Login

Murphyv310 · 20-06-2021, 08:11 AM

Hi.
There is an improvement Adrian, the ringing isn't quite as bad.
I do see a small amount of multiple ringing with the first one more noticeable.
Check out the cathode components on the video amp and any correction chokes, some are wound on a resistor for damping and the resistors may have increased in value.

AdrianPH · (This post was last modified: 20-06-2021, 01:43 PM by AdrianPH.)

I have included a scan on the video detector and output on the 991 Schedule C/D.

It does seem to be a very basic circuit lacking in lots of components, detection and cathode load are both done with diodes. OK a metal one in the cathode.
L23 and L24 are wound coils on formers. R33 & R34 were replaced when I worked on the set as was C28 in the noise limiter all though this is wound off.

The Resistors and caps from the brilliance control to the R1123, R114 & R118 and the two caps C88, C89 were also replaced.

I have just looked at the 3K9 resistor R28 that was high so I decided to change it, in doing so I also found that the coil L21 was open circuit with a wire off at one end of the resistor, this was sorted and it all put back together. No noticeable change to the picture that I can perceive.

Adrian

If there is anything one should consider I look at please let me know.

peter scott · (This post was last modified: 10-09-2021, 01:09 PM by peter scott.)

Hi Adrian, I was puzzled by the twinned verticals at the top left and bottom right of the test card. I assume that the two different positions of the verticals are from differences in the two 202.5 line half scans. This suggests to me that HT ripple might be the cause. That said, I was wrong to suggest that it was an effect on the frame scan. If the effect is varying then perhaps mains ripple on the line scan but if static then the frame rate modulating the line scan.

Do you have an isolating transformer and a 'scope? If so perhaps you could look for ripple on the line output HT?

Peter

https://youtu.be/DIbJUKFLPzs

Apologies for starting a separate thread:
https://golbornevintageradio.co.uk/forum...p?tid=8548&pid=93928#pid93928

peter scott · 10-09-2021, 01:35 PM

Hi Adrian,

It looks like I have an obsession with replacing HT smoothing caps in these little Ferguson sets. Here I am at my "work bench" replacing that component 57 years ago!

Peter

peter scott · 10-09-2021, 01:57 PM

I see the 991T has flywheel sync and there is supposed to be a long time constant between the discriminator V17 and the grid of V15. It might be worth checking or replacing the 1uF cap. C55.

Peter

AdrianPH · 10-09-2021, 02:56 PM

Hello Peter,

The mains capacitors were replaced as well as others around the video and frame and line stages. Trying to think what else I did after that video was originally posted in Feb. I changed the CRT for a better one in May, also did the alignment for the phase osc alignment.

I believe C55 was also replaced with a film style 1000 Volt one when I did caps C45 and C66 in the same area as it would have been a paper one, but I can happily do it again.

I just need to get the work area back, I use the lads bedroom when he is at University and he is at home at present.
I still have to get any isolation transformer so I could use with a scope, I will generally not remove the earth on my scope.

When I get chance I will post another picture as it is now. I think the last images taken from it in June were these: -

Still ringing.

Adrian

AdrianPH · 10-09-2021, 04:10 PM

Peter just a question, if the line sync was firing at the wrong time on half of the frame (202.5 lines) and this was consistent would the two vertical lines be the same brightness/contrast. i.e. be the same intensity?

Just trying to figure out how to better fault find.

Adrian

peter scott · (This post was last modified: 10-09-2021, 04:37 PM by peter scott.)

My thoughts were that the average line speed was correct for a straight left hand side but without the 1uF would be able to rapidly change line speed by the time it had reached the first test card vertical line. Having said that the photo of the test card in your #36 post doesn't appear to show the problem so perhaps you've fixed it since making the YouTube video which I think is dated Feb. 2021. The 1000 volt rating of your 1uF is a bit higher than you really need. 100 volts would be more than enough but that shouldn't matter. Did you replace C65?

Peter

AdrianPH · 10-09-2021, 07:29 PM

If it was a paper cap it got replaced also the brown Hunts caps were replaced as a matter of course.

Login
Username:
Password:	Lost Password?
	Remember me