Started digging into the broken BBC Master 128 I have. Previously I’d replaced a failed Video ULA, which at least got the right clock signals in the right places. This time around I observed that the /IRQ input pin was stuck low on the CPU - ie, an interrupt was being generated. That signal appears on the 6850 (IC45 pin 7), the user 6522 VIA (IC6 pin21), the system 6522 VIA (IC8 pin 21), and the custom memory controller IC (IC20 pin 29). ...
So the current state of play is that IC42 (which was a VC2069) has been replaced with a VC2023 borrowed from a model B. They’re pin compatible (and I believe functionally compatible). I now have: pin 8 – 16Mhz in pin 7 – 8MHz out pin 6 – 4MHz out pin 5 – 2MHz out pin 4 – 1MHz out BUT, the 2MHz out, while it makes it to IC34 pin 2, doesn’t make it to IC28 pin 3, which results in no 1MHZE signal being generated – and that’s used by the VIA’s, they keyboard, and in various other places. So a bodge-wire corrected that, and now I get the 1MHZE signal in the relevant places. ...
I’ve been testing with it powered from the bench PSU today (albeit without the -5V, but IIRC that’s only required for audio/serial/cassette). As far as progress, I removed IC42 (destructively – cutting pins) and socketed a spare video ULA – now I’m getting the output clock signals that I’d expect to see, and the CPU is receiving a 2MHz clock. Initially RST was stuck low on pin 40 of the CPU, yet was going high everywhere else (tube socket, cartridge slots, etc). Closer inspection showed some noise on the RST signal at 2MHz, which made me suspect some sort of connection between pins 39 & 40 on the CPU. A clean of the PCB in that area resulted in RST going high on pin 40 as expected – so perhaps that “dust” on the board is more conductive than it looked… or something else was going on that cleaning somehow resolved. ...
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. ...
After the last BBC stream I pondered that maybe the suspect disc drive’s heads were just dirty, so we clean those. Then investigate a couple of Master Compact systems, and end up opening the Amiga again to work out which Kickstart ROM it had (1.3) and realised it had a mono video output! Streaming PC bluescreened towards the end, but we picked up where we left off. ...
A look at my “main” BBC model B and the upgrades that I’ve done to it, re-assembling the other model B, checking out the double floppy drive unit, and an update on what’s been happening with the Z80 build. Live streams on Twitch
More bits arrived so the Z80 boards are starting to fill up. Gave the BBC keys a bit of a clean, and with the soldering iron fixed, took the A501 apart to see what was going on inside, and a quick run down of things to come. ...
Time to remove these old capacitors… …from this BBC model B power supply, circa 1983… …and put these new ones in their place. The expectation being that the smoke will stay in these ones (and so far it has).