1967 Decca CTV25

The “Battersea” Chassis

Model: Decca CTV25

Year:1967

System: 405/625 Line Colour

Original List Price : £377 gns

Valves:
V200 PL802 Luminance output
V301 ECC82 Frame Oscillator
V302 PL508 Frame output
V303 PCF802 Line Oscillator & Reactor
V400 PY500 Efficiency diode
V401 PL509 Line output
V402 PD500 EHT Stabiliser
V403 GY501 25kV Rectifier
V600 PLC84 R-Y Amp & clamp
V601 PCL84 B-Y Amp & clamp
V602 PCL84 G-Y Amp & clamp

Transistors: 35

Diodes: 40

CRT: A63 11X

General Info:

Dual standard television incorporating 11 valves, 35 transistors, 40 diodes and providing the following:-
Colour and Monochrome on the 625-line system on U.H.F channels 21 – 68 inclusive and V.H.F. Channels C – H inclusive and suitable for colour modulation to the PAL system. In addition monochrome on V.H.F. channels 1 – 13 inclusive in bands 1 and 3 operating on the 405-line system. The set is provided with a multi-band tuner controlled by a six push button channel selector mechanism which also selects the transmission standard appropriate to the desired channel. The tuner however, has the attribute, ” any button, any channel, any system”.

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Powered on, I cannot get a good locked colour. When fine tuning to the testcard I can get a good B&W picture, but tuning to tune it for colour sync and colour lock go out is impossible. Looks like it might have decoder issues/ref osc.

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To get to the bottom of colour issue I need to disable the colour killer to shed some light on the fault condition. Checking the manual I found the colour killer can be overridden by shorting D600. I fed in colour bars from the Philips 5515T, powered on and the truth be confirmed, the reference oscillator is off hence unlocked colour.

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Now what’s needed is to follow the oscillator tuning procedure, this entails scoping pin 1 of the decoder and checking the waveform present matches the one in the manual. The amplitude needs to be 2V p-p. Then I need to connect the scope to the left hand of R658, then adjust L604 for maximum output which should be 6V. Again a sine waveform is given as an example. Fingers crossed this sorts it. Checked the waveform at pin 1 of the decoder, it looks correct and reads about 1.8V p-p, it should be 2V but I guess that’s close enough.

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Next up is setting the tuning of the oscillator and peaking it to the stated 6V p-p unfortunately the colours are still unlocked need to put my thinking head on now, I wonder if I need to now perform Burst tuning followed by setting up the phase discriminator.

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Checked the burst, it should be 30V p-p +/- 5V I have roughly 23V p-p again pretty much within spec. I will check the phase discriminator next.

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Success, adjusting the phase via VR602 has given colour lock. I think I need to sort the delay as I have blinds. Still much improved on what I had.

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4 thoughts on “1967 Decca CTV25

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