Nasty fault on an old Ekco - UK Vintage Radio Repair and Restoration Discussion Forum

thyratron · 3rd Oct 2007, 10:31 pm

Here is a good one.
Ok, it is an Ekco TGC316. It has a gated vision agc fault. The contrast control seems to have little effect on the picture apart from a slight change in width!
The picture goes negative as the whole video stage gets overdriven at normal signal levels although I can get a good picture by de-tuning or fitting an attenuator in the aerial lead but then there is far too much valve noise/snow and weak sound so I can't get around it that way. I have looked for line pulses on the scope from the line output transformer as far as R90 (120k) but nothing beyond. I have changed all caps in the circuit and most of the resistors. Does anyone have any experiance with these gated agc systems? It is a a real teaser!

plaka78 · 4th Oct 2007, 2:38 pm

I had a similar fault on a 70s B/W portable (Can’t remember make) from what I can remember it was a 220µF Electrolytic Cap in the AGC cct

FERNSEH · 4th Oct 2007, 3:05 pm

Check that 0.1mfd capacitor in the turret tuner. You might recall from other topics on the Ekco TMB272 that the same capacitor causes similar problems in the portable set.

DFWB.

thyratron · 4th Oct 2007, 5:11 pm

Thanks,
I have already changed all the capacitors in the set
(apart from the main smoothing bomb) including the one hidden in the tuner.
It is not easy to work on the chassis with it running. I have rigged it up on top of the cabinet with the only two leads left unconnected being the gram plug and the sound output transformer (don't worry I looped the ht back to the anode as the screen is supplied ).
I have not tried changing the video output valve but I have swapped the two double triodes around (one is the cathode follower video) and the two double diode bottles , no change either way.
It is a pig of a fault!
cheers
Michael

Steve_P · 4th Oct 2007, 6:14 pm

Go back and check your work! It's amazing how many faults are caused by incorrect fitting!!

Then, round the valves and check DC conditions, change any gain affecting resistors, try a new valve rather than swapping them over (you never know!) and then do full realignment.

If any slugs appear to do nothing, look carefully...

Cheers,

Steve P.

murphyv310 · 4th Oct 2007, 6:15 pm

hi.
Have you actually replaced the pulse cap from the loptx for the agc, it goes faulty under load, but will check ok i think its a 120pf or 60pf depends on the production date.
trevor

thyratron · 4th Oct 2007, 9:55 pm

Yes , that was one of the first suspects. I have popped a nice 3kv 120pF disc in place of the original.
It looks like I will have to go valves next.
In the old days we always used to go VALVES FIRST it was so easy.
Because we are moving soon I have packed hundreds of spare TV valves into a depository 15 miles away!
Somehow I don't think it will help.
many thanks for these good tips.
Michael

thyratron · 9th Oct 2007, 9:48 pm

SOLUTION:
Would you believe it! This one had me really thinking it was time to hang up the soldering iron for ever.
The thing that really got me was the set worked and all the voltages seemed to check out, but by disconnecting various stages, I deduced that the voltage on V7b cathode was too high by about 50 volts for the stage to rectify the line pulses/back porch signal.

I found that the connection between C92 and R90 is made on pin 1 of the line output valve; a spare, unused pin. Unused if it is the right valve! The set had a PL36 in it rather than a 30P4 and because it was in it when I bought the set, I assumed it was an equivalent.

The PL36 has an IC (internal connection) on pin one to the screen. This was not too dissimilar to the voltage that was required at the junction and not enough difference to cause any damage. I lifted the joint away from the valve and it works a treat!

Perhaps this was an old repair attempt when the old 30P4 died, and when the set started up again with a negative picture the set was written off!
Anyway job done and a lesson or two there I think!

Took about 6 hours to find this . In the old days at the bench, we would have gone for a pint

after a set like that!
regards to all.

Michael

Chris55000 · 9th Oct 2007, 10:30 pm

Hi!

Just a quick note from me - WELL DONE! - it would have took me hours to trace that out, and illustrates the point both Mullard and Mazda made in their Valve Data Books about leaving valve-base connections annotated as i.c. blank - you do not know for *certain* what part of the electrode system a pin so marked is connected to, and therefore you won't easily guess what unrelated parts of circuitry you've inadvertently coupled together if you use an (i.c.) as an anchor-point, and too many setmakers have made that howler as you found!

Well done!

Chris Williams

thyratron · 10th Oct 2007, 8:25 am

I will post some pictures soon.

P.S. Of course the biggest clue was there all the time. As I said at the start the contrast control was slightly affecting the width.
(just bringing it in by about half an inch with the control at full contrast and still giving a little more gain as there was a trace of agc working).
I had put this down to some interaction between the line pulse integrator circuit and the way it is connected to the contrast control. Of course it was actually changing the line output screen voltage by +/- 10 volts !
Of I had had spare valves around I might have got there earlier but why change a line O/P valve that gives full width?
cheers once again!

3rd Oct 2007, 10:31 pm	#1
thyratron Tetrode Join Date: Mar 2007 Location: Bath, Somerset, UK. Posts: 69	Nasty fault on an old Ekco Here is a good one. Ok, it is an Ekco TGC316. It has a gated vision agc fault. The contrast control seems to have little effect on the picture apart from a slight change in width! The picture goes negative as the whole video stage gets overdriven at normal signal levels although I can get a good picture by de-tuning or fitting an attenuator in the aerial lead but then there is far too much valve noise/snow and weak sound so I can't get around it that way. I have looked for line pulses on the scope from the line output transformer as far as R90 (120k) but nothing beyond. I have changed all caps in the circuit and most of the resistors. Does anyone have any experiance with these gated agc systems? It is a a real teaser!

4th Oct 2007, 2:38 pm	#2
plaka78 Pentode Join Date: Jul 2007 Location: Salisbury, Wiltshire, UK. Posts: 174	Re: Nasty fault on an old Ekco I had a similar fault on a 70s B/W portable (Can’t remember make) from what I can remember it was a 220µF Electrolytic Cap in the AGC cct

4th Oct 2007, 3:05 pm	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Nasty fault on an old Ekco Check that 0.1mfd capacitor in the turret tuner. You might recall from other topics on the Ekco TMB272 that the same capacitor causes similar problems in the portable set. DFWB.

4th Oct 2007, 5:11 pm	#4
thyratron Tetrode Join Date: Mar 2007 Location: Bath, Somerset, UK. Posts: 69	Re: Nasty fault on an old Ekco Thanks, I have already changed all the capacitors in the set (apart from the main smoothing bomb) including the one hidden in the tuner. It is not easy to work on the chassis with it running. I have rigged it up on top of the cabinet with the only two leads left unconnected being the gram plug and the sound output transformer (don't worry I looped the ht back to the anode as the screen is supplied ). I have not tried changing the video output valve but I have swapped the two double triodes around (one is the cathode follower video) and the two double diode bottles , no change either way. It is a pig of a fault! cheers Michael

4th Oct 2007, 6:14 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Nasty fault on an old Ekco Go back and check your work! It's amazing how many faults are caused by incorrect fitting!! Then, round the valves and check DC conditions, change any gain affecting resistors, try a new valve rather than swapping them over (you never know!) and then do full realignment. If any slugs appear to do nothing, look carefully... 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...?

4th Oct 2007, 6:15 pm	#6
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,421	Re: Nasty fault on an old Ekco hi. Have you actually replaced the pulse cap from the loptx for the agc, it goes faulty under load, but will check ok i think its a 120pf or 60pf depends on the production date. trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

4th Oct 2007, 9:55 pm	#7
thyratron Tetrode Join Date: Mar 2007 Location: Bath, Somerset, UK. Posts: 69	Re: Nasty fault on an old Ekco Yes , that was one of the first suspects. I have popped a nice 3kv 120pF disc in place of the original. It looks like I will have to go valves next. In the old days we always used to go VALVES FIRST it was so easy. Because we are moving soon I have packed hundreds of spare TV valves into a depository 15 miles away! Somehow I don't think it will help. many thanks for these good tips. Michael

9th Oct 2007, 9:48 pm	#8
thyratron Tetrode Join Date: Mar 2007 Location: Bath, Somerset, UK. Posts: 69	Re: Nasty fault on an old Ekco SOLUTION: Would you believe it! This one had me really thinking it was time to hang up the soldering iron for ever. The thing that really got me was the set worked and all the voltages seemed to check out, but by disconnecting various stages, I deduced that the voltage on V7b cathode was too high by about 50 volts for the stage to rectify the line pulses/back porch signal. I found that the connection between C92 and R90 is made on pin 1 of the line output valve; a spare, unused pin. Unused if it is the right valve! The set had a PL36 in it rather than a 30P4 and because it was in it when I bought the set, I assumed it was an equivalent. The PL36 has an IC (internal connection) on pin one to the screen. This was not too dissimilar to the voltage that was required at the junction and not enough difference to cause any damage. I lifted the joint away from the valve and it works a treat! Perhaps this was an old repair attempt when the old 30P4 died, and when the set started up again with a negative picture the set was written off! Anyway job done and a lesson or two there I think! Took about 6 hours to find this . In the old days at the bench, we would have gone for a pint after a set like that! regards to all. Michael

9th Oct 2007, 10:30 pm	#9
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,864	Re: Nasty fault on an old Ekco Hi! Just a quick note from me - WELL DONE! - it would have took me hours to trace that out, and illustrates the point both Mullard and Mazda made in their Valve Data Books about leaving valve-base connections annotated as i.c. blank - you do not know for certain what part of the electrode system a pin so marked is connected to, and therefore you won't easily guess what unrelated parts of circuitry you've inadvertently coupled together if you use an (i.c.) as an anchor-point, and too many setmakers have made that howler as you found! Well done! Chris Williams

10th Oct 2007, 8:25 am	#10
thyratron Tetrode Join Date: Mar 2007 Location: Bath, Somerset, UK. Posts: 69	Re: Nasty fault on an old Ekco I will post some pictures soon. P.S. Of course the biggest clue was there all the time. As I said at the start the contrast control was slightly affecting the width. (just bringing it in by about half an inch with the control at full contrast and still giving a little more gain as there was a trace of agc working). I had put this down to some interaction between the line pulse integrator circuit and the way it is connected to the contrast control. Of course it was actually changing the line output screen voltage by +/- 10 volts ! Of I had had spare valves around I might have got there earlier but why change a line O/P valve that gives full width? cheers once again!