Resurrecting an MTX500.

About original Memotech hardware.

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 11 May 2019 21:51

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

Mark

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 11 May 2019 22:12

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

http://www.primrosebank.net/computers/m ... mavram.htm
Oh! Yes :-)

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 11 May 2019 22:51

So, I removed the original chip, fitted a socket and modified a 4164...
IMG_1456.jpg
IMG_1456.jpg (1.36 MiB) Viewed 1204 times
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???)

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 11 May 2019 22:59

There is no problem using faster RAM

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 11 May 2019 23:04

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.

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 11 May 2019 23:15

That’s good progress :D

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

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 11 May 2019 23:16

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

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 11 May 2019 23:40

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.

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 12 May 2019 19:43

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.

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