Thread: Philips T-Vette
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Old 23rd May 2007, 1:11 pm   #40
TNC
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Join Date: Apr 2003
Location: Westcliff-on-Sea, Essex, UK.
Posts: 385
Default Re: Philips T-Vette

Have finally managed to spend sometime on diagnosis...and I think the results clear the AGC circuit.

As suggested, I substituted a 9v battery + pot (actually a 50k seemed to be work best) in the AGC feed to the tuners.

On VHF - without AGC, just a trace of a signal displayed - obviously floating around - no sound, no vertical locking.
With 5+ volts of agc, as before with good contrast and sound and continual, severe instability - the scope reflecting this in varying signal strength - but
no detectable distortion of the video.
Below 5v, obviously contrast and audio fading away but much greater stability - not perfect but almost watchable.

On UHF a somewhat different reaction. With the AGC line disconnected from c of T2018,
the picture/vision still appeared quite strong - but considerable instability.
On connecting battery power (5v), contrast and sound up further but video signal much distorted and screen display unintelligible.
Disconnected battery but coupled agc line to chassis via .002mf and 100k in parallel and vision/sound restored but still quite unstable - as reflecetd on the scope.
I also took the UHF feed direct (rather than via the VHF tuner) to the pcb and got broadly similar results.

I think all of this confirms that the problem lies between the tuners and the final vision output but with the agc in the clear...the instability worsens as the signal strength rises which looks like leaking of some sort - so I will now turn my attention to Can A!
Trevor
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