I've spent the last hour or so scratching my head with this A1200 problem as it isn't the usual composite video problem with a dodgy delay line.
I have an A1200 board that works fine on RGB, but not on composite. I initially diagnosed a faulty delay line (checked by shorting pins 16 & 18 on the video chip and replaced it with a replacement that I believed good. This again failed to work, so I decided to replace the video chip and associated circuiry with a more modern CXA2075. This however still didn't solve the problem. I can see a video output on pin 20 of the chip (and indeed can connect directly to that chip to get a very bright, slightly unstable display. However, while the video signal gets to C237 +ve, there is no corresponding signal on the -ve pin. I have tried a number of capacitors there and even just connected a capacitor to pin 20 of the video chip with the other side of the capacitor going directly to the monitor with no success whatsover. Removing the capacitor and replacing it with a temporary wire link gives a very bright output, but I simply can't get the video signal to pass though a capacitor. I temporariy removed C236 in case something on the modulator circuit was causing a problem, but again with no better result - indeed disconnecting the output from C237 and connecting it drectly to the monitor still doesn't work.
Seemingly the composite video signal is simply not able to pass through a capacitor - and I don't think I've ever seen that before.
I should add that this is a PAL board, and I am using a DiagROM for testing. Also that the RGB signal is completely fine and no other faults seem to be present on the board.
Any ideas?
Thanks
Graham
I have an A1200 board that works fine on RGB, but not on composite. I initially diagnosed a faulty delay line (checked by shorting pins 16 & 18 on the video chip and replaced it with a replacement that I believed good. This again failed to work, so I decided to replace the video chip and associated circuiry with a more modern CXA2075. This however still didn't solve the problem. I can see a video output on pin 20 of the chip (and indeed can connect directly to that chip to get a very bright, slightly unstable display. However, while the video signal gets to C237 +ve, there is no corresponding signal on the -ve pin. I have tried a number of capacitors there and even just connected a capacitor to pin 20 of the video chip with the other side of the capacitor going directly to the monitor with no success whatsover. Removing the capacitor and replacing it with a temporary wire link gives a very bright output, but I simply can't get the video signal to pass though a capacitor. I temporariy removed C236 in case something on the modulator circuit was causing a problem, but again with no better result - indeed disconnecting the output from C237 and connecting it drectly to the monitor still doesn't work.
Seemingly the composite video signal is simply not able to pass through a capacitor - and I don't think I've ever seen that before.
I should add that this is a PAL board, and I am using a DiagROM for testing. Also that the RGB signal is completely fine and no other faults seem to be present on the board.
Any ideas?
Thanks
Graham