Decca Bradford Pincushion info

[dominicbeesley]

Hello all,

I've been wrestling with a few frame faults on my Decca and have managed to get the frame back to full size be replacing lots of high value resistors but am still having trouble with a bowed in pincushion distortion (mainly at the top - fairly good at the bottom) and a pincushion control that doesn't appear to do anything!

I've been poking round the L404 region of the board looking for suspiscious components but am at a bit of a loss mainly because I don't have a decent schematic. The only schematics I have a from a '37 chassis without pincushion correction and another schematic from a photocopy I think of a "Television" article, which just doesn't match my boards at all.

All the resistors appear correct and caps measure close. Coils all have continuity in the right places (I think).

Thing is for the type of distortion I have I'm not even sure I'm looking in the right place! The distortion looks roughly like that attached sketch (I've tried all the adjustments I can think of )

Any help would be great I'm starting to get a bit stuck! Also a bit clumsy - managed to break the top cap on the PL509 valve tonight - an expensive mistake - the only valve for this set that I don't have a spare for!

Cheers

Dom

[Studio263]

I'd start by replacing the transductor (if you can find one). This is the special transformer that links the frame and line circuits together to perform the correction. It's on the sound panel (long thin one down the back) at the end closest to the line output stage (I think...).
It is important to to expect too much from sets like these, the smaller models in the series had no geometry correction at all (in common with a lot of British sets of the period), it was common to have proper correction arrangements on the 26" models only. This, combined with minimal EHT regulation, gave sets of this period a "distinctive" performance while those imported models with proper systems to perform the required corrections were dismissed as over-complicated.
There also wasn't so much stuff in the programmes that made the shortcommings of early colour sets so obvious back then, no straight lines of text scrolling across the top and bottom, no numbers shown right in the corners and plenty of room around the edges to allow for a bit of over-scan.

[dominicbeesley]

I've got a spare transductor on another board so I'll try swapping that over (when I get the new l/op valve).

I know these sets are not always 100% on geometry but this really is quite bad - I'd send a picture if I hadn't had my accident!

Cheers

Dom

[dominicbeesley]

At least they had the decency to put all the convergence controls in a reasonable position - unlike the field/line shift and pincushion: there doesn't seem to be any way to get at them that doesn't involve getting a nip from either the EHT, back of the tube or the power resistors on the timebase board!

Could the distortion be down to a shorted turn in the field coils?

Dom

[dazzlevision]

Hello,

If your set is a 22" or 26" (and possibly a 20") - but not 17", 18" or 19", then it will have pincushion correction components fitted.

There is only one adjustment - L404. This inductor ("pincushion phase") will shift the horizontal bowing along the screen. Use a non-inductive trimming tool of the correct size, or you'll split the tuning slug.

There is usually a variable resistor in most contemporary sets from other manufacturers, which adjusts the amplitude of the correction (connected across the frame windings of the transductor). In the Bradford, it is not fitted.

If you've checked the peripheral components, then the transductor is probably faulty. The usual type fitted in a 90 degree CRT is the Mullard AT4041/07.

Regards,

Dazzlevision

[Tazman1966]

Originally Posted by dazzlevision

There is usually a variable resistor in most contemporary sets from other manufacturers, which adjusts the amplitude of the correction (connected across the frame windings of the transductor). In the Bradford, it is not fitted.

Hi Dom.

The resistor that Dazzlevision mantions is fixed resistor R481 and is 10R in this chassis. Try increasing the value to 39R or 47R. That will increase the pincushion amplitude and may well solve your problem.

Good luck!

[dominicbeesley]

Success!

Thanks to everyone for the help. For info I replaced the transductor got a bit of adjustment. Then increased the resistor to 30R gave me enough effect to get more or less spot on. (A strange side effect was a couple of dead spots at the top of the screen where it looks like the blanking circuitry is getting affected).

All looking good now!

Cheers

Dom

[dominicbeesley]

And here's some pictures....

Still haven't quite got the teletext mod working 100% and the convergence is still a bit pants down the left and bottom right edges but a nice, fairly straight, lively picture!

Now to sit down and watch it without fiddling! I've no idea whether this is good or whether better can be expected from such a set?!?

Now I've got into this I'll never be able to sit in front of a TV set without wanting to fiddle with the convergence and purity controls.

Dom

[Hybrid tellies]

Great pictures and looking at the first one with the crosshatch pattern, I'd say you won't improve things that much with any further adjustments. Convergence linearities and tube look good too.
I did notice the unusual style cabinet; looks like it is one of the final versions of the 30 chassis, possibly the 31 chassis.
Now all you have to do is sit back and enjoy this set and restrain yourself from making any further adjustments, as you will probably only make things worse.