Philips Z26C512-80 no picture but sound is OK

[Red to black]

Originally Posted by Red to black

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

oops I made a mistake with this bit

It is actually meant to be that U.... are the voltages measures when that module is fitted/present and actually plugged in, The U..X are the voltages measured with that particular module unplugged.

I only saw this after looking at the K35 manual, apologies for any confusion.

[Agent24]

Does anyone have any more ideas as to why the signal going into the RGB amps is not coming out? It seems strange that all 3 may have almost failed at once, though I suppose it is possible. Could the failure of C464 cause that?

Or perhaps I measured the signal at the CRT cathodes incorrectly? - How are you supposed to do it?

Is there any significance that my RGB amp board Pin 6 voltage is -1.22 volts instead of 0.4 as in the manual? Or is this likely irrelevant considering my set is not a K35?

[Red to black]

The cathodes appear to be cut-off (if I am reading the previous posts correctly).
Here are a couple of faults listed for the KT3/K30 wrt to no/dark picture, some of these will have already been posted by others previously.

10V ripple on the -20V line as C1586 (100µF) was open-circuit.

TDA2571Q chip IC367

68nF smoothing capacitor C1565 in the beam limiter circuit, this can read okay on test but may either breakdown under load and/or be physically damaged/cracked and should be replaced if suspected. I think sideband mentioned this previously as tripler failure can damage this device.

Other causes of no picture with these sets are loss of line pulses due to dry-joints, failure of the TDA2560 or TDA3560/1 chip (depending on decoder type),

failure of the flyback blanking transistor (T1535) and, on teletext versions, failure of either fuse on the teletext power supply mounted at the base of the cabinet.

[AndrewM]

It's unlikely that all three video amps have failed simultaneously so whatever is common to all three will be the most likely cause of the fault. It does appear that the video amps are being shut down (biased off) you just need to figure out the cause.

Are you sure the replacement C464 (100µF) is fitted correctly and working ok? This capacitor is fitted to make the video amps shut down when the TV is first turned on. When the -20v rail stabilises the amps function normally again. It is important that there is very little ripple on the -20v rail as it will be passed through C464 and shut the video amps down. Make sure C586 (100µF filter for the -20v rail) is working properly, if in doubt replace it.

The -1.22v on pin 6 of the RGB module is probably enough to bias the video amps off. The only likely causes I can see are excessive ripple on the -20v rail or C464 is leaky.

[Agent24]

When I looked at the -20v rail it did not seem like there was much ripple but I don't know how much would be too much. I will check the amount of ripple and check C586 tomorrow.

I tested the new C464 for capacitance and ESR which seemed fine but I did not check leakage. All I had in 35v was some old ones, so I guess that could be a problem.

[AlanC]

Hi, Just a thought based on the theory; the third transistor in the RGB amps is an emitter follower and if the output of the amps are all in the high output voltage condition, then the second stages, whose collectors are directly coupled to the third, must be cut off. If that is the case, the first stages, which are a PNP transistor and so 'upside down', must be hard on, depriving the second stages of their bias. Since the video signals are AC coupled to the first stage, the only way their bases can be pulled down sufficiently would be via the diodes, from board pin 6. D245 could also be short circuit, which might do the same thing.

To eliminate pin 6 being the issue, you could try lifting the wire off pin 6 and see if that restores the picture. If so, you're looking for a fault elsewhere.

Cheers, Alan

[Agent24]

Well, I re-checked the ripple on the -23v rail and it turned out to be 4v pp! - Pulled C586 and found it was open-circuit.

Replaced C586 and ripple went down to around 80mV and the picture came back

A big thanks to everyone for their help, advice and explanations. I have learnt some more things about CRTs\TVs and electronics in general. Cheers!

[AndrewM]

A happy ending! Glad to see it's working again.