Cramped picture Philips (Techniek Radio/TV)

door Richard_M @, 13.01.2023, 23:14 (16 dagen geleden)

Hello. I hope you don't mind that I'm writing in English.

I've been working on a Philips X20T731 TV (Chassis type F6) and it has a very squashed picture in the bottom half. The linearity controls can't solve it.

I have tried a new PCL805 (B407), no change. Cathode voltage measures 20.8V. The cathode resistor R914 is OK. Cathode capacitor C650 was defective, but replacing it doesn't change anything either.

What should I suspect?


[image]

Cramped picture Philips

door ruudtx @, 14.01.2023, 17:33 (16 dagen geleden) @ Richard_M

In the cathode circuit is the line oscillator, could be faulty. But apparently still working.
Normally it is the cathode cap.
Does it have the correct value?

Cramped picture Philips

door Richard_M @, 14.01.2023, 18:03 (15 dagen geleden) @ ruudtx

That was the first thing I checked. It was bad (measured almost double value, and huge ESR), but replacement with a new one did not change anything.

Even experimenting with larger/smaller value does not do anything. Neither does running the TV without the cathode capacitor.

It's ignoring it totally!

Cramped picture Philips

door Goldline @, Oranjedorp, 14.01.2023, 19:45 (15 dagen geleden) @ Richard_M

Do you have 0 volts on pin 9 (g1 Penthode) ?
[image]

--
Cees PA1DBA

Cramped picture Philips

door Richard_M @, 14.01.2023, 21:23 (15 dagen geleden) @ Goldline
Bewerkt door Richard_M, 14.01.2023, 21:46

Not sure to test AC or DC so I did both:

When TV is warming up the DC reaches 1.2V, then when the picture appears it goes down to 0.025V. If the TV is tuned to static, then voltage is 0.2V

Switch off the TV, goes to 0.6V then slowly to 0.0V.

AC with picture on screen 14.25V, with static 18.3V. Does not go up when TV switched off.

The DC measurement is the important one? Any DC who is on this pin must come through C647 or C648?

Cramped picture Philips

door Maurice ⌂ @, Dordrecht, 14.01.2023, 23:34 (15 dagen geleden) @ Richard_M

0V is at DC measure.
Follow dotted line and find fig.9 scope signal, that is AC on pin 9 PCL805

But there is way more to the left of that tube.
Apparently the sawtooth at pin 3, cathode triode PCL805 is not ok.

So trace back till TS422 (BC158) where it all starts.

Without a scoop , it is only some try and find an error by luck.

--
"I haven't even turned this on to see if it works because as long as I don't it might"

Cramped picture Philips

door Maurice ⌂ @, Dordrecht, 15.01.2023, 00:05 (15 dagen geleden) @ Maurice

I often go to the colour option in schematics.
As by then I get the idea how the signals follow the path.

Also get idea what is important to specific components.

You see the main red line what is the signal path.
Rest are corrections, feed backs and all kinds of helpful to get a steady picture.

[image]

--
"I haven't even turned this on to see if it works because as long as I don't it might"

Cramped picture Philips

door Maurice ⌂ @, Dordrecht, 15.01.2023, 00:18 (15 dagen geleden) @ Maurice

If you can manage to get the picture right.

I found that F6-chassis to be one of the best B/W TV sets.
For me it sure can compete with modern flatscreen.
With a good CRT (As I can see yours is very fine also) it is a bright
as a 100W light bulb

Here mine X20T731
For detail , look at that shiny sharp black hair at the right.
[image]

--
"I haven't even turned this on to see if it works because as long as I don't it might"

Cramped picture Philips

door Richard_M @, 15.01.2023, 14:48 (15 dagen geleden) @ Maurice
Bewerkt door Richard_M, 15.01.2023, 14:58

Thanks for colouring the signal paths. Some was obvious, some was not

The measurement being at DC is something I know, I'm just making rookie mistakes because I don't normally work with valves. I'll get used to it soon enough.

I can't afford an oscilloscope at the moment, so I'm going test every component in this section individually.

You're right it's a very good TV. With this one the previous owner said she used it from new until 1995, so when I powered it up and got a good picture it was a very happy surprise. It looks like the original CRT too, no replacement or regun sticker.

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