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Saturday, 9 August 2014

GEC 2028

Been after one of these old girls for a while now. Picture the scene. It's 1967. Colour TV has just started in the UK. Sets are an arm and a leg. This was the cheapest colour TV available at he launch of the Colour TV service (BBC 2 only at the time) as it used a Mullard 19" tube (A49-11X) , whereas the competition all used the bigger 25" A63-11X.

It's a dual standard 405 line/ 625 line set, and much in keeping with bodges done once everything was in colour this one has had it's system switch permanently set to 625 lines. This I had to reverse to restore the set to full operation.


The set arrives....








Off with the back... system switch lever missing...








One of the RGB output valves is down to air (the getter has turned white). It's a PCL 84, but no worry, I have a NOS "Pinnacle" branded one to fit.







Then, my heart sinks.... I'm just about to check the condition of the CRT, when I spot a crack all the way round the CRT neck....





The CRT is made from purest unobtainium. I'm gutted, but hook up the B&K CRT analyser to check it , but I don't hold out any hope....






As usual with a CRT that's been standing for years, there's almost no emission, but the CRT heaters glow about the right colour when connected to the tester, which is good, as if the tube was down to air, they would burn bright. Slowly, oh so very slowly, emission starts to occur, and after a few hours, the CRT tester shows we have a very good tube. I disconnect the main reservoir capacitors and leave them to reform on the "MK87B Dreadnaught Capacitor reformer"

I'm still very nervous of that crack in the CRT, but gently apply some mains to the set (after replacing the reformed electrolytics). There's a smell of warm dust from the dropper resistor as it wakes from it's slumber... The valve heaters light .... There's a hiss from the speaker ... but no life from the line output stage. Easy fix, the anode wire has broken off the top cap of the PL509 line output valve. A quick dab with a hot iron, and I start the procedure again...

This time, there's some life from the line output stage, followed by some fizzing, popping and crackling from the line output "cage". Not Good. Mains quickly removed. The line output cage houses the line output transformer and a semi-conductor EHT "Tripler" or multiplier. The EHT tripler creates the 25KV used to drive the final anode of the tube, and in this set it also feeds a VDR dividing network to derive the 7KV focus voltage.

I ran the set again in the dark, with the top off the cage, and it was obvious the tripler was arcing to earth where the EHT lead exited the case, so I removed the tripler and the focus VDR.






I removed the damaged insulation, and soldered a new piece of EHT lead to the tripler. I then filled the join with epoxy resin to insulate it. 25KV is difficult to keep insulated, so I also expoxy bonded two pieces of perspex to the tripler case to prevent any corona or arcing.




Once the tripler was installed again, it was time to apply mains again....







.... and after a few minutes, a bit of fiddling, a test card was displayed, low in height and no frame lock at all! Checking around the frame stage revealed 7 resistors well out of spec, and two suspect electrolytic capacitors. Once these were changed....





... the set resolved a very reasonable picture.









Manually operating the standards switch and feeding in a VHF 405 line signal in, the set displayed nothing but interference. No amount of trying could get the tuner to move. It was seized solid. Removal is not an easy task, and I was still wary of that crack in the CRT neck...




The screw threads which move the slugs in and out of the tuning inductors were seized solid. Some cleaning with IPA to remove the hardened grease, and application of petroleum jelly soon freed everything up and restored the operation of the VHF tuner.

Once the tuner was refitted, I could tune in the output from my 405 line test card generator, but line lock was difficult, and getting a lock on 405, threw out the line lock on 625.

After much head scratching , and checking of quite a few components in the 405 section of the line stage, I find the fault .... in the 625 section! It's obvious that the set developed a line sync fault, and was fudged to work on 625 only (what's the betting that's when the standards switch lever was removed). The 625 section of the line oscillator was loading down the 405 section. It was a faulty polyester film capacitor, very much outside of tolerance.

EHT was adjusted to 25KV on both standards, and I set about setting up the convergence...







 ... and snapped the blue lateral magnet in the process ...








.... which was duly repaired with some very nice orange heatshrink sleeving....








... and allowing a very nicely converged colour picture.








I fabricated a standard switch lever (which is operated by a cam on the VHF tuner. It operates the lever when switched to the VHF position). A tag soldered on to a piece of wire coathanger at the top....





 ... and a catch at the bottom to hold it in place.









 ... allowing us to switch between 405 lines monochrome...
... and glorious colour 625 lines at the turn of a knob!
video

Flicker here is caused by the difference in frame rate of the camera and the TV screen. It's not evident in real life!

Post-scipt....

THE CRACK!
After some research (mostly on http://www.forum.radios-tv.co.uk ) , and a chat with Peter from http://www.notobsolete.co.uk , it turns out that the crack is just a weld in the neck of the CRT. This was done by the original manufacturers of the tube (Mullard) when producing a factory re-gunned tube. Now I can relax!

2 comments:

  1. Brilliant set Andy, thank goodness the "crack" was not a crack !!

    Andrew (PYE625 on VRATS)

    ReplyDelete
  2. I have the same set I picked it up in 2006. Well done in sorting the faults.

    Marcus 3500.

    ReplyDelete