Bush TV24 EHT problem

[mark pirate]

I am currently restoring a TV24 mk2, I have replaced most of old caps, a broken thermistor and the 8k power resistor on the R/H tag strip.
I am not hearing a line whistle, but am able to draw a small spark from the anode of the EY51 (heater not lit).

The EY51, PZ30 and PL81 have been changed, along with the ECL80.
I have over 200v HT, sound stage is working and heater voltages are about right.

The LOPT looks to be in good nick, but not above suspicion
Any ideas where to go next would be much appreciated!

Mark

[Nuvistor]

Check the frequency and amplitude of the Line drive at the grid of the PL81.

[mark pirate]

I have to say that this is not a nice set to work on, especially when trying to access the bottom of the upper deck to take voltage readings. The CRT on these are only secured at the neck, if the clamp is loose the CRT will drop out. Or even worse, snap the neck!

Leaning the main chassis on the tube face, with the receiver chassis on it's end it is just possible to take the voltages, which are as follows:
PL81 pin 2= -35, 3= 0, 8=172 DC
ECL80 pin 1=123, 2=-49, 6=160, 8=23, 9=1.1 DC

HT is around 250v, and I have finished recapping the frame stage and fitted a rebuilt blocking tx. I can now hear the frame tx working, it is varying with the frame hold as it should.

Reading through LLJ's article on these sets, The symptom can also be a leaky 470pf cap. I have checked this and it is fine. All resistors are also reading about right.
I have no access to my scope at present, so am stuck with my DMM.

Mark

[Nuvistor]

The drive to G1is about right at -35v and the screen voltage looks reasonable, I don't have a circuit, you could check the screen feed resistor if it is carbon in case it is high, but doubtful.
We don't know if the frequency of the drive is correct, presuming it is ok then you need to check items including the LOPTX, which I think you are thinking on those lines anyway.
I don't know the set so advice is generic.
Frank

[mark pirate]

I am beginning to fear the LOPT is duff, although I was told the set was working when retired back in the 60's
I do have a TV22 chassis, but the condition of the LOPT is unknown. When I get my scope back, I will give it a ring test.

Mark

[Lloyd 1985]

This all sounds very similar to my TV22, which packed up on new years eve! Mine shorted between the primary and secondary, and can be measured with a DMM. I'm currently deciding whether I should attempt a re-wind or not! The EHT overwind is perfectly OK, so it's possible to repair it. If you need it, I've got a doodle of the LOPT and the resistances I measured before and after failure.

Regards,
Lloyd.

[Ed_Dinning]

Hi Mark, the classic symptoms of a failing LOPT are usually declining EHT over a period of half an hour or so as the pri warms up and the resistance between turns/ layers and windings reduces.
When it is well carbonised there is very little of anything, EHT, Boost, line whistle.
Cold resistance checks are not always a good indication.

In re-winding great care must be taken with the insulation between windings and layers because of the pulse voltages. There is also the constraint of limited space within the outer bobbin.

Ed

[mark pirate]

The strange thing is that the LOPT gets barely warm after a 20 min run, I have seen LOPT's burn up when shorted!
This is only the second time I have had LOPT problems, it is usually the CRT that spoils the restoration
If it turns out to be duff, I would certainly not consider rewinding it myself, a rewind would be an option if my spares chassis does not have a working LOPT.

I will remove the spare and compare readings with the one in the set, before attempting to replace it.

Mark

[Heatercathodeshort]

Originally Posted by Lloyd 1985

This all sounds very similar to my TV22, which packed up on new years eve! Mine shorted between the primary and secondary, and can be measured with a DMM.

This is a very common fault with the TV20 series transformers. I have a TV22 in my collection that was presented to me about two years ago. It has had very little use since new and the LOPT appeared to be pristine performing 100% after restoration. [I must admit I was expecting something to happen..] Unfortunately it broke down primary/secondary dead short within hours. Mike Barker rewound it to my complete satisfaction.
I think the shorted windings are more common than overwind failure. John.

[mark pirate]

It has had very little use since new and the LOPT appeared to be pristine performing 100% after restoration.

The LOPT on this set looks in very good condition. Unlike my TV22, this showed signs of overheating, it had dripped most of it's pitch on to the receiver chassis below, but is still working perfectly after many hours use since restoration.

Mark

[Ian - G4JQT]

TV22/TV24 line output transformer info table attached. Compiled myself to track down similar problems.

Hope someone finds it useful.

Ian

[mark pirate]

Thanks Ian, very useful info, it came in handy today as I removed the LOPT from both the TV24 and the scrap TV22 for analysis.

The original LOPT matched the resistance on all readings apart from one, pins 2 & 3 read 10.1 ohms instead of 14.2 so a short is likely.

The LOPT from the scrap chassis was damaged at the bottom where it sits on the paxolin base, part of the former tube was missing, as was one of the mounting lugs. Despite it's poor state (and the fact two of the terminals were hanging off!) it read to spec on all windings.

After very carefully glueing the terminals back with superglue, I managed to glue the bottom to the paxolin base and gently tighten the one remaining mounting, it seems to have done the trick!

I fitted it to the set, using the EY51 from the other LOPT, as the one on it had a broken lead.

I carefully wound up the volts, and was rewarded with the EY51 being lit, and a nice fat spark could be drawn from the anode.
After adjusting the ion trap a full screen raster was present

Thanks to forum member Jerryw22 for supplying a new bobbin for the blocking TX, it was a fun job to fit, but is obviously working fine.

No sound or vision yet though, the receiver has not been touched, so the next job is to clean it up and recap it before the heater decoupler goes S/C!

I must be the only person using parts from a TV22 to restore a TV24

A quick question, the receiver has been fitted with the Bush band 3 converter.
I noted that the aerial socket is marked band 3, so I am guessing the band 1 aerial still connects to the original terminals?

Mark

[Barry Lloyd]

I am guessing the band 1 aerial still connects to the original terminals?

Yes - at least it does on my TV22.

Barry

[mark pirate]

I have more progress to report, I have removed the tuner unit and replaced most of the caps in the receiver deck.
The tuner unit did have three connections under the chassis, one of which went to the contrast pot, so I reinstated the original wiring and now have both sound and vision via the Aurora on channel 1.

I will refit the tuner unit, but will not reconnect it.

There are a fair few jobs still to do, such as sort out or replace the brightness pot and set up the picture properly, but the set is coming along nicely.

The LOPT is barely warm after half an hour, so hopefully it is a good one.
I am thinking about leaving the screening cage off the finished set, as the LOPT would have less heat to stress it out, Any thoughts on this?


Mark

[Ian - G4JQT]

Looks like you have quite a good tube too.

By 'the screening cage' I guess you mean the metallised card that sits around the top in side of the cabinet that's supposed to reduce RF interference. Well I've never tested its effectiveness (though an easy enough job) but I'd guess it doesn't do too much!

Perhaps its removal may help dissipate heat a bit better, but I doubt it'll make much difference.

Ian

[mark pirate]

No, I was referring to the three part screening box that covers the LOPT, PL81 & PZ30.
The set seems to run fine without it in place.

I will wait until I have sorted the brightness pot, as it is way to bright to assess the condition of the CRT yet, but I think it should be ok.

Hopefully will have more progress to report later, I have spent all morning replacing rotten rubber wiring and cleaning the chassis.

Mark

[mark pirate]

I dismantled the wire wound brightness pot, cleaned away the old hardened grease and reassembled it with a smear of vaseline on the wiper, it now works perfectly.

I am still waiting for a few caps to arrive, but all the critical ones have been replaced. I ran the set for nearly five hours last night, and it is nice and stable.
EHT is measuring a steady 7.6Kv.

I had to readjust the contrast and brightness a few times as the CRT improved.
By the end of the evening the picture was really good.
Another bonus is that the LOPT was barely warm after it's long run, so I have decided to refit the screening can, but may leave the lid off to aid cooling.

The speaker rotting is a common problem with both the TV22 and TV24, this one was no exception, it is way beyond repair

A rummage in the workshop turned up a scrap Roberts R600, the speaker was the right size and impedance. offering it up to the set it was a good fit, apart from two of the mounting studs being too far apart.
This was solved by making a couple of square washers.

I will get the set back in it's cabinet today, and set up the picture properly.

Mark

[mark pirate]

After a deep clean of the cabinet, safety glass and mask. I replaced the chassis and fired it up, although working well I have noticed the CRT heaters are running at 5.6v, it also takes nearly two minutes to come on from cold.
I am sure my TV22 warms up faster than this. But it is very stable, the picture is locked before it is visible.

I am waiting for some 0.003uf caps for the receiver chassis, and will test the valves before I can call this one done.

Mark

[Lloyd 1985]

It looks good!

I guess that my TV22 was doomed to fail from the start, the LOPT used to get quite hot, the core especially, the overwind stayed cool though.

That speaker looks a bit knackered!

Regards,
Lloyd.

[mark pirate]

Hi Lloyd, yes the speaker had certainly seen better days!
The new speaker certainly sounds good
It certainly is not as nice to work on as a TV22, made worse by the amount of work needed. I have lost count of how many times the CRT has been removed and refitted as the restoration progressed.

It's saving grace is that the cabinet is in very nice condition and has cleaned up a treat
I was very disappointed to find the original LOPT was duff, A rewind would probably cost more than the set is worth.

I will keep an eye out for another cheap example as a spares set, as I now need a LOPT, blocking tx, CRT and back for my spare TV22.

Mark