BBC Model B - Issue 4 saved from the skip Jan 14, 2020 8:47:36 GMT cercamon, 8bitbaz, and 2 more like this
Post by Vyper68 on Jan 14, 2020 8:47:36 GMT
I've been a bit quiet of late but I thought I would share this with you as some of us also like the Acorn computers as well as our favourite Atari's. So I managed to pick this up over Christmas, it's an Issue 4 BBC Micro and it was destined for the landfill until I persuaded this person not to and to let me have it. I traded some C64 & Sinclair items as a payment in kind as even though it was in a sorry state it had potential and I love a challenge. The first thing was to change the Three main capacitors on the PSU before I even turned it on as most Beeb owners know these tend to go "BANG!" these days after spending the last 35 years inside the PSU. Once done I tested the voltages on the PSU and all looked fine. Next step was to connect the flyleads to the mainboard and power on connected to my TV. This produced One solitary beep and a flashing cursor in the top left. So this told me that the 6502 and CRTC & Video ULA were working to a point so i'm making progress.
So back onto the workbench to take the a better look at the Beeb. So it's an Issue 4 and along with the Issue 7 ( Issues 5 & 6 were never produced ) so by Issue 4 most of the bugs were iorned out and it's a fairly stable board. This one was manufactured and sold with a disk interface already fitted. It is the original Intel 8271 FDC with assorted logic IC's. This was a bonus as some old BBC games like the original disk version "The Hobbit" which has pictures as well as text relied on the 8271 as a copy protection and will not run on the later 1770/2 FDC Boards. It was also fitted with EPROM versions of BASIC2, DFS0.9 and Disk Doctor 1.09 as well.
I eventually tracked the fault down to a corrupted DFS EPROM and BASIC2 EPROM, once they were replaced the BBC booted to the normal startup screen. It then became apparent that something was wrong with the keyboard or the BBC again. I originally thought it was one of the 6522 VIA's that were the problem but it turned out to be the keyboard PCB itself. Some of the points where the contacts were soldered to the PCB had broken and come away from the PCB and were not making a circuit when pressed. So a small repair was needed in a couple of places to fix those points. There was evidence of a previous repair done some time ago by someone and that was what helped me realise what was happening to the rest of the PCB.
So now it has a working keyboard, new BASIC and DFS EEPROM's fitted and had a good scrub we have ended up with a nice Issue 4 BBC that works!!! I am pleased I was able to save it and it didn't take a lot to get it up and running again.
So today I am going to build a SPI/MMC interface using parts I have to hand and see if I can get that working on the Beeb. It should be just like a games Jukebox so I will follow up with that ...
In the meantime here are some pictures of this Beeb.