working on 60KV/mm, I thought I would have got at least 0.5mm build up. Even 0.25mm should give 15Kv insulation in theory. However, with the transformer working, I can draw a large arc from the surface of the EHT winding with a screwdriver. This was a bit of a surprise so all I can do is to add more coats.
What does the spar varnish perform like as an insulator? Can you draw an arc from the surface of the overwind?

The LOPT I am currently working on (VT161) has now been denuded of it's wax coat. The original problem was arcing near to where the wire enters the EHT winding (PL81 top cap end) I need to power it up so I can see exactly what is happening. Obviously, I don't want to insulate it at this stage, so my plan is to power the line stage from a variable power supply to reduce the energy in the transformer and prevent too much arcing That's the theory anyway!

Keith, I found after a day or so the white spirit seemed to loose it's softening power. I guess it gets saturated with wax so I renewed it half way through the process.
Good luck with your one,
All the best
Nick

[Argus25]

Originally Posted by 1100 man

I can draw a large arc from the surface of the EHT winding with a screwdriver. This was a bit of a surprise so all I can do is to add more coats

Nick, I would advise never to do this, to put a metallic object close to a working overwind in a vintage valve TV, regardless of the type of insulation it has, it runs the risk of damaging the overwind if an arc strikes.

The 0.5mm or so of varnish should be fine, as there is nothing in close proximity to the overwind to arc to. The application of the overwind insulation, (pitch, wax or varnish) is to prevent localized effects at the surface of the winding with corona discharge to the atmosphere around it with humid atmospheric conditions, and localized surface tracking and to prevent the ingress of moisture into the winding. It is not supposed to insulate the overwind from a grounded metallic object placed close to it.

There have been some overwinds manufactured that could withstand a grounded object placed on their surface, these are overwinds sealed in at least 5mm or more of fibreglass resin or those on modern completely potted assemblies in transistor TV's, but vintage TV loptys are not like this and rely on a good spacing around the winding to nearby objects. So avoid putting a screwdriver anywhere near them.

One thing other thing to watch out for:

Avoid any sharp edges, for example where the EY51 is soldered in avoid any sharp spikes of the leads projecting from the solder tags, sharp edges on the connections result in increased localized field intensities an encourage arcing or corona discharges. If there is any tendency to this you can put small blobs of black non acid cure silicone rubber over the solder joins where the EY51 connects and the EHT lead solders in. You can check this by looking into the EHT cage in the dark for any violet colored corona discharges.

[1100 man]

Hi Hugo,
Thanks for your input and advice. I am talking hand held, insulated screwdriver, not connected to the chassis. Although I guess that potential wise it would be much the same.
I have found this to be a useful technique in the past to pinpoint where corona discharge is taking place but I realise that large arcs are to be avoided!
There is leakage from the transformer in question as there are classic white dots on the screen, but I've not been able to pinpoint exactly where from yet. I feel that the varnish coating is not yet sufficient so will add some more layers.
All the best
Nick

[Argus25]

Originally Posted by 1100 man

There is leakage from the transformer in question as there are classic white dots on the screen, but I've not been able to pinpoint exactly where from yet. I feel that the varnish coating is not yet sufficient so will add some more layers.
All the best

Sounds like a good plan to add more varnish. It can be hard to find where there are small discharges generating those white dots, Listen and look in a very quiet dark room might help find it. Also a huff of humid air from your mouth via a long plastic drinking straw might exaggerate it, if blown into the right area.

[Argus25]

Originally Posted by Keith

In the meantime, I've been re-forming the electolytics (OK so far) and examining some of the very dodgy repair work done over the years..

Keith,

I'm not an advocate of re-formed electrolytics, partly because most of the old electrolytics I have opened over the years for re-stuffing have severe internal issues that make them fail or be untrustworthy.

In any event there is a superior way to re-stuff electros which is seldom done, it is the un-stuffing that is the problem.

Most people cut the aluminium canister within a cm or two of its base. This severely weakens and compromises the strength of the thin aluminium cylinder. The aluminium electrolyic canister gets its integrity and strength from its flat top at one end of the tube and the circumferential folded lip at the other, which retains the rubberized base and contents.

The better way to remove the base is to place a hand file parallel to the base and tilt it just about 5 degrees and carefully hand file away the folded lip. As it gets thinner and the base material shows in places it can often just be peeled that that point. Then the base and capacitor contents removed from the canister. Then the edge of the lip can be smoothed just a little with a fine file and then with 600 grade paper. This way the circumferential fold at the canister base is largely intact.

Ideally then a new base is machined of phenolic material about 5mm thick or more with new tags which can be done with screws & lugs. But without the facility, to machine the new base, you can save the old base and simply re-fit it into the area it was previously in with black non acid cure silicone rubber. This method also works well for the restoration of radio vibrators with aluminium canisters and folded lips also.

This is by far and away the better method to rebuild chassis mount electrolytics. Of course if they are just standard sizes etc there is the option to simply replace with new, but in the TV22 and other sets, as you know, there is the unique large capacitor canister. I have seen so many of these capacitors in TV's and radio restorations damaged by people just sawing through them near the base in the rush to re-stuff them.

Scroll to page 5 of this article to the photos and look carefully at the capacitor canister, you can see that it has been un-stuffed using the method described above, and the replacement base (made of a white insulating material) where screws have been added to create the new terminals for the re-built capacitor:

http://worldphaco.com/uploads/BUSH_TV22.pdf

[Keith]

Argus25

Thank you for the advice. It certainly sounds like a superior method for re-stuffing electolytic cans.

[Keith]

Progress. There is light at the end of the tunnel (although not yet at the end of the CRT!). The LOPT cleaned up nicely after a couple of days in white spirit and a bit of gentle help from a brush. I let it dry out on top of the hot water tank for a couple of days and then had a close look at the overwind with an eyeglass. Sure enough there were two broken ends sticking out of the edge of the winding - looks like it had had a poke from a screwdriver. Measuring from the start I could get ~600 ohms to one end and from the finish ~200 ohms to the other. Very delicately linking the break and the whole winding was about 840 ohms which I believe is about right (see pictures). Still no leakage between the other windings so I shall let it dry out further before giving it a try. The bad news is that the frame output transformer has an o/c primary. If I can get line scan and EHT, I may substitute an audio output transformer for now. I assume this will give me some sort of vertical deflection to allow me to evaluate the CRT.

[HamishBoxer]

Frame TX's go O/C very often. Easy rewind for Ed Dinning.

[Argus25]

Originally Posted by Keith

Progress. There is light at the end of the tunnel (although not yet at the end of the CRT!).

Keith,

That is a good photo and looks very much like the lopty in my TV22 set after I removed the pitch with mineral turps. Although, in this case, there is obviously more heating damage to the former as it has darkened. However, the windings may be ok (after you have repaired them). The assembly too will look a lot better and better than new, once you have varnished it.

When I executed this process to my TV22 transformer I was remiss in not taking detailed photos of it to show forum members. However, it is obvious to everyone on the forum now, that this is the better way to deal with the pitch issue, rather that heating the assembly and picking at the pitch. Because it is a molecular removal process, not chunk by chunk. So thank you for documenting this and vindicating my approach to this problem, which otherwise looks like a suspicious Antipodean solution.

One reason I have sorted a lot of restoration issues and techniques out, is that I started vacuum tube equipment restorations around 1972, so I have traveled many long roads with different techniques to settle on the better approaches to tricky problems we face in this interesting and challenging area.

Hugo.

[Keith]

Good news. We now have light at the end of the CRT! The repaired LOPT was reinstalled (temporarily without any coating) and the set was brought up on a variac (initially with no AI connection). Eventually good line pulses appeared at the anode of the EL38 and EHT was detected. Temporarily replaced the 1N4007 that a previous repairer (?) had thought was a suitable metrosil replacement and was rewarded with a horizontal line on the CRT (kept the brightness well down). There was no frame deflection as frame output and blocking oscillator transformers both have o/c windings. With an audio o/p transformer in place of the former and a 20Hz sawtooth from a sig gen into the grid of the ECL80 pentode, there was enough vert deflection to show that the CRT has no significant ion burn so I'm now happy to continue with the restoration proper.
Many thanks to all those that have helped me get this far - I shall probably start another thread for the rest of the project.

[Boater Sam]

Spiffing good news!
An 1N4007 with how many volts on it? Does it work?
Unlucky to have both frame transformers o/c, green spot? If so, look after the scan coils.

[Keith]

Hi Sam,
Thanks for the comment. Using the 1N4007 puts around 400V on the A1 which I think is too high so I won't run it for too long. I'm expecting a VDR in the post today (EPCOS B72210S151K101). Transformers to be investigated.

[Graham G3ZVT]

Originally Posted by Argus25

The better way to remove the base is to place a hand file parallel to the base and tilt it just about 5 degrees and carefully hand file away the folded lip. As it gets thinner and the base material shows in places it can often just be peeled that that point. Then the base and capacitor contents removed from the canister. Then the edge of the lip can be smoothed just a little with a fine file and then with 600 grade paper. This way the circumferential fold at the canister base is largely intact.

Ideally then a new base is machined of phenolic material about 5mm thick or more with new tags which can be done with screws & lugs. But without the facility, to machine the new base, you can save the old base and simply re-fit it into the area it was previously in with black non acid cure silicone rubber. This method also works well for the restoration of radio vibrators with aluminium canisters and folded lips also.

This is by far and away the better method to rebuild chassis mount electrolytics. Of course if they are just standard sizes etc there is the option to simply replace with new, but in the TV22 and other sets, as you know, there is the unique large capacitor canister. I have seen so many of these capacitors in TV's and radio restorations damaged by people just sawing through them near the base in the rush to re-stuff them.

Scroll to page 5 of this article to the photos and look carefully at the capacitor canister, you can see that it has been un-stuffed using the method described above, and the replacement base (made of a white insulating material) where screws have been added to create the new terminals for the re-built capacitor:

http://worldphaco.com/uploads/BUSH_TV22.pdf

Argus25, I followed your advice and I cannot commend your de-stuffing method highly enough!

After weakening the lip with a file I elected to drill a hole in the bung and insert a wood-screw, which I clamped in a vice and pulled out the bung+contents. Considerable pulling force was required.

I always suspected there was a better way than cutting the can where it would be hidden by the retaining clamp. I am glad I had put this condenser to one side.

<Edit> Further examination shows that the woodscrew had firmly embedded itself into the capacitor's structure.

I am of the opinion that had it not done so, only the bung would have been removed, snapping the wires and leaving the contents firmly stuck in. There is some hard debris visible in the top of the can, I suspect it is adhesive used in manufacture.