Grundig CUC 2401 CRT TV - Brightness is too high

[Flybacks]

Hello, I'm trying to fix a Grundig CUC 2401 with picture brightness too high.
The problem seems to be in the brightness voltage supplied by pin 16 of TDA8442 that should be in the 1-3V range instead it is about 3.2V.
If I manually lower the brightness the voltage goes in the right range of about 2V and doesn't increase but I don't understand why at start up it is set wrong ?
I've already changed TDA8442, the video output IC TDA3505 and the microcontroller IC811 but it didn't fix.
The collector of RGB transistors on crt board are about 170V for red and blue but 150V for the green, shouldn't be all equal ?
Could someone please help me to troubleshoot it ?


Schematic: https://elektrotanya.com/grundig_cuc.../download.html
The TDA8442 is at page 4

Many thanks!

[R_J]

Re: Grundig CUC 2401 CRT TV - Brightness is too high

Too high brightness can be due to a bad screen bypass capacitor, this cap is usually near the hi voltage/flyback transformer and is usually an electrolytic with a value of 2.2µf ~ 10µf 250v ~350v (C515?)
Check CT741 2.2µf/350v on the crt drive board.
The voltages on the rgb outputs will not be equal because the ratio to make white is not equal. roughly 30% red, 59% green, 11% blue

[Flybacks]

Re: Grundig CUC 2401 CRT TV - Brightness is too high

Hello, the screen voltage is correct I don't think it is related to that because lowering the brightness manually from the menu option fixes the problem and pin 16 of TDA8442 goes in the right range of about 2V.
But every time I switch on the tv it comes back to 3.2V and I have to lower brightness again.

[dskall]

Re: Grundig CUC 2401 CRT TV - Brightness is too high

Is that the only setting that does this?

[R_J]

Re: Grundig CUC 2401 CRT TV - Brightness is too high

Hello, the screen voltage is correct I don't think it is related to that because lowering the brightness manually from the menu option fixes the problem and pin 16 of TDA8442 goes in the right range of about 2V.
But every time I switch on the tv it comes back to 3.2V and I have to lower brightness again.

Could it be a bad cap on that line? C9548 (4.4µf/16v) [page 28]

As for the screen voltage, even if it is close to correct, the high frequency ripple from the supply can effect the screen.

[Flybacks]

Re: Grundig CUC 2401 CRT TV - Brightness is too high

Is that the only setting that does this?

The voltage of contrast and colour of pin 1 and 2 of TDA8442 are also a little higher but don't disturb the picture as much as the brightness.

Could it be a bad cap on that line? C9548 (4.4µf/16v) [page 28]

As for the screen voltage, even if it is close to correct, the high frequency ripple from the supply can effect the screen.

I've already replaced C9548 but didn't fix it

Thanks

[dskall]

Re: Grundig CUC 2401 CRT TV - Brightness is too high

Are these settings digitally set? Values stored in eeprom.

[Flybacks]

Re: Grundig CUC 2401 CRT TV - Brightness is too high

Are these settings digitally set? Values stored in eeprom.

Yes they are digitally set.

On the tv there are two eeprom, one is SDA2216 inside the tuner unit (page 11 of schematic) and the other is on the teletext board (page 17, there are two SDA2216 on the schematic but my teletext board only has one SDA2526).

Could any of these store the brightness value ?
Or could be stored inside IC811 and sent to TDA8442 at start ?

Thank you