*** M E M O R U M ***

Oh, you mean just like this then ?

http://www.primrosebank.net/computers/m ... mavram.htm

You can also use HM4816 or equivalent DRAM chips...

Mark

Dave wrote: ↑11 May 2019 21:41 Oh, you mean just like this then ?

http://www.primrosebank.net/computers/m ... mavram.htm

Oh! Yes

So, I removed the original chip, fitted a socket and modified a 4164...
Success!

However, it's not reliable. The machine starts up with either a blank screen or corrupted graphics on screen a lot of the time. It's probably due to the 4164 being 150ns RAM and the ITT stuff being a heck of a lot slower (they look like they're 300ns???)

There is no problem using faster RAM

OK, so I don't know what's causing the issue then.

Anyway, 99% there. Once the new 74LS273 appears on Tuesday hopefully all the keyboard will work. I'm not sure about this one as it came off the machine I fixed a couple of years ago and seemed iffy, so I swapped the keyboards over. Anyway, we'll see.

That’s good progress

Is the power on reset circuit working okay? It’s formed by R18, D13 and C11 (10uF) on the input to IC 9C (4070B).

Mark

If the machine previously worked well, then the following may be a red herring, but . . . . .

Have a look at page 46 of the Service Manual available on this page :

http://www.primrosebank.net/computers/m ... anuals.htm

Memotech made a modification to the timing chain circuit to help intermittent start up problems

Probably not needed, but since you have the machine in bits anyway, it might be worth doing

My machine is from after October 1984 so should already have the modification.

I've just noticed that I need to change Link 7 though!

P.S. Having checked the GAL programming this isn't necessary as that pin is ignored anyway.

Having had the machine burning in running Andy Key's "Reveal" (loaded from the CFX interface) all afternoon it's been stable.

It smells a bit warm, but that might just be the new components evaporating off the volatiles as they get gently roasted by the power circuitry and VDU.

I hope the switch cleaner I have on order will fix the keyboard switch "chatter" so that I don't have to replace most of the keyboard switches.