UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Philips T-Vette

TNC · 8th May 2007, 12:00 am

Back again - but this time not so much progress!

Taking your advice Dazzlevision, I readjusted the video bias as per the manual - however, I could only wind it up to 10.3v. - after that the vision and sound just fade out so its just on the limit at 10.3v. Adjusting the AGC after that was little different from before- lowering it brings a fair degree of stability but at the expense of a lot of contrast.

At this point I replaced the OC75 with an AC126 (looks to be a fair equivalent) - this resulted in very little difference to the AGC action and no change to the instability.
I also checked HT4 and that is steady at 83.3v.

After this I tried a few experiments and voltage measurements. The first interesting reading was the voltage on the collector of AGC T2108 which I had previously noted was low - with the new component it was still only 5.3v (spec. 10v) - but that was with a signal...remove the signal input and it rises to 9.3v.
I then removed the tuner feed to the IF panel - that results in clean blank raster- no instability whatever but contrast control has no effect. Partially re-connecting the plug produces a low-contrast picture with no instability...replacing firmly/fully and the picture contrast returns with the instability.
I have a Gould 30mhz scope - on the tuner output the signal appears and is unstable in time with the CRT display - however I feel this may be coming up from the IF board rather than from either of the tuners - although I am not sure I can really justify that thought.
Placing the scope further into the circuit around T2017 (Aurora/Test Card C) gives a pretty clear view of the waveform - no crushing but the instability takes the form of narrow blocks of the waveform appearing to jump up and then back down again randomly/rapidly in time with the CRT instability - I am assuming that's caused by a variation in signal voltage. These results have made me re-assess the instability with I now think must be a random fluctuation in the contrast level. Should I be looking at any particular set of components - I am assuming a leaky cap. must be the culprit?
I have resistance checked C2114 - it reads 26.1k/27.5k depending on the polarity of the test leads - but without removing one leg I am not sure I should draw any firm conclusion from that. Could a faulty cap. affect the signal along its entire path from tuner to crt?
I wonder how I can narrow down the faulty component without having to unsolder many pcb components - which is something I try to avoid. Any thoughts on this or the process so far would be much appreciated.

Thanks again,
Trevor.

8th May 2007, 12:00 am	#37
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Philips T-Vette Back again - but this time not so much progress! Taking your advice Dazzlevision, I readjusted the video bias as per the manual - however, I could only wind it up to 10.3v. - after that the vision and sound just fade out so its just on the limit at 10.3v. Adjusting the AGC after that was little different from before- lowering it brings a fair degree of stability but at the expense of a lot of contrast. At this point I replaced the OC75 with an AC126 (looks to be a fair equivalent) - this resulted in very little difference to the AGC action and no change to the instability. I also checked HT4 and that is steady at 83.3v. After this I tried a few experiments and voltage measurements. The first interesting reading was the voltage on the collector of AGC T2108 which I had previously noted was low - with the new component it was still only 5.3v (spec. 10v) - but that was with a signal...remove the signal input and it rises to 9.3v. I then removed the tuner feed to the IF panel - that results in clean blank raster- no instability whatever but contrast control has no effect. Partially re-connecting the plug produces a low-contrast picture with no instability...replacing firmly/fully and the picture contrast returns with the instability. I have a Gould 30mhz scope - on the tuner output the signal appears and is unstable in time with the CRT display - however I feel this may be coming up from the IF board rather than from either of the tuners - although I am not sure I can really justify that thought. Placing the scope further into the circuit around T2017 (Aurora/Test Card C) gives a pretty clear view of the waveform - no crushing but the instability takes the form of narrow blocks of the waveform appearing to jump up and then back down again randomly/rapidly in time with the CRT instability - I am assuming that's caused by a variation in signal voltage. These results have made me re-assess the instability with I now think must be a random fluctuation in the contrast level. Should I be looking at any particular set of components - I am assuming a leaky cap. must be the culprit? I have resistance checked C2114 - it reads 26.1k/27.5k depending on the polarity of the test leads - but without removing one leg I am not sure I should draw any firm conclusion from that. Could a faulty cap. affect the signal along its entire path from tuner to crt? I wonder how I can narrow down the faulty component without having to unsolder many pcb components - which is something I try to avoid. Any thoughts on this or the process so far would be much appreciated. Thanks again, Trevor.