Philips Z26C512-80 no picture but sound is OK

[Agent24]

My Philips Z26C512-80, a KL9-based TV set, the same as I posted in here: https://www.vintage-radio.net/forum/...d.php?t=108439 has gone on the blink again. This time the picture has vanished.

It was working great after I cleaned the boards, reflowed all the cracked joints, and re-seated the ICs. No more random pink picture and no more horizontal lines through the image.

But then I got home yesterday and was told there had been a click\pop or some such noise and the picture cut out though the sound was still fine. I suspect a blown component but when I had a quick look today and didn't see any blown.


I powered it up and saw nothing on the screen.

The heaters are going and I can hear the 15kHz. Nothing sounds overloaded. The B+ voltage is fine, as are the others, though I did not check the higher voltages (600v and above), nor the EHT (don't have a HV probe), but I believe that is OK as I do get what I believe is a collapsing raster at switch off (used to happen before, too), and when I discharged the tube I got a good spark.

I can't confirm if I get a raster as I don't know where to find the screen control on these sets, but going by the image produced at switch off I am assuming it's OK - does anyone know otherwise?

I have the schematic from here: http://elektrotanya.com/philips_chas.../download.html and that's it. I also have a KT3 schematic on paper which is likely more inaccurate.

I have uploaded a video of the current state of affairs here: https://www.youtube.com/watch?v=d8Ez3brfo0M Oddly enough that was the only time when I saw a bright circular dot at the end (no idea what causes that)


Any ideas where to start?

[Nuvistor]

I can only give generic advice, never seen on of these. the CRT seems to be "cut off", check the DC conditions on the tube and work back from there. Some sets blank the tube with no signal although you have sound it could be the video has been lost and the circuits are cutting off the tube.
Hopefully someone know the chassis and can give you better advice.

[Agent24]

Originally Posted by Nuvistor

check the DC conditions on the tube and work back from there.

Not sure exactly what you mean, I assume you mean the cathode voltages? Bear in mind I am not incredibly familiar with CRT sets. What should they be? Schematic shows 140v.

G1 should be what, around -30v?

I can't measure G2, I tried measuring G2 last time I worked on this but my meter is too low impedance and it shuts the set down. Can't measure focus or EHT as no HV probe.

Can't measure heaters either as meter doesn't do true RMS (I have read this is what you need) - But I could use an oscilloscope, though I doubt there is any point as they seem to be working just fine anyway.

[Agent24]

Just checked: At the tube socket pins, the cathode voltages are about 130v each, and G1 is -52v

I don't know if that is correct or not for this tube: http://www.radiomuseum.org/tubes/tube_a66-540x.html as I can't download the datasheet there.

[Nuvistor]

I don't know the tube either but with effectively -180 on G1 I would say the it is biased to cut off.
You will need to work your way through the video circuit and any protection circuits that are in use.
As I said I don't know the set, the noise that was heard could have been a flash over in the tube or other high voltage area and damaged components.

[Agent24]

I am guessing then that the G1 voltage is wrong? It seems derived from Pin 18 of the LOPT and I don't see how it could change unless something went wrong. I have uploaded a picture of the circuit that appears to generate it. Line A24 goes straight to the tube (Well, through a 1k5 resistor first)

I inspected more carefully and found an electrolytic capacitor, 100uF 35v, C464 with its positive lead corroded off. It came to my suspicion when I saw it was on a slight lean, while other capacitors were straight. Though it looks like it may be have been like that for a long time. It's the filter capacitor for the -23v Rail and replacing it made no difference.

There may be something funny with the voltages going on indeed. A couple of times now I have heard a click like a small static discharge a few seconds after the set has shut off. On the last time it happened I saw a small white dot flash on the screen, so maybe there is a HV problem and some protection circuit is activating. But I don't understand the protection circuits so I don't know what to look for on the schematic.

[Agent24]

I think I get it now after looking at the waveforms on the schematic - the cathode voltages are too high, right? And G1 is probably fine?

It looks like most things must be working actually. I started checking the waveforms marked on the schematic, but I was having problems with my pattern generator and trying to tune in the signal (annoying because it's old too and has no audio tone, just video)

Then I spied a very faint image of the vertical green tuning line that goes across the screen while searching for signals.

[Agent24]

Looks like I might have found something. Assuming my thinking is right (It could well be totally wrong!)


Got the test pattern generator tuned in and checked the waveforms. Colour bars signal coming out of the TDA2541 IF Amp looked good.

Then I checked the RGB signals coming out of the TDA3560 PAL decoder and they seem to match up (mostly) with what the schematic says.

Now, I believe a corresponding staircase-looking luminance waveform is supposed to come from module U430 Pin 12 - which on that plug-in board goes to Pin 10 of a TDA2560, and the datasheet calls that the "Luminance Output". Then it goes through R425 into the base of TS426.
At least that is the case on both the KL9-S and KT3 schematics I have, but on my KL9 it is not so.

I have a TDA3560 which has no such "Luminance Output" pin. My U430 module board has no Pin 12.

I do have a TS426 and other associated parts nearby though, so I assume this circuit is the same. I checked the base of TS426 (can't find R425, so no idea where the signal actually comes from yet) but I expected to still get some kind of staircase waveform. However what I actually got was pretty much nothing. Well, not nothing (I think I see some part of the sync signal or something) but everything else is just flat.


I guess this is why there is nothing on the screen - no signal to TS426 means the RGB amps are being told to drive the cathodes with RGB signals at basically zero brightness, so they are effectively outputting nothing. Does this make sense so far?

[Nuvistor]

You seem to be getting there, the TDA2560 appears to have seperate circuits for colour and luminance. Your set is different but probably very similar, try to find where the luminance drops out and hopefully find out why.

[Sideband]

A common problem on KT3/K30/K35 which I think are the similar European sets was a capacitor going O/C (after a flash-over) in the beam limiter that effectively cuts off the RGB drive from the luminance/chroma chip. I'd need to see the circuit of the RGB drives and chroma stages to remember where the capacitor is located.

[Agent24]

Originally Posted by Nuvistor

You seem to be getting there, the TDA2560 appears to have seperate circuits for colour and luminance. Your set is different but probably very similar, try to find where the luminance drops out and hopefully find out why.

Have to figure out where the base of TS426 goes... it certainly doesn't go to Pin 12 of U430!

Originally Posted by Sideband

A common problem on KT3/K30/K35 which I think are the similar European sets was a capacitor going O/C (after a flash-over) in the beam limiter that effectively cuts off the RGB drive from the luminance/chroma chip. I'd need to see the circuit of the RGB drives and chroma stages to remember where the capacitor is located.

That sounds bad. What would cause a flash-over in the first place?

Mine is very similar to the KT3. In fact the mainboard is silkscreened with voltages for both KL9 and KT3.

The KL9-S schematic from here http://elektrotanya.com/philips_chas.../download.html is also pretty close, it seems.

I will scan and upload the KT3 schematic I have somewhere too.

[AndrewM]

I've attached a page from the Australian KL9A-3 manual and circuit which sounds very similar to your set. The European Philips K35 chassis is also very similar.

There are several things that could cause the video drive to be cut off but I'd look at the brightness and contrast control voltages before going much further. The beam limiter circuit connects to both these control lines and is a likely area of failure to investigate.

[Agent24]

OK, I will check those signals.

But, I do not understand this KL9A-3 schematic. The RGB amp circuit is different and there is no TS426. I assume 7257 is doing the same job, but it does not appear to receive a luminance signal from anywhere. I don't get it.


My RGB amp is much the same as the KT3 NZ version schematic which I have attached. Except for the base drive to TS426, apparently.

Can you tell me where the beam limiter circuit is? I don't know what I'm looking for.

[Agent24]

I think I got it (from reading http://books.google.co.nz/books?id=9...ircuit&f=false)

Is this the right circuit? (See attached image)

[AndrewM]

Yes, that's the beam limiter. However, that circuit uses the early decoder so it does not work the same way on your set.

I've uploaded the KL9A-3 circuit because it uses the TDA3560 decoder chip which you said your set uses. It functions very differently to the earlier TDA2560 and TDA2523 IC combination as the luminance signal is matrixed within the TDA3560 and puts out proper RGB signals that are then amplified before driving the CRT cathodes.
I'm assuming that Philips NZ followed the standard design when they did their version.

I've just had another look at the K35 circuit and I can see why your confused. They use the original RGB amp with the later TDA3560 decoder. TS426 no longer functions as a luminance amplifier from what I can tell.

The KL9A-3 colour decoder circuit should still be the same as your set, it's just the RGB amp that's different. The output from the TDA3560 is RGB not R-Y, B-Y and G-Y so the waveforms are different. There is no external luminance signal path, it is all done within the chip. If you look at the block diagram of the TDA3560, pin 10 is the luminance signal going into the chip and the block marked Y in the corner is the luminance amplifier. Pin 7 is the contrast control line for the amplifier. The output then goes to the RGB matrix before being amplified and the RGB signal exits on pins 12, 14 and 16. It all occurs within the decoder IC.
Pin 11 of the TDA3560 is the brightness control line, it varies the gain of the RGB driver amps before exiting the IC and being fed to the final RGB amp module.

Hope that makes sense.

[Sideband]

Yes if you follow the line 'A35' back along the bottom thick black line you find it takes you to connection 'D' from the tripler. C565 decouples this to chassis and this is the cap that can go O/C if the tripler or the CRT arcs.

[Agent24]

Originally Posted by AndrewM

Hope that makes sense.

It sounds OK to me.

Though the beam limiter circuit with TS422 is almost exactly the same in mine, even if it operates differently here. Except that C423 appears to be deleted. However the output of that circuit (ie: Pin 19 of U430) does not go straight to Pin 7 of the TDA3560 but instead goes through a 4053 multiplexer first. I think this has something to do with handling the input from the teletext board.


The Brightness and Contrast signals going to Pin 19 and 18 seem OK actually. I can control them with the front buttons and Contrast changes from 2.155v to 3.48v while Brightness changes from 1.145v to 3.39v, which matches up mostly with the schematics.

In any case, I assume if the brightness limiter was faulty the value would not change at all.

I suppose I need to check if those signals get all the way to the TDA3560 though.

Originally Posted by Sideband

Yes if you follow the line 'A35' back along the bottom thick black line you find it takes you to connection 'D' from the tripler. C565 decouples this to chassis and this is the cap that can go O/C if the tripler or the CRT arcs.

C565 doesn't appear to exist in my set. But C566 does and it tests as OK on my capacitance meter anyway.

I might keep looking at the components there because the old tripler I replaced a few months ago had been arcing from a crack in its side.

[Agent24]

Well, I soldered a couple of wires to Pin 7 and Pin 11 pads under the TDA3560 and get the same voltage ranges as I did probing Pin 18\19 of U430 connector. So I guess those at least are not the problem.

[Agent24]

I went around in circles all afternoon, then looked at the K35 schematic and I believe that the implementation there with TS426 is the same as mine. The waveform I saw on my scope is therefore probably correct.

Instead of a Luminance signal feeding the base of TS426, the base drive to that transistor starts as a 140v p-p square wave from the LOPT, reaching TS426 much smaller. Earlier on that signal is also fed into the RGB amp on Pins 3,11,18 of the RGB board (U440) which do measure 19v DC as said on the schematic. I don't know what this signal is for, though. Is it something to do with the retrace blanking?


The K35 schematic also shows extra video signal waveforms as they go into the RGB amp board on pins 1, 7 and 13. These also match up approximately when I view them.

I don't think TS426 is doing anything wrong as the same signals do end up coming out of the first transistors in the RGB amp circuits (BC558) which is after that TS426 circuit.
But, I have found they do NOT come out of the first BF422.

I also scoped the RGB connector on the neck board and there is nothing there either, just solid 130vDC.

So it would seem that all 3 video amps are not working. But their supply voltages all seem OK. The only other common thing between them is the connection at Pin 6. Which goes to C464. Sound familiar? This is the faulty capacitor I replaced. I thought this was supply decoupling for the -23v rail but it is not. I confused myself with C586 somehow
It actually goes in series between the -23v rail and Pin 6 of the RGB amp board.

I don't know what is supposed to be on Pin 6 of the RGB amp board. The schematic does not show a voltage or waveform. With the scope there is a small amount of ripple. This exact ripple is also present on the -23v rail. I guess that makes sense, it would be coupling through the capacitor.

With the DMM, it measures -1.22 volts. Interestingly the K35 schematic has voltage charts for all the plugin modules and indicates Pin 6 of the RGB Amp (They call it U10 there) should be 0.4 volts. Or -20 volts. I am not sure why two voltages are labelled. I have uploaded a picture of the U10 voltage table, hopefully someone can interpret it. I don't understand why one column has "U..." and other "U..X" or what this means.

Either way my Pin 6 voltage is neither 0.4 volts nor -20 volts so perhaps that is why the amps are not working.

Can anyone enlighten me as to what C464 does and what the circuitry around Pin 6 of RGB amp board is for? I also saw on the K35 page for the RGB amp board that Pin 6 is labelled "ON/OFF SUPPR. INFORMATION" - I can guess what that means, but am not sure.

[Red to black]

Can anyone enlighten me as to what C464 does and what the circuitry around Pin 6 of RGB amp board is for? I also saw on the K35 page for the RGB amp board that Pin 6 is labelled "ON/OFF SUPPR. INFORMATION" - I can guess what that means, but am not sure.

C464 looks like it is a decoupling capacitor for the -20V line, the + of that capacitor is to the chassis ground or 0V line, the - to the -20V line, the minus 20V line is referenced to the 0V line (or Chassis ground).
On/off suppr. is I believe for spot suppression at switch off.

The two columns for voltages marked U and U..X are for normal running conditions from the tuner (internal) and when the set is in external mode (external video/AV mode) respectively for the K35.

I hope this helps