Dynatron - why did I open my big mouth? CDA fault

[Tazman1966]

The title says it all really .

In another thread that asked for the good, the bad and the ugly as far as TV sets from 1960 to 1990 were concerned, I extolled the virtues of my recently restored Dynatron CTV16 (Pye 697 chassis). I wish I hadn't said anything now.

The very next day there was some arcing going on and a small section of print got blasted on the LTB/Power panel where the mains goes into it. I easily repaired that but previous to this, there was occasional red flashing on the screen. The picture would flash up very red except where anything bright was which would be alright in a horizontal band from that object and then calm down just as quickly. This got more and more frequent but could be cleared by quickly flicking the contrast or brightness up and down. Now however it is permanent - see attached photos (the colour is turned down to help see the fault more clearly and the pictures explain it better than I could anyway).

Previously I had done some preventative maintenance on the CDA panel and changed the grid leak resistors from 2.2M to 8.2k. I also replaced all but one of the "Callins" capacitors as they were notoriously poor.

I tried swapping the PCL84 valves over but the fault remains. The A1 voltage is okay and can be varied with the pot on the convergence panel. I've checked the resistors on the tube base panel, checked the spark gaps for muck.
What's next? Is it okay to swap the green and red leads (SK17 & SK18) to the grids of the tube to see where the fault lies?

Look forward to hearing from you soon.

All the best ,

Tas

[Steve_P]

Yes, you can do that. It'll show up whether the fault is in the tube or somewhere else in the set.

Old Tellys do this. It's part of the fun....

Cheers,

Steve P

[Tazman1966]

A-ha! Thanks for that Steve.

The fault is now in green so phew, not the tube!

When put back the right way there is red chroma information getting through not like when you disconnect the red lead and it goes bright red but with no red chroma to it.

At least I now know it's in the final part of the red output on the CDA panel. I should've taken Lee up on his offer of a complete fire damaged chassis although that would've been cheating .

All the best,

Tas

[Mike Phelan]

Tas
I'd look around the clamp circuit - the triode part of the PCL84. Just a thought, but have you tried swapping the blue and green leads? The fault might be on the green, not red. It might be easier to turn the contrast right down.
HTH

[Tazman1966]

Hello Mike, Steve, Simon and the Doctor (and as they used to say on the radio "and anyone else who knows me!")

The fault is definitely in the red channel. If you slide the contrast right down, there is just an overall bright red raster. However, before I could do anything else, it righted itself and steadfastly refused to go wrong again How am I supposed to check it now?

Thanks anyway guys.

Tas

[Hybrid tellies]

You are lucky for two reasons, first the ltb/psu did n't go up in flames and second you appear to have eliminated the tube.
If you have access to a pattern generator, use the grey scale bars and check and note all the crt & relevant cda voltages with the set running correctly so you are prepared when the fault re appears. You could also swap the PCL84 in the R-Y channel with one of the others to see if the fault re appears but effecting the green or blue channel. Like everyone else has said I would suspect a red clamp fault on the cda or possibly the red A1 voltage going high.
Good luck as you will probably need it together with a lot of patience, but then thats the joy/fun of restoring and running old colour sets.

[Mike Phelan]

Originally Posted by Hybrid tellies

Like everyone else has said I would suspect a red clamp fault on the cda or possibly the red A1 voltage going high.

Tas has eliminated the A1 supply by swapping the leads to the CDA. As the PCL84 pentode anode is AC coupled to the clamp, it is probably the capacitor, valve base or a DJ.

[Focus Diode]

Originally Posted by Tazman1966

Previously I had done some preventative maintainance on the CDA panel and changed the grid leak resistors from 2.2M to 8.2k. I also replaced all but one of the "Callins" capacitors as they were notoriously poor.

Perhaps a typing error, but the resistors should be 680K!
Which Callins capacitor could you not replace? Might have a sutiable replacement somewhere.

I had a similar fault on an ITT CVC5 due to a dry jointed clamp diode. The Dynatron uses colour difference drive which is a different kettle of fish.

The usual cause is the valves themselves. I suspect when you had the CDA panel out a dry joint has occurred somewhere around the red PCL84. This is very easy to introduce seeing these panels are so brittle. Also check those wirewound resistors are making good contact with the PCB and of course not o/c. I'm sure I once read about a fault not unlike this caused by one of the wirewounds going o/c but unlikely in your case as the fault is intermittent.

Also- do the spring clips make good contact with the chassis? This is essential.

These sets are capable of reliable service after some attention. I had a PYE 205/2 running as domestic for two years with only one breakdown. I wouldn't run it unattended though!

Cheers,
Brian

[Tazman1966]

Thanks Brian.

It was a typo sausage fingers at work again . They were changed to 820k as I didn't have 680k handy.

I've given the clips a good scrape but at the moment I can't instigate the fault.

The "Callins" that I couldn't change was C367 (4uF) which goes from the grids of the amplifier sides of the PCL84's.

I'll let you know how I get on. Off to work now .

Cheers,

Tas

[Tazman1966]

Hello again.

Seeing the as the Pye is fixed I though I'd better find out why the red problem on the Dynatron. If you remember the fault had seemed to right itself but after I swapped the CDA panels over with the Pye and then swapped back again the next time I fired up the Dynatron the red problem was back. I just thought I'd share the fault finding with you.

Now the strange sort of red colour banding depending on the brightness of various objects to me and many of you made it seem like it was in the area of the R-Y clamp circuit.

As stated previously I first swapped PCL84 around with no change.

This is where the story takes up from. I changed C368 and R394 in the anode circuit of the red PCL84 and as a matter of course checked R390 and R399 although if those two had conked out surely that would mean that there was no red chroma going to arrive at the tube grids. I tried it out again and...grrr...no change!

Time to change tactics now and start clutching at straws. I changed C364 in the amplifier section and checked all the associated resistors (R377, R383, R386) and I then went on another dry joint/cracked print hunt. I found nothing conclusive but thought I'd switch it on again... still as red as my face.

I then remembered that when I tried this CDA panel in the Pye there was excessive brightness and quite purple. Now what on earth else could it be? The penny then dropped. In order to make the panel work in a Dynatron the links between the cathodes of the clamp sections of the valves are cut on the panel and are fed to the tint control via a red plug (PL31) - this has nothing plugged in it on the Pye. I pulled the plug out of PL31 and sure enough the fault was now in red and blue ie overly bright and purply. Using two screwdriver blades to remake the severed links brought the picture back to its former glory. Checking the tint contol revealed that it was faulty. Not having a spare control to hand I left the plug disconnected and remade the links permanently. Obvously that means that the tint control is now in operative but hey so what. (Waits for any Dynatron purists to throw rocks at me!)

Problem solved!

For the sake of the completeness I'll post a picture of the links and the plug but for now...

...cheers,

[Focus Diode]

Well done Tas, great news indeed! I wondered why they bothered fitting a tint control in the first place. Certainly Philips were very quick to remove this feature in their G6! The ITT CVC5 also has a tint control which gives either a blue or red bias, set in the middle and the grey scale is perfect!

Perhaps they were a gimmick to make B&W programmes/films colour tinted?! Certainly on a correctly adjusted set this feature is poinless in my opinion.

Cheers,
Brian

[Steve_P]

There were a number of sets had this. Including the BRC2000. If the set drifts about on Colour, just disconnect it.

The NTSC - PAL sets, such as the Sony's, needed it because the NTSC System does not have a reference point the way that PAL does. So they stuck it on the front and called it a feature!!

Cheers,

Steve P

[Tazman1966]

As promised here are the pictures of the CDA panel differences if ever any of you need to put an old Pye/Ekco/Invicta CDA panel into a Dynatron. I've use my meter lead prod to show the two links (re-made) and the red plug (disconnected)that goes off to the tint control.

I had my first attempt at computer doctoring the circuit diagram of a normal CDA panel to show what the difference is. I saved it as a word doc - mods, I hope that's alright, if not can you convert it or let me know how to.

Thanks,