The Hertfordshire Rediffusion MK1s

[malcscott]

Brioken print around the pal delay line, or the delay line itself can often cause this problem, Malc.

[Tazman1966]

Good evening!

Well I got around to having a look again. The lack of R-Y was traced to R202 the B-Y amplitude pot which had gone open circuit although quite how I really don't know. With a replacement fitted all was well for a short while until the blue faded out before my very eyes. The curse of the Lockfit transistor strikes again! This time TR304, the blue amplifier emitter follower a BC147. I fitted a nice new BC108 and Bob's your uncle!

Now to turn my attention to the cabinet.

Tas

[Tazman1966]

Wow! A whole month has passed since the last update.

Well as promised, the cabinet on my CU2214 needed some serious work. I sanded it down using various grades of sandpaper and then gave it a couple of coats of spray varnish, flatting it down between each coat. Not my best effort but a zillion times better than it was!

I'm left with one remaining fault however, and it's a bit of a weird one. The black level floats up and down depending on picture content. It is however the opposite of an AC coupled monochrome set if that makes sense. When the overall scene doesn't have much light in it, say a night time scene, the brightness is too low. If in this same scene the camera pans across a lighted window for example, the brightness increases to a normal level and things that were previously lost in the dinge can now be made out. The fault can be illustrated in such a case by switching on the "Sky" banner on the Sky box when a dark scene is being shown and the brightness improves. I have set up the beam limiter correctly, the CRT Grid Bias (Preset brightness) is at maximum as per manual. I've checked around the DC restorer area including the diode D304 which reads correctly although I haven't got a suitable replacement. Am I in the right area? All suggestions are welcome.

Meanwhile, here are a some shots of it in action on our sideboard. The footie didn't stay on long by the way!

Tas

[crustytv]

Looking superb now Tas, well done

The black level floats up and down depending on picture content.................

I've checked around the DC restorer area including the diode D304 which reads correctly although I haven't got a suitable replacement...............

Am I in the right area? All suggestions are welcome.............

Tas

Obviously Malc is your man on this one but I would also be checking D303 (pulse limiter) with a Black level clamp fault.

Both D303 & D304 (pulse DC restorer) as you probably know can be replaced with BA145, BA157, BA159, BA245, BA248, BY206 and BY207.

Could one or both be failing under load?

D303 & D304 outlined in green

[Tazman1966]

Thanks Chris. I do suspect the diodes but have nothing (yet) suitable to replace them with. When I do, I'll replace them and see what happens. They test okay cold....

Tas

PS Thanks for the equivalents list

[malcscott]

Hi Tas, the 100mfd cap on the PL509 base is first suspect. There is a brill mod to the t/b panel which vastly increases the brightness range. I will dig it out for you, Malc.

[Tazman1966]

Hi Malc

I'll check that out first then. As for the timebase panel mod, I think it may have been done already. There's a bit of print cut to one end of the preset and a wire connecting it to a point further up the voltage rail but yes, please dig out the mod. It might be something else.

Thanks again,

Tas