Pye 697 chassis - poor luminance

[Hunts smoothing bomb]

Hi all,

I am trying to iron out some probs with a Dynatron CTV, uses the Pye 697 chassis I think.

Someone in the know said that you can test tube emission by grounding the tube cathodes to chassis, If grounding one at a time you should get a pure raster of the primary colours, and if all three, a white raster, they then said that the brightness of the rasters will be the tube working at max emission and therefore you can judge how good the tube is.

Is this correct as I am a little wary of doing it, it just sounds a little "hairy" to me.

Thanks all

Lee

[Hybrid tellies]

Hi Lee
Personally I would not try grounding the CRT cathodes of any set, won;t do the tube any good and could overload the Line output stage. As the Pye 697 chassis uses colour difference drive, rather than RGB drive so you would have to seperate out the three cathode feeds as on your set they are connected together and driven by the PL802 luminance output valve.
The best way of assessing the tube on these sets is to use a colour bar generator or perhaps a recording of test card F, then with a suitable meter check the cathode grid and first anode voltages. If these are ok then I think you can turn off each gun on the convergance panel by the First anode controls.
There are quite a few of us on this forum who have experiance of this chassis and will be only to willing to help out if we can. However it would be helpful to let us know what problems you are trying to iron out first.

[Mike Phelan]

I reinforce what Simon says - dont do it!
If one or more guns is low emission, it will be fairly unmistakeable with just a normal picture.
If you have a dark picture or a missing primary colour, it is something else.

We really need to know what the actual fault is, then we can go a bit further (probably towards the CDA panel!)

[Focus Diode]

I also reinforce what Simon and Mike say!!!

A quick way to check the emission of the three guns is to collapse the frame using the switch on the CRT base, switch off two of the three guns (on convergence panel), then turn up control. If all ok they should be a red horizontal line at the centre of the screen (if only red gun on). Repeat with other two primary colours.

If there's a primary colour missing check volts as explained by Simon as the lack of one colour could be, as Mike says, due to another course. If all volts are ok and the emission obviously differs between each of the primary colours it can usually be set up ok for three coloured lines of equal brightness. Inevitably in time re-adjustment will be neccessary as the emission of the faulty gun deterioates further.

I kept a 22" CRT going this way for a couple of years, despite a label on the degaussing shield saying the CRT was naff, until it got so bad it had to be replaced.

Cheers,
Brian R

[Hunts smoothing bomb]

Thanks for your pointers chaps.
I started resurecting this set a month or two ago under the thread Dynatron CTV10
I got as far as getting a picture on the set but thats about it, the picture is very dim and practically no contrast but all primary colours are apparent.
I know I have probs in the convergence panel as the controls hardly function at all. Also the convergence board according to the service data has a couple of AC128's on there, I have found two metal cans that look like AC128's but they only appear to have two legs! I was just trying to think of a quick way to test the CRT before I spend too much more time on it,

Regards
Lee

[Focus Diode]

The AC128s on the convergence panel are "Diode strapped" hence the two legs!

From the sounds of it it doesn't sound like the CRT. These series of sets have a very active contrast control, so the problem is more likely to be on the CDA board. Are you sure the PL802 luminance output valve is ok? If you don't have a spare a 6F28 or EF184 can be tried, but ensure the print to pin 6 of the valve is disconnected first, this doesn't need re-connecting when a PL802 is later fitted. Many thanks Mike Phelan for the tip.

Cheers,
Brian

[Focus Diode]

For poor brightness/luminance check R356, R357 (both 2K7), VT4 & VT5 (both BF184), in addition to the PL802 of course. Info from the J.McCourt book.

Cheers,
Brian

[peter_sol]

Hi. If the picture looks milky and /or slightly negative it may very well be a low emission crt.

[Focus Diode]

Another possibility for the poor picture is low HT caused by the main multicap which is strapped at the bottom of the LTB/PSU. If it's a PYE labelled type it is suspect.

Cheers,
Brian