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Old 18th Apr 2021, 4:22 pm   #1
Chris55000
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Default Decca Bradford Preset Brightness Modification?

Hi!

There seem to have been at least four distinct printed Decca Bradford Service Manuals in circulation, and in one of these, the PCB for the sound o/p and PSU had a modified Preset Brightness Control circuit, with R611 (1M8) added from the cathode of the h.t. rectifier diode to the top end of VR601, plus R612 (47k) added between the +25V l.t. line and the top end of VR601.

These resistors were properly designed in the relavent PCB, not simply added on the copper side with track cuts, etc!

Can anybody suggest the reason behind this modification?

(I only found out about it after I'd sent Chris Field's New Telpro panel for production, but I have now made a second revision to incorporate these resistors!)

Chris Williams
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Old 19th Apr 2021, 9:22 am   #2
Hybrid tellies
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Default Re: Decca Bradford Preset Brightness Modification?

The Decca Bradford which was in production from 1971 to 1977 did undergo quite a few changes over this period.

The original was know as the Decca Bradford 10 which used a a fully discreet decoder/RGB output board and was a 25Kv chassis driving 19" 22" and 26" CRT's. This was followed by the Bradford 30 chassis which used a MC1327 Matrix and driver chip on the decoder and RGB board. There was two versions of the 30 chassis the 25Kv version for 20", 22" and 26" CRT's and a 20Kv version for the smaller 17" and 18" models.
The 30 chassis had a few tweaks made to it over its production run from 1972 to 1977.

At a guess the circuit diagram you have shown is an early version Decca 10 chassis.
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Old 19th Apr 2021, 7:37 pm   #3
Chris55000
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Default Re: Decca Bradford Preset Brightness Modification?

Hi!

AH!

I've read thro' the manual again that showed R611 and R612 and it is indeed the all-transistor decoder Bradford 10 Version!

The penny dropped earlier this evening what they were for - I suspect those two resistors may have been added to help with switch-off spot suppression, relying on the principle that the h.t. line takes a second or two longer to discharge on switch-off than the +25V l.t. line, thence taking the common CRT grids a few volts positive of the normal preset potential from VR601 at the moment of switch-off, assisting the e.h.t. to discharge more rapidly and suppress any switch-off spots!

Presumably Decca found the change to the MC1327 RGB matrixing i.c. design in the Bradford 30 decoder rendered special switch-off spot suppression unnecessary because the outputs of the i.c. dropped towards chassis-potential at the moment of switch-off rapidly enough to rapidly raise the CRT cathode potentials towards the h.t. line voltage, and therefore blank the CRT beyond cut-off whilst the e.h.t. discharged!

Chris Williams
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Old 20th Apr 2021, 12:11 pm   #4
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Default Re: Decca Bradford Preset Brightness Modification?

As with all long-ish production runs (except for the Pye hybrid) there were lots of improvements along the way, such as a spring-fusible resistor instead of the unreliable fuse in the HT feed to the line stage. I think the last versions (the 33 chassis) had a resettable 'red button' cutout in the feed. These black-fronted 22" sets gave a fine picture, though perhaps the discrete decoder in the10 chassis had the edge.
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Old 21st Apr 2021, 7:43 am   #5
No.1 Labrat
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Default Re: Decca Bradford Preset Brightness Modification?

Just a little bit of information. Here in Australia we got the series 33 and series 39.
Both had a mains isolation transformer and a V.H.F. rotary tuner.
The series 39 had a voltage doubler whilst the series 33 with the 22" and 26" tubes had a trippler. Both had a thermal cut out on the rear cover.
They were, the only hybrid colour TV sold here. I remember their introduction in 1974, prior to colour transmissions officially beginning in 1975. Only three more years until I retire. Well, that's the plan.

Wayne.
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Old 21st Apr 2021, 8:41 am   #6
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Default Re: Decca Bradford Preset Brightness Modification?

The original 10 chassis had a double pole thermal cuttout on the back protecting the mains input and the LT supply. Decca returned to the thermal cuttout on the later and final 31 and 37 chassis with a single pole cuttout protecting the Mains input.
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Old 23rd Apr 2021, 5:55 am   #7
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Default Re: Decca Bradford Preset Brightness Modification?

Quote:
Originally Posted by No.1 Labrat View Post
Just a little bit of information. Here in Australia we got the series 33 and series 39. ......
They were, the only hybrid colour TV sold here........

Wayne.
It would be interesting to hear how they coped with the Australian heat. Were any modifications made to the original 30 series to take this into account?


Cheers. SimonT.
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