A friend bought a faulty Kairn and asked me if I could fix… Sold as ‘doesn’t power on’. Now the first thing that was spotted is that someone’s been messing, with a regulator completely missing:
and it doesn’t get any better (battery removed, clearly faffed with to try get working, even looks like they swapped the CPU for some reason – unlikely to be faulty but I guess desperate measures and decided to get rid when it didn’t fix it):
What a mess!
Cleaned up what I could and installed a temporary LED to test as not even that worked
and the back:
after a long winded search through the usual places, and getting power to where it should do, managed to see some boot progress in about 1 in 10 power up cycles:
Probing EPROM to check data was being accessed
I noticed the CPU is a faster version (HD63A03) than the usual normal HD6303, could there be a timing issue?
After scrutinising the CPU datasheet there is a multiplex mode which when disabled provides for 8 pins to be more address lines with a separate data bus or when enabled, for them to be used as purely I/O ports, and then multiplexes data/address lines on the lower half of the address bus through use of an address strobe output.
This mode is set via state of three I/O pins during startup/reset and is expected to be a certain configuration (the Kairn uses multiplex mode). Turns out the culprit was a bad HEF4066 analogue switch IC with a dodgy pin that was connected to one of the I/O ports randomly disabling multiplex mode therefore being unable to address the EPROM, now it boots up every time!
A LM340T5 regulator was temporarily tacked to the main board for testing (LM2940CT-5 was in short supply and had to wait for delivery):
Now we have lift off and communication to main board, but all is still not right (notice half of the display is brighter) :
This was down to a faulty LMC660CM, which was replaced:
Also tidied up the LM317T regulator which was missing a screw and isolation bush, and fitted new green LED
Some green conformal coating applied:
Tidied up the HEF4066BT:
New battery fitted to complete the front panel repair, having also programmed a new EPROM with the most recent firmware.
Newer firmware provides user functions U10-12 option:
Just one thing left to do:
All nice and fully restored functionality now!
So that probably was the worst Kairn seen (so far)!