I recently acquired a “may not be working, for parts” BBC Master 128. So I’m not entirely surprised it doesn’t work. Was pretty filthy, and for the most part still is.
Battery pack was 3 x Duracell’s that had sweated fairly badly, but not corroded in the normal way they do. That’s been removed (and not replaced yet). Part of the top of the PSU was rusty, and the top of the video modulator is rusty.
There’s no visible corrosion on the board itself.
It had a Logotron ROM fitted (removed), and a ROM marked “C”, that dumping showed was from BSoft (or BeeSoft – can’t remember) – looked to be some sort of printer/plotter driver ROM – about 5k of code. Removed in any case.
I’ve replaced the RIFA’s in the PSU – both of which were pooched and ready to drop their smoke. The rest of the caps look OK for now. +5V rail tested at +5.07 unloaded, and -5V rail was similarly good. 12V rail was down a bit at 10.7V, but the motherboard doesn’t need that so I’m not too concerned by it for now.
Plugged into the board, the voltages under load are still OK. Ripple on +5V was about 240mV. Power-on and caps lock LEDs illuminate, and the cassette relay engages. I’m leaning towards these being the default state on power-on, and not set by the boot code…. because…
I see 16MHz clock coming from the 74LS04, and I see it arriving at pin 8 of IC42 (the video ULA). Also indicates that LK60 is intact.
BUT, I get 9/10th’s of nothing on the clock outputs FROM the ULA – pin 7 (8MHz) is mostly floating around 1.8V, but the logic analyser is picking up at least some pulses periodically – but very short, and definitely not 8MHz. pin 6 (4MHz) is stuck around 0.6V, pin 5 is stuck at about 1.7V, and pin 4 is stuck around 1.86V.
I’m getting a solid +5V on pin 16 of IC42. The V2 pin in most circuit diagrams shows C25/D14-16/R60 which seem to set this pin to about 1.8V, but none of those components are fitted on this machine (and would appear they never have been), and the V2 pin sits at 0V.
This is all with no battery pack, and with the keyboard (unproven though) fitted. I wouldn’t expect a broken keyboard, or missing battery, to result in no clock signals from the video ULA though.
I’m leaning towards a dud video ULA, since it would appear unlikely that another failed component would be causing issues with all 4 video ULA clock outputs.