Moderators: Dave Moll, Andrew Davie, Steve Anderson
Klaas Robers wrote:I see that you have problems with the line synchronisation of your wave form picture. And that is funny, because your picture (right) is well synchronised. Why do you not use that sync signal?
. I see that your picture (right) is focussed well, because now and then I see the individual lines. This is contrary to the wave form, which is unsharp. You should be able to focus the left tube as good as the right one. I hope that you have not just one focus control....
Good luck!
Andrew Davie wrote:Loving it! Well done, it's a beautiful thing
Klaas Robers wrote:I think Harry, that making a separate focus control and a separate brightness control for both picture tubes is a better solution. Then you can optimise both at the same time.
I like the video signals with the sync pulses negative (down). This is how video signals should look like. Then in the video signal high is white and low is black (dark). Intuitively it is like that.
And for the line width: I think that seeing one line, a sync pulse and the beginning of the next line is enough. That is indeed the situation where you came from.
Steve Anderson wrote:The poor focus on the 'waveform' CRT does need addressing. That circuit was created around a 'nominal' tube, but with age, maybe use and other things this CRT is not quite nominal.
The simplest thing to try is to swap the two CRTs and see what happens.
Also check the resistors and pot that form the focus circuit are actually correct. Older parts, especially pre-70s, are notorious for going high in value, particularly resistors, simply with age. Capacitors, especially those wax-covered paper variety are not worth salvaging.
Try the tube-swap first - let us know the result.
Steve A.
Actually, having played the videos rather than looking at them still, the focus on the waveform CRT isn't that bad. If the focus control is at one end of the track a small adjustment to either of the two resistors each side of the focus pot may be required. i.e. try changing the 470k to 560k or 390k. Or the 220K, try 180k or 270k.
There is the possibility of the CRT going 'soft'. Over the years a small amount of air has leaked in - and it only needs a small amount to cause de-focusing. Let's hope that is not the case. If the 'getter' has a hint of white then that's probably the cause. The getter is the (usually) silver coloured metallic gunk sprayed into all tubes just prior to final sealing. In most CRTs it's usually near the base on the inside of the glass. If it's starting to look a bit white then that's a sign of air getting in. This applies to all tubes/valves, not just CRTs. The photo shows a tube with what a getter should look like.
Steve Anderson wrote:Yep, that generally looks better. Even with brand-new resistors I check the resistance before I fit them. Often the colour code can be 'off'. What should be a red looks more like a brown for example. With 'reclaimed' resistors I would always check them, it only takes a few seconds and can save you hours later trying to find what is causing you a problem.
There are many places in a circuit where a resistor could be twice or half it's supposed value, e.g. pull-up resistors in logic circuits. But in an analogue circuit like this they should be near the intended value...what is near? Say 10% or better still 5%. Many resistors used in domestic gear were 20% up until the end of the tube/valve era...and that's when they were new!
When measuring resistances (or anything else) remember that your meter has an accuracy specification too. Most modern yet inexpensive DVMs aren't too bad in that respect, but often the resistance ranges are those with the greatest error. Keep that in mind. Just because it's digital doesn't mean it's accurate.
i have seen others using DVMs where the lo-battery symbol is showing, "It's still working, so I'll use it until the display dies." The problem there is that the DVM chip generates a reference voltage - all measurements use this to calculate what it should display. If the reference is 'off' because of a low battery the result will be too. If you live right next door to a 711 - no problem, otherwise always keep spare batteries.
Steve A,
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