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Dick Smith Super 80 computer kit


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I forgot to mention that with the program, it doesn't touch the memory between 0-FF otherwise it would overwrite itself.

 

Yes, although that range does get modified, when I set the video page to 00 I can see bytes changing, actually it appears as though single columns of characters are modified at the same time. That might tell me something.

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Hopefully it will help. Let me know when you progress further.

 

I managed to replace RAM bank 0 with new sockets. It took me about an hour and a half to prise off the housings and desolder the pins from all 8 sockets. The problem still persists even after I replaced the RAM sockets. One other symptom is this, initially the CAS line on bank 0 is logic hi and after 20 seconds or so it starts to oscillate with what appears to be a short lo pulse and that's when the characters start to change at random. The other CAS lines on bank 1 and bank 2 remain logic hi while this is going on. This eliminates the RAM sockets, next I'll replace all the sockets for the chips involved in memory address decoding and multiplexing. I'll trace the CAS line signals further upstream when I get a spare moment.

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It is an odd problem. I suspect that 50Hz hum may have snuck back in, without looking at the CRO trace I remember that the 50Hz hum often took a while to manifest. I'd have a nice clen signal and then after a minute or two it would appear and that's similar to this random character problem. I may have removed the hum from the video generation circuit but isolated it to some other part of the system.

 

I installed a new 74HC165 chip in place of the TTL LS165 (which are hard to find) and the video dislay is now perfect so the original chip had a bad input causing the last dot or column of dots on each char to not display. I figured that an HC165 would work in this situation considering that it was being driven by a CMOS EPROM. I haven't put the 2513 back in yet but it may not work correctly if I do. When I get around to doing the EL-Graphics mod the HC165 would be Ok there.

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Any progress yet?

 

None yet, My house is missing a roof and I've had to spend all my spare time mucking around doing work on the house. I've observed that the memory corruption problem is also temperature sensitive, cold days it takes a while to show up but on hot days it's there almost instantly. Hopefully I'll get a chance this weekend and over christmas to spend some time on it.

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Seasons Greatings!

 

Hopefully you'll have a roof by Christmas!

And, hopefully the New Year will bring a fully working Super-80 !!

 

We're getting there, slowly.

 

I've spent a few hours since Christmas resocketting to no avail. I feel I've solved most of the socket troubles and the memory corruption problem may be unrelated. It appears that the RAS and CAS signals become unstable and the corruption starts at the lower RAM addresses and moves up eventually messing up the video page and you start to see the results on screen. Sometimes the system reboots by it self or goes into test mode automatically as the RAM locations get changed. I'll have to step out and get some more IC sockets as I seem to have run out.

 

Peter.

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