Bush TUG24: No picture

[jooles405]

Hi. I have just registered with UK Vintage Radio and hope that some of you experienced 405 TV experts may be able to help. I am currently restoring a Bush TUG24 receiver that belonged to my Grandparents. I've moved it around with me over the years since I left home some 35 years ago and finally got round to pulling the chassis out of the cabinet and starting the restoration. After a thorough clean and re-forming/replacing the elctrolytics and paper caps on the main deck I have had some success in establishing the HT rails, line & field output. This did require a tricky & time consuming re-wind of the field oscillator transformer, but it was worth the effort. I've been able to borrow a 1000:1 probe which connects to a DVM and have established that the A1 voltage is at 260V and the EHT is 6.2kV. This is lower that quoted in the service manual, but close to the 7kV in the tube spec.

The problem I have is that I have been unable to get a raster, or any illumination of the CRT. I've looked arround the cathode and grid and meaure voltages of 88V on the cathode and 0 to 76 (full brightess) on the grid. As far as I can see this should give me some sort of display, but nothing so far - even in a dark room.

Any thoughts & ideas would be greatly appreciated.

[ppppenguin]

Welcome to the forum. You've certainly put a lot of methodical work into your Bush.

Let's hope it's not a faulty CRT. A few things to check before condemning it.

Is it a "bent gun" ion trap tube? This is pretty obvious if you look at it and if so you'll need to adjust the ion trap magnet before you stand a chance of seeing anything on the screen.

Measure the heater voltage. I think it should be 6.3V but check with the CRT data sheet. It's not unknown for a heater to go partially short circuit though this would normally mean much reduced rather than zero emission.

Short the grid to the cathode briefly. This will ensure that the tube is defintiely biased on.

When you've got a raster you'll need to think about a source of 405 line pictures. The Aurora standards converter is available for about £150 and is used by many of us.

[jooles405]

Thanks for the welcome and the comments. The tube does have an ion trap, although I have not attempted to adjust this at this stage. The heater voltage is slightly low at around 5V, but is glowing very brightly. All the heater voltages are slightly down - this could be due to the fact the receiver is fitted with a band III tuner which I believe adds the 2 valves in series with the heater chain. I thought it best just to leave these under running for the time being. I'll have a go a shorting the grid and cathode briefly as you suggest.

Thanks also for the information on the standards converter - I'll look at getting one of these when I get a bit further.

[Mike Phelan]

Welcome to another West Yorks member, jooles.

I think that the only thing left is the ion trap, as Jeffrey suggests, unless the CRT has no emission at all or an open-circuit electrode. If the heater voltage is reasonable and it looks bright, you should get some sort of raster.

Keep us posted.

[ppppenguin]

Mike and I are agreed here. The ion trap must be the most likely cause. If adjusting it doesn't help then it's possible that the magnet is damaged - it's easy enough to check if it's still a magnet.

As for the underrun heaters, it's almost as bad to underrun heaters as to overrun them. ISTR this is something to do with cathode poisoning but I'm sure that somebody with greater knowledge will put me right. If you have a decent AC ammeter then set the heater chain current, otherwise check the voltages on a few valves. Not urgent at this stage, nothing that will stop you getting a raster, but try to put it right fairly soon as it might cause all sorts of odd problems, especially with valves whose emission may not be 100%.

[jooles405]

Thanks to all for your comments.

The cabinet is a dark wood veneer - it needs a bit of attention, but not in bad condition - that's for a bit later though.

The ion trap magnet has never been moved as far as I know and I was going to leave it this way, but it sounds like I should have a go at adjusting it. I take your point about underrunning the heaters - I'll definately sort this out before leaving the TV on for extended periods.

I'll have another look at everything tonight and let you know what I find.

Julian

[beery]

Hi Jooles,
I would say the A1 is a bit on the low side.
Try...
Check that you have replaced the cap for this (0.1uF 500V), it is located under the tag board that is located unter the center of the main chassis and is hard to spot.

Replace the Metrosil. There is a thread about this somewhere on this forum.


On the subject of the Ion trap. Well assuming you have re-capped the set and that the line output valves have been changed etc. the EHT may be a little different to what it was when the set was last used. You may find that you do need to move the magnet slightly from where it is now.

Cheers
Andy

[jooles405]

Hi Andy & all

Thanks for the suggestions. I did eventually find the A1 cap and have replaced it. I think you are right about the Metrosil the signal on the drive side has ~500V pulses sat on ~200V dc. I am still running with the original valves, but do intend to get replacements to try/keep as spares.

I carried out some tests last night by setting the brightness control to minimum (0V on the CRT grid) and briefly disconecting the video feed lead from the receiver unit. The voltage on the cathode resistor measured ~10V indicating a beam current of 50uA, which seemed to suggest the tube should be OK. Having replaced the video cable I then increased the brightness control and as if by magic a raster appeared. I can't quite explain this as no components had been changed, so can only assume the tests I carried out stimulated the cathode into emmission. I was then able to carry out a few rough adjustments of the scan controls to check the effect and all are basically OK.

I am finding that restoring old TVs is a bit of a roller coaster ride - flushed with my success I started working through checking the dc levels on the receiver unit and at some point during this work I noticed the raster had disappeared and to my dismay found that the line o/p transformer overwind has gone oper circuit!! I thought I noticed a small flash of light around this time and the compound on the outside of the winding appears to have bubbled, so my hope is that I can remove some of the material and find the end of the winding not too far in that shows continuity. Any thoughts on repairs and spares of this LOPT greatly appreciated. Does anyone know the best solvent to remove the coating which won't damage other materials in the transformer.

I'll carry on testing the front end in the meantime - if this looks good it will encourage me to keep at it!

[jooles405]

I thought it was time I updated you all with progress. Having checked out the front end with a signal generator and after replacing the frequency changer valve, I was ablle to confirm operation of the original band 1 and the additional band III turner. I checked audio and video stages using a 1kHz tone and all worked OK - even with mostly the original waxys.

I then decieded to re-check the line output stage more carefully and to my great joy found the overwind was OK ( I must have measured between the wrong points earlier) and as I probed around one leg of the PY51 came adrift - resoldering it (properly this time) brought the EHT back to life.

So after a bit more cleaning & tidying up I put the chassis back together and completed this just as the Aurora converter arrived. After connecting up and tuning and a few minutes adjustment I was delighted to get test card C on screen. It must be some 40 years since this TV displayed picures and it was very rewarding to see.

I then went on to connect the coverter to a Freeview digital set-top box and was of course able to tune through all the UK digital services. There is something fasinating about the mix of technologies spanning over 50 years or more. I've attached a few photos of the finished product. Still needs a bit of fine tuning as any adjsutment of the contrast upsets the line and frame syncs and the EHT is still very low.

Once this is sorted I'll start on the cabinet and a couple of other TVs waiting in the wings.

Thanks to everyone for their help to date.

[Tazman1966]

Hi Jools

That's a great result so far. It looks very reassuring to see those new yellow caps in place...waits for purists to shoot me down

It is especially good considering that you thought the EHT overwind on the LOPT assy had gone up the spout.

...and the EHT is still very low.

Have you tried a new EY51 EHT rectifier and a PL38 Line output valve?

Well done and I'm looking forward to the pictures of the finished article.

All the very best,

[jooles405]

I haven't tried replacing the PL38 & EY51 as yet - they are on order. Also I only just noticed in the service manual that TC1 sould be adjusted to give maximum width & EHT - haven't tried that yet, but will do tonight.

I have mixed feelings about replacing the wax caps, but I decided it was a must on the main deck. I've left in as many as possible in the tuner, for now at least. I have replaced the hum bucker cap with a modern x-type but really need to mount this inside the original waxy as this is a very visible feature on the top side.

A few more pictures attached: You'll probably recognise the picture from the TV programme "Friends"

[jooles405]

I've been putting some finishig touches to the TUG24; checking waveforms, voltages and heater cutrrents. The mains supply in my area is quite high at 248V so I have had to adjust the dropper tappings to avoid the HT going too high. At 240V mains I was geting >200V HT & was concerned about this going any higher.

I've also been looking at the EHT and having replaced the line output valve this is now higher than before and gives a brighter picture. The service manual refers to the EHT being 8.5kV open circuit. I've not tested this condition, but with the brighness set to minimum the EHT reads around 6kV, dropping to around 5.5kV on a "normal" testcard picture.

I would be interested if anyone with a TUG24 could let me have typical operating figures for the HT, A1 & EHT of their samples for comparison.

Another point is that the vertical hold appears to operate as a height control for a good part of its movement within the locked range. I've set this mid-way between the extremes of loosing lock and then adjusted the height control to give the correct picture. Does this sound right?

As always, help is greatly appreciated.