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#1 01-10-2017 20:15:40

witchy
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Registered: 23-09-2017
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UK and French 'brain repair

Hi folks,

After a friend of mine lucked out with a boxed UK 'brain he ran into difficulty with the PSU, exploding RIFA mains filter notwithstanding. I've had my 'brains out on display for years but hadn't powered either of them since at least 2003. I'd been lucky back then myself when someone in Northern Ireland found me online and sent me a load of 'brain bits including spare 'parts only' boards and PSUs. One of the PSUs worked fine apart from letting the magic smoke out so I figured if I was going to help him fix his 'brain I needed to sort my own first smile

The UK one *almost* worked for 20 minutes before the screen inverted and the characters became unrecognisable. Searching around found Tom/Scouter3d's heimcomputer.de site and here, and Tony/tjewell99 sent me the schematics which were great for hints.

With an oscilloscope everything LOOKED ok, no stuck address or data lines, CPU clocking nicely, ROMs active. The VFD showed correct characters. I spent a while looking for a dump of the character ROM since I suspected an issue there and in the end pulled one from a spare board and dumped that. Replacement didn't help so I traced the video signal back from the RCA connector and put my logic analyser on everything between that and the Character ROM.

I was chasing the source of the RV/RVF signals thinking they may have something to do with screen inversion and also tested every LS04 hex inverter. Inputs matched outputs in the chain until I noticed clocking on IC436 (74LS166) before RESET was asserted so nothing should've been happening on those data lines. Swapping IC436 brought that machine back to life big_smile

The French machine was dead apart from a reverse screen even after swapping the PWRUP and RESET caps so I spent a while testing the components around the startup circuit, even swapping the CD40106 that ultimately asserts those signals. Still nothing so I pulled the major components and built the circuit on a breadboard, worked first time. The CD40106 I pulled from a spare board also worked first time. Turns out I'd missed some leaked electrolyte on the holes from the original startup caps so after a thorough cleaning I soldered everything back in and.....it worked.

PRINT FREE only showed 13K though so Colin's (RIP) service manual to the rescue with the POKE 20000,0:?PEEK(20000) command returned 1 and looking at that chip on the store board it was obvious it had been damaged also by leaking electrolyte. I pulled those components and gave it a thorough cleaning. Replaced the caps and fitted a new IC416. - bingo! The VFD doesn't work though, voltages on the converter board are low to say the least so that's obviously died. Still, the machine itself is happy.

I took many pictures doing this which are on my collection's FB page - http://facebook.com/binarydinosaurs but I'll upload them here if people would like.

UK s/n is 102084/FDS, French s/n 614666/GDS

Witchy

 


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