TV24 - no picture - UK Vintage Radio Repair and Restoration Discussion Forum

dominicbeesley · 2nd Sep 2007, 2:17 pm

After a marathon recapping session yesterday I finally plugged in at about 4.30am.

All seems well except there is no picture, I do have:
- all valves including EY51 glowing
- line whistle
- a 50ish hertz buzz from the field coils
- life from the CRT heater
- set left on for 5 minutes and no smoke or glowing plate
- a telling off girlfriend

I've tried touching a grounded screwdriver to the EHT CRT cap and no spark after set is switched off - do I have a duff LOPT?

How do I test / where should I look first?

Cheers

Dom

Steve_P · 2nd Sep 2007, 2:28 pm

What's your Girlfriend telling you off for? You've done nothing wrong!

Anyway,

The EY51 could be faulty or dry jointed.
The lead to the EHT cap on the tube could be faulty.
The connection to the EY51 Anode could not be there.
Is the Brightness Control OK.
The Ion Trap could be out of place. (Magnet on tube neck. See if it's loose!)
The A1 could be absent. Metrosil and C31.

Also, if you've been up that late, in the cold light of day check everything again!!

Cheers,

Steve P

evingar · 2nd Sep 2007, 4:08 pm

Quote:

do I have a duff LOPT

Unlikely if its not getting hot and you have a line whistle. I agree with Steve, the EHT rectifier and associated wiring needs investigating.

dominicbeesley · 2nd Sep 2007, 4:44 pm

What are my options for testing the EHT?

When checking by trying to get a spark is it safe to do this with the set on, or does this run the risk of burning out the lopt? Is the fact I'm not getting a spark due to the fact that I've switched off and its discharged?

Wiring round the EY51 should be ok - I had to resolder it after a leg dropped off but I'll check again

I have AVO8 or reasonable digital multimeter - neither go as high as 8kv - is it safe to make a chain of 1M resistors as a potential divider?

Cheers

Dom

Ian - G4JQT · 2nd Sep 2007, 4:53 pm

If the EY51 is glowing and you have line whistle, there is a good chance you have EHT. Have you checked the 2uF cap (C32 on Trader sheet 1-91/T38) in the middle of the tag board under the main chassis? I’d change all the electrolytic caps too anyway with the possible exception of the large main smoothing cap, unless it is faulty.

Have you cleaned with meths or similar all around the tube EHT connection? If this is VERY grubby, it's possible some of the EHT is leaking across to earth.

Have you moved the tube and/or focusing magnets? If the gun in the tube looks a bit angled, there should be a small magnet on a clamp between the tube neck and focusing magnets. Have you inadvertently moved this?

On my TV22, there’s between 0 and 100 volts on the centre of the brightness control depending on setting, about 300 volts on the tube side of the metrosil and about 500 on the other side of the metrosil.

If you are brave, (I wouldn’t recommend it!) experienced hands would draw a spark from the EHT tube connection using the screwdriver blade of a very thick, well insulted handled screwdriver. Spark should be almost invisible, but about 1cm long if all is more-or-less ok.

Ian

Reading

evingar · 2nd Sep 2007, 5:06 pm

Quote:

When checking by trying to get a spark is it safe to do this with the set on, or does this run the risk of burning out the lopt?

Perfectly OK on equipment using a valve EHT rectifier / LOPT stages . I don't recommend doing it on solid state stuff (triples fitted etc).

dominicbeesley · 2nd Sep 2007, 5:53 pm

Mmm no spark when on either, got a very dirty curved triangle wave at the grid of v8 (line driver) - is this normal no signal behaviour?

G1 ~ 100V
G2 ~ 220V
K ~ 120V

Will test v8 and ey51 on a characteristic meter and report back later.

Dom

dominicbeesley · 2nd Sep 2007, 11:46 pm

I've checked the PL81 (involved repairing my flood damaged AVO Char. meter!), its just shy of 100%.

I happened to have an EY51 in a box of lucky dip valves I got years ago! Well worth the ten quid!

I've now go a big fat spark at the EY51 anode (to the side of the shielding tin until a bit of resoldering) and a 1/2 inch spark at the EHT cap.

Still no picture

I've tried fiddling with the ion trap - no joy

Measured the following voltages at the tube at max brightness:
k: 128V
g1: 111V
g2: 260V

At min. brightness
k: 120V
g1: 0V
g2: 267V

These look a bit suss to me. I would have expected cathode to rise as more current is drawn but I'm pretty ignorant when it comes to CRTs

Also the magnet in the ion trap is a bit feeble, it attracts a screwdriver tip fairly weakly, is that likely to be a problem.

The magnet appears to be a sort of torroid affair around a bolt. Is there a particular orientation of this magnet?

What do i do with the ion trap, I've tried sliding it back and forth and round and round to no avail.

Slightly frustrated, but enjoying it really!

I really hope I can get it going for next week - I'd love to watch the Grand Prix on it though I'm not sure the girlfriend will approve of the line whistle!

Cheers

Dom

dominicbeesley · 2nd Sep 2007, 11:54 pm

Quote:

Originally Posted by Ian - G4JQT

If the EY51 is glowing and you have line whistle, there is a good chance you have EHT. Have you checked the 2uF cap (C32 on Trader sheet 1-91/T38) in the middle of the tag board under the main chassis? I’d change all the electrolytic caps too anyway with the possible exception of the large main smoothing cap, unless it is faulty.

Yes, I think I've got all the caps, even the ones hiding away under the tag boards and the extras that don't appear on the trader sheets!

Quote:

Originally Posted by Ian - G4JQT

Have you cleaned with meths or similar all around the tube EHT connection? If this is VERY grubby, it's possible some of the EHT is leaking across to earth.

Yes, did that as a matter of course, I've fixed a few modern tellys rescued from skips that needed nothing more than a vacuum and a wipe round the tail!

Quote:

Originally Posted by Ian - G4JQT

Have you moved the tube and/or focusing magnets? If the gun in the tube looks a bit angled, there should be a small magnet on a clamp between the tube neck and focusing magnets. Have you inadvertently moved this?

Yes - I think so!?

Quote:

Originally Posted by Ian - G4JQT

If you are brave, (I wouldn’t recommend it!) experienced hands would draw a spark from the EHT tube connection using the screwdriver blade of a very thick, well insulted handled screwdriver. Spark should be almost invisible, but about 1cm long if all is more-or-less ok.

Yes cheers, a bit scary first time is a huge fat purple spark that maintains itself over a fair distance.

Cheers

Dom

Mike Phelan · 3rd Sep 2007, 7:56 am

Dom

Quote:

I've now go a big fat spark at the EY51 anode (to the side of the shielding tin until a bit of resoldering)

Don't do that - you will risk the LOPT! You should be able to draw a 1/2" spark from the anode of the EY51 without earthing it.
OK to earth the cap on the CRT, though, with or without disconnecting the lead.

dominicbeesley · 3rd Sep 2007, 9:06 am

Cheers Mike,

It wasn't deliberate I've got an aversion to sparks in general! It was just arcing over by itself because there was a (very small) solder spike.

I'm a bit dubious of the design of the shielding tin in that area though - it seems a bit close to the lopt for comfort!

Any ideas about the ion trap / no picture problem?

Cheers

Dom

2nd Sep 2007, 2:17 pm	#1
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	TV24 - no picture After a marathon recapping session yesterday I finally plugged in at about 4.30am. All seems well except there is no picture, I do have: - all valves including EY51 glowing - line whistle - a 50ish hertz buzz from the field coils - life from the CRT heater - set left on for 5 minutes and no smoke or glowing plate - a telling off girlfriend I've tried touching a grounded screwdriver to the EHT CRT cap and no spark after set is switched off - do I have a duff LOPT? How do I test / where should I look first? Cheers Dom

2nd Sep 2007, 2:28 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: TV24 - no picture What's your Girlfriend telling you off for? You've done nothing wrong! Anyway, The EY51 could be faulty or dry jointed. The lead to the EHT cap on the tube could be faulty. The connection to the EY51 Anode could not be there. Is the Brightness Control OK. The Ion Trap could be out of place. (Magnet on tube neck. See if it's loose!) The A1 could be absent. Metrosil and C31. Also, if you've been up that late, in the cold light of day check everything again!! Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

2nd Sep 2007, 4:44 pm	#4
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: TV24 - no picture What are my options for testing the EHT? When checking by trying to get a spark is it safe to do this with the set on, or does this run the risk of burning out the lopt? Is the fact I'm not getting a spark due to the fact that I've switched off and its discharged? Wiring round the EY51 should be ok - I had to resolder it after a leg dropped off but I'll check again I have AVO8 or reasonable digital multimeter - neither go as high as 8kv - is it safe to make a chain of 1M resistors as a potential divider? Cheers Dom

2nd Sep 2007, 4:53 pm	#5
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,323	Re: TV24 - no picture If the EY51 is glowing and you have line whistle, there is a good chance you have EHT. Have you checked the 2uF cap (C32 on Trader sheet 1-91/T38) in the middle of the tag board under the main chassis? I’d change all the electrolytic caps too anyway with the possible exception of the large main smoothing cap, unless it is faulty. Have you cleaned with meths or similar all around the tube EHT connection? If this is VERY grubby, it's possible some of the EHT is leaking across to earth. Have you moved the tube and/or focusing magnets? If the gun in the tube looks a bit angled, there should be a small magnet on a clamp between the tube neck and focusing magnets. Have you inadvertently moved this? On my TV22, there’s between 0 and 100 volts on the centre of the brightness control depending on setting, about 300 volts on the tube side of the metrosil and about 500 on the other side of the metrosil. If you are brave, (I wouldn’t recommend it!) experienced hands would draw a spark from the EHT tube connection using the screwdriver blade of a very thick, well insulted handled screwdriver. Spark should be almost invisible, but about 1cm long if all is more-or-less ok. Ian Reading Last edited by Ian - G4JQT; 2nd Sep 2007 at 4:59 pm.

2nd Sep 2007, 5:53 pm	#7
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: TV24 - no picture Mmm no spark when on either, got a very dirty curved triangle wave at the grid of v8 (line driver) - is this normal no signal behaviour? G1 ~ 100V G2 ~ 220V K ~ 120V Will test v8 and ey51 on a characteristic meter and report back later. Dom

2nd Sep 2007, 11:46 pm	#8
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: TV24 - no picture I've checked the PL81 (involved repairing my flood damaged AVO Char. meter!), its just shy of 100%. I happened to have an EY51 in a box of lucky dip valves I got years ago! Well worth the ten quid! I've now go a big fat spark at the EY51 anode (to the side of the shielding tin until a bit of resoldering) and a 1/2 inch spark at the EHT cap. Still no picture I've tried fiddling with the ion trap - no joy Measured the following voltages at the tube at max brightness: k: 128V g1: 111V g2: 260V At min. brightness k: 120V g1: 0V g2: 267V These look a bit suss to me. I would have expected cathode to rise as more current is drawn but I'm pretty ignorant when it comes to CRTs Also the magnet in the ion trap is a bit feeble, it attracts a screwdriver tip fairly weakly, is that likely to be a problem. The magnet appears to be a sort of torroid affair around a bolt. Is there a particular orientation of this magnet? What do i do with the ion trap, I've tried sliding it back and forth and round and round to no avail. Slightly frustrated, but enjoying it really! I really hope I can get it going for next week - I'd love to watch the Grand Prix on it though I'm not sure the girlfriend will approve of the line whistle! Cheers Dom

3rd Sep 2007, 9:06 am	#11
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: TV24 - no picture Cheers Mike, It wasn't deliberate I've got an aversion to sparks in general! It was just arcing over by itself because there was a (very small) solder spike. I'm a bit dubious of the design of the shielding tin in that area though - it seems a bit close to the lopt for comfort! Any ideas about the ion trap / no picture problem? Cheers Dom