Resurrecting an MTX500.

About original Memotech hardware.
stephen_usher
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Joined: 27 Nov 2016 19:58

Re: Resurrecting an MTX500.

Post by stephen_usher » 16 May 2019 20:44

Dave wrote:
16 May 2019 20:41
stephen_usher wrote:
16 May 2019 20:40
Dave wrote:
14 May 2019 23:19
I wonder if there might be a problem with the DRAM timing circuit?

Assuming that all the chips in the circuit are working, it might be worth checking C5 and C6 for damage. With all the work on the machine, they might have taken a knock? I had a weird RAM fault that turned out to be a cracked capacitor
Both the capacitors C5 and C6 are in pristine condition.

Can you read the values on them?
I may need a microscope for that. :-)

stephen_usher
Posts: 205
Joined: 27 Nov 2016 19:58

Re: Resurrecting an MTX500.

Post by stephen_usher » 16 May 2019 21:04

Dave wrote:
16 May 2019 20:41
stephen_usher wrote:
16 May 2019 20:40
Dave wrote:
14 May 2019 23:19
I wonder if there might be a problem with the DRAM timing circuit?

Assuming that all the chips in the circuit are working, it might be worth checking C5 and C6 for damage. With all the work on the machine, they might have taken a knock? I had a weird RAM fault that turned out to be a cracked capacitor
Both the capacitors C5 and C6 are in pristine condition.

Can you read the values on them?
C5: 47P C6:68

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 16 May 2019 21:35

OK, that confirms that the machine doesn’t have the timing chain mod described in the service manual. C6 and R14 were replaced in later machines.

It likely won’t help your current problem but you might like to do it anyway. It’s supposed to help with poor start-up response and crashes after running for a while

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 16 May 2019 23:16

I think that I've possibly found the problem:

(a) The lower case is somehow not being earthed by the screw-bolt going through the heatsink.
(b) The TV is sending a 50Hz, 50V (very low power) noise down both the TV or composite video connection.
(c) The CFX data signals are only marginally above the TTL threshold.

The combination of these was causing the data lines to/from the CFX to jump around enough that they weren't always going above the TTL '1' level. Data generated on the data bus from the internal chips had a higher voltage and hence didn't see the issue.

Adding a (temporary) wire between the modulator shielding and the base of the machine seems to have solved the problem. CP/M doesn't crash all the time for a start.

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 16 May 2019 23:32

Do you have a wire connecting the regulator bolt to the ground terminals on the AV connector? I didn’t see one in your photos.

Again, probably nothing to do with the current (but hopefully fixed) problem but if it’s not there it can give display problems

You can see it in this photo http://www.primrosebank.net/computers/m ... moVid4.jpg

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 16 May 2019 23:37

Bother! Spoke too soon. It's back to its old ways.

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 16 May 2019 23:38

Dave wrote:
16 May 2019 23:32
Do you have a wire connecting the regulator bolt to the ground terminals on the AV connector? I didn’t see one in your photos.

Again, probably nothing to do with the current (but hopefully fixed) problem but if it’s not there it can give display problems

You can see it in this photo http://www.primrosebank.net/computers/m ... moVid4.jpg
No, but I was looking into it. I need to find a suitable wire with an eye on one end as I don't have the crimping stuff.

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 16 May 2019 23:40

It’s easy enough to create an eye by looping a bit of bared wire and soldering it, without letting the hole fill with solder

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 17 May 2019 00:35

The MTX is not connected to mains earth. If the double insulated TV is causing problems due to the internal Y type suppression capacitor leaking current, connect the MTX case to mains earth.

Mark

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 17 May 2019 20:04

I s'pose it could be a RAM paging issue...

I ran Toado from SDX BASIC, immediately reset into CP/M mode and when CP/M crashed the game loop of TOADO started, with the logs going across the screen and the "fly" appearing and disappearing and the sounds being produced.

Other than the CPU, PAL, multiplex chips and LS273 (all completely replaced with new parts within the last week) what other chips are involved in the RAM paging?

P.S. I'm guessing that the 74LS08 (7E) is the only chip in that whole part of the circuit that I've not yet replaced.
Last edited by stephen_usher on 17 May 2019 22:49, edited 1 time in total.

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