Decca CTV25. Replacement flyback trans..

[FERNSEH]

Hi All,
Some time ago I was given a Decca CTV25 to restore. The general condition of the set is not good. Worst of all was that the line output transformer had failed and it unlikely a replacement could found.
As a replacement I decided to use a Decca Bradford transformer, a much better component and should prove reliable in the long term.
The results are encouraging, on 625 the EHT is correct at 24KV and the raster is bright so I know know the CRT is OK, (try finding a A63-11X these days) Even better there is a good chance the set might operate on 405 lines. The owner is hoping that set will work on both standards.A number of turns has been added to the transform core to determine the volts per turn. The original transformer had a winding to supply the receiver with pulses for clamping etc.

There is stil a lot of work to do on the set.

I'll keep you informed how things progress on this project.

Regards,
DFWB.

[FERNSEH]

Hi Everyone,
I've had not too much time today (Wednesday) to do any extensive work on the Decca, however, my attention was turned to the receiver section of the set. As mentioned before, the set displays a full width raster on 625 lines, but there is no picture and sound on 405 and 625.

The receiver section employs silicon transistors. The power supply is 18 volts negative which is rather unusual. Working backwards the first transtor to have it's voltages measured was the post detector video phase splitter transistor TR106 (BF184)
The voltages are all wrong. The transistor is faulty. I'll look for a replacement tomorrow. (Thursday)

Regards,
DFWB.

[FERNSEH]

Oh dear, it seems that the IF board was a disaster area. I have replaced five transistors. Still I can report some good news, the set now displays a snowstorm on the screen, and I think the multi-band tuner is OK.

I will post pictures soon.

DFWB.

[FERNSEH]

Good morning,

I can now report that the Decca displays a monochrome picture on 625 lines, however, the sound is bad. No results on 405 lines.
The next stage of the restoration will involve winding the pulse windings on the LOPTx. The volts per turn has been determined, it's 9 volts per turn.
The colour difference amplifers require a positive 70volt pulse for the triode grid of the PCL84s, for the croma line blanking it is +24V. The bistable trigger is negative 24volts and for the luminance amplifier line blanking it is -70V.

There is a good chance that the set could be displaying colour pictures by the weekend.

The Decca is the first of three colour set restoration projects for 2006.
There is my own GEC 2028 and the best of all the 1956 Pye 405 line colour receiver.

DFWB.

[Dave Moll]

Originally Posted by FERNSEH

The Decca is the first of three colour set restoration projects for 2006.
There is my own GEC 2028 and the best of all the 1956 Pye 405 line colour receiver.

DFWB.

(my emphasis above) I for one will be very interested in the progress of this one, as these are very rare, having never been in commercial production.

[FERNSEH]

Hi,
With regard to the Pye 405 colour set. Pictures of the set can be seen on the Early Television Website. www.earlytelevision.org

DFWB

[Danny]

Hi its amazing just how many of these pre production sets are still in existance. just out of interest do you know the crt type no?.
Danny

[FERNSEH]

Hi Danny,
The original CRT in the 405 Pye CTV was an RCA 21AXP22A metal cone type, it had gone down to air and was recently replaced with an all-glass Mullard AX53-10. All the valves in the set are normal Mullard types except for two special RCA types employed in the colour demodulators.

Returning to the Decca CTV25. I've wound on the additional pulse windings on the Bradford LOPT. The windings will produce +55-0-55 volts, tapped resistors will supply the +&- 24 volts for the colour decoder.

DFWB.

[FERNSEH]

Hi,
I can not report any good news today about the restoration of the Decca CTV25. The positive pulse for the LOPT operates the colour difference clamp triodes OK. The problem is that negative line pulse has not got a sharp enough leading edge to trigger the bistable in the PAL switch. Also, I don't think it is necessary to supply line blanking transistor with such a high amplitude pulse, it's -70 volts!
It might be worthwhile studying other manfacturers circuits.

DFWB.

[Ed_Dinning]

Hi David, try a C-R differentiator to sharpen the pulse.

HTH Ed

[FERNSEH]

Hi Ed,
I'm back in the workshop. I'll spend an hour on the CTV25. I'll try sharpening the trigger pulse to the bistable.

Regards,
DFWB

[FERNSEH]

Not a great deal of progress has been made on the Decca, the bistable still does not work and the pulse inverter thing does not work properly.

However, I can report some good news, unlocked colours can be seen on the screen, so that's encouraging. The IF amplifier unit will have to be realigned, it's pretty certain that it has been "twiddled" some time in the past. This work can be done on the bench away from the set.

The date for the completion of the restoration is to be April.

DFWB

[FERNSEH]

I've removed the pulse inverter, wound three turns on the line output transformer to provide the the bistable with a -30V trigger pulse and now it works.

My attention will now turn to the IF board.

DFWB.

[FERNSEH]

The pictures shows the modified Decca CTV chassis. The Bradford transformer and the tripler can be seen on the right side.
The focus control is at present the original CTV25 arrangement and features separate adjustment of both standards. For reliablity this system may well be replaced with a more modern focus module.

DFWB