PhilCs blog / the latest things hes broken(fixed)

[PhilC]

Thanks @stephen_usher thats where I found the Cisco PSU replacement guide

[PhilC]

My replacement power supply for the MTX has arrived today, so I've got to carry out a few mods to the MTX motherboard to allow the use of a DC PSU instead of the original AC type PSU as seen on this web page

https://archive.sundby.com/mirror/primr ... psu_dc.htm

I also removed the 32K ram that was fitted to the MTX and fitted 64K instead. Looks like this computer had an external 32K upgrade board fitted but that's long gone now. I'll have to try and locate the Jed file for a new memory GAL if it doesn't boot with the memory as is.

[stephen_usher]

Yes, you'll need the new PAL/GAL The description is at the end of the manual. You'll also have to adjust the links on the motherboard.

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In this case (because the MTX500 was using "low" versions on the RAM chips) the top link needs to be moved so that it links middle and "4" vias.

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[PhilC]

Thanks @stephen_usher I’ll have to find where to track down the GAL jed file.

I’ve got it sort of working now as well

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I’ve got to work out what the links are presently set to as well.

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[stephen_usher]

Looks like one of the 4116 video DRAM chips is dodgy. That should say "Ready", so that's bit 3.

I've attached the GAL stuff.

[PhilC]

@stephen_usher its not just the one 4116, I’ll socket them all and then test and replace.

Thanks for the GAL stuff

[stephen_usher]

I'd replace them with hacked 4164s as they're more reliable, run cooler and don't need the extra voltages.

[PhilC]

I'd replace them with hacked 4164s as they're more reliable, run cooler and don't need the extra voltages.

I've got loads of working 4116's at the mo, otherwise I'd probably do that and am running low on spare 4164s as well.

[PhilC]

Once again thank you @stephen_usher, new GAL and 8 new 4116s fitted and we have a nice garbage free screen and 64Kb ram.

There is another error to track down unsurprisingly, the key repeat delay is pretty much zero, so pressing any key briefly usually ends up with 5 or 6 lots of that character. Should be something simple to find

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[stephen_usher]

That'll be slightly corroded key switches. Electrolube EML down the key post and exercise the switch will fix that. It's the same as BBC Master keyboards.