What to do with a Philips T-vette...
I think the sync separator is an OC44. I know these seem to go on forever as mixers but I did have one a long time ago give odd sync problems in a T-Vette
Cheers, you both gave me some idea's what to look at, I stuck the scope on the collector of the sync separator (T4021) which is also the point for waveform number 3 in the manual, there was the required train of pulses, but with noise overlaid, particularly bad when bright objects appeared on screen, which also distorted the pulses, and when the distortion was present, the sync was interrupted, resulting in the frame dropping on screen. Cathovisor's mention of bias made me remember there is a video bias pot on the main board, and the manual states how to adjust it, meter between chassis and the base of the video amplifier transistor (T1220 on the tube base), set for 11V with no signal, so I did what it said, it needed quite a tweak to get it to 11V, but now I have a rock solid picture, and the waveform (3) looks a lot healthier!
I'll give it a good long test with various video sources just to make sure, but so far it's looking good! I've not checked 405 again yet, so I've probably ruined that to fix 625.. Oh, and it's working now without an attenuator!
I'm guessing that something must have drifted off value somewhere to make it need such a large adjustment, so I'll keep prodding at it.
Thanks again for your help 🙂
Spoke too soon, it's doing it again!
Well I suppose you could try another transistor. Don't know how critical it is but an RF transistor will probably work OK. Don't forget the electrolytics though...
I've probably got an OC44 somewhere.
When I had a T-vette some years ago, like John, I found it was rather critical of 625 signal strength. I know that if used outside in the back garden with it's own telescopic aerial it gave good stable results. But using the main roof-top aerial, it was quite unstable with similar results you have with your set now.
However, this does not mean that it is meant to be like this, because surely it would have been designed to accept the widely varying signal strength a portable set would encounter.
Are there any tired electrolytics that could be slugging the AGC action perhaps?
Sorry for the slight absence over the last few days! Not been feeling too good, spent far too much time sleeping than anything else..
I did get back to this little horror yesterday though, and did some more poking of things. First thing, the cap I replaced back near the start, the 4uF that cured the frame collapse, I’d not noticed it’s supposed to be a non polarised one! I’ve changed it now to 2 back to back caps, not made any difference to the sync issues though.
After lots of messing about I think I’ve found why the sync is lost, it’s more false lock I think, I had the scope on the collector of the sync separator again, and if I slow the scope down and set up the trigger for vertical TV signals, I can see this:
looking closely at the start of the waveform, there is the little negative pulse, which I’m guessing is the frame sync pulse, when the frame does it’s false lock thing that pulse disappears, and the waveform ends up like this:
interestingly I can reproduce the same on 405 sometimes too, but it seems more resilient than 625. I’ve pulled all the electrolytic’s around the AGC video and sync and replaced a few, but most test good on the component checker, I did get some interesting results swapping the sync separator transistor for a silicon PNP, it then would have a blip in both line and frame sync when changing from light to dark! But it would recover a lot quicker.
that’s as far as I’ve got!
at least the test card is looking better!
Sorry for the slight absence over the last few days!
That's nothing... It has taken me nearly two years to get back to the frame fault in the PAM set. 😉
I’m tempted to shove this thing back on the shelf for 2 years!! Maybe some of these really were mediocre at best on 625?! I did wonder if it was because I was displaying 16:9 content with black bars top and bottom, so I switched it to 4:3, and it still does the same.
Maybe some of these really were mediocre at best on 625?
Hard to say, but at the time (1966?) 625 was not common-place so most would have been used on 405. This system is ideal for a portable, especially just using the telescopic aerial from room to room. 625 was a bonus as a selling point perhaps, but may not have been refined enough to what we are used to in later UHF only portables.
I'm guessing of course and tricky to say for sure if there is a definate fault, but it would certainly seem that these set's are fussy on 625.
It might be worth posing the "625" question over on UKVRR too, so as to gain an even wider opinion of the set. If they generally really are a bit tricky on 625, it could save you lot's of time.
My T-Vette Works reasonably well on 625 although I get the impression that the contrast is lower...not really a problem since you just increase it a bit. I certainly don't have any sync issues but I seem to recall that some could be a bit tricky and have subtle faults. The expert I knew at Philips has long since retired or possibly even departed this world by now since he'd be in his late 80's. When I'm next in the workshop, I'll power mine up and give it a run via a modulator.
Just given mine an airing after several months. The contrast levels on both 405 and 625 are good although a slight readjustment (increase) is required on 625. There is more than enough contrast available on both systems.
Cheers for trying yours out! It sounds about the same as this one here, it certainly doesn't struggle with contrast, although it is a bit up near the top end of the control, there's still a little play in it.
I was reading the Practical Television article from Mikey's old technology website the other day (can't seem to find it on my PC, nor will it let me download it again now?!) and it mentioned a few suspects to try for jittering frame and false lock, all of which I tried, but it still insists on doing it! I am beginning to question my component tester now though, I replaced 2 caps in the frame stages with new ones, even though the old ones test good, and there was a large increase in the height on screen after replacing them, the height control now sits at just below half way, whereas before it was less than a quarter turn from the top end, so I'm now thinking I should just order up a load and replace the lot. Before that though I think I'll stick it back together as it is, and give it a good long run and see how it goes, maybe it'll fix itself! (I can dream....)
I was reading the Practical Television article from Mikey's old technology website the other day (can't seem to find it on my PC, nor will it let me download it again now?!)
It is available via our B&W servicing the sets' section here
Cheers Chris! I'll make sure I transfer it to the iPad this time, before my PC decides to get rid of it again for whatever reason!