Repairing a dead A520 modulator
 

I have just received a dead A520 modulator. There's no sign of fault on its board.
It shows no picture at all. How to fix it? :confused

Get a working replacement? There's bound to be plenty out there since hardly anyone is using them nowadays (RGB-SCART looks much better) and most A500 packages included one. :)

Use A520 schematics ( http://members.iinet.net.au/~davem2/overclock/A520.html ) and MC1377 datasheet - or replace PCB with something more modern.

I wouldn't bother fixing it. These things produce the worst picture quality ever.

Yeah, there has to be a lot of them sitting around unused everywhere since so many people use RGB output these days for its much superior quality. If you really did want to troubleshoot it however, you need to look at the signals with an oscilloscope. Check that the 1377 chip is getting power, then check the clock signals and sync signals are appropriate. No clock signal would suggest looking at the 74 logic ID and/or the crystal on the board. Make sure the RGB signals look ok too, then see if you're getting anything out from the chip's outputs. If you are, you can trace them through the extra analogue stages and see where they stop.

1337's power was ok but I don't have oscilloscope =/