Hi
I thought you all would like an update.
I’ve found two problems.
The cause of the loss of the video output, is a broken PCB track on the underside of the board. Between transistor TR1 collector lead and resistor R51. From R51 the track goes to TR2 base. Hence TR2 having no signal, is just turned fully on all the time due to resistor R51. Therefore there is no valid video signal going to the output socket (this machine has been composite video modified).
The cause of no colour is a missing PCB track between capacitor C65 + pad and a through-hole via under capacitor C40. This track is (well, was!) on the top side of the board. It links C65 + with R72, which in turn is linked to pin 13 of the LM1889 colour encoder chip. With this missing track, there was no way the colour video carrier signal could be mixed into the monochrome video signal.
But it appears that the system is running, as if I simulate key presses, the bleeper clicks, and if I fill up the input line, eventually I get the error rasp sound (which is BASIC saying, I’m not having that as an input!). And on my oscilloscope, I get a good video waveform at TR1 base.
So I fired up the soldering iron and set to repair these two problems.
Here’s some photos...
The relevant part of the schematic diagram:
The damaged track causing the loss of the video signal:
What, can’t see the break? Try this photo...
Okay, so no you can’t really see in the earlier photos. So I cleaned the old flux and dirt off the board. Now you can see the track. It does not look like it is broken. But I get intermittent continuity when I test with the multimeter.
Having confirmed that the track is the problem, I desoldered the relevant solder pads ready to fit a wire to replace the track. Look at the track now:
And here is a shot showing the wire that replaces the broken track:
This is the missing track that resulted in no colour in the picture (well, the bit of the board where the track used to be, you can just see the light green line where the copper track has been lifted away):
And the fix (put on the bottom side so it does not spoil the top of the board and is easier to fit on this side anyway
):
A screen shot showing it now does colour (sorry, without a keyboard attached, I could only be bothered to poke wires in the keyboard sockets to type in BORDER 3)...
So as you can see, it now works okay.
When I have got a keyboard membrane to hand, I’ll do a few tests to see if it’s running as a 48K machine.
Mark