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The Memotech MTX Series

Memotech MTX Video RAM Fault Diagnosis / Repair

Documents the repair of David Kimberlin-Wyer's MTX500s

 

System Description : MTX 500 Serial No.18439, 4000-04 computer board with 32k DRAM chips.

Problem Description : Corrupted video screen

Background

David bought two MTX500s, sold on eBay as pair, but with only one power supply between them. The ebay description included the words :- "Have powered both up but cant remember how to get them going, they both showed screen with ready in bottom left corner, so sold as untested."

In reality, when they arrived, both machines displayed similar corruption on the screen, a sceptic might doubt the veracity of the statement in the advert, but anyway  . . . . . . .

MTX500 Serial Number 18439

4000-04 computer board with 32k DRAM chips.

(Output from the Monitor connection)

MTX500 Serial Number 16153

4000-05 computer board with 32k DRAM chips.

(Output from the Monitor connection)

[This machine also loses colour (blue fading to grey) 10-15 minutes after being turned on. * ]

 

MTX Display Hardware

Memotech MTX computers use a dedicated Video Display Processor (VDP), either a Texas Instruments TMS9929A (PAL versions) or TMS9918 (NTSC versions), with 16kb of dedicated TMS4116 or equivalent Video RAM (VRAM). An overview of the VDP hardware can be found on this page, and full circuit diagrams can be found in the MTX User and Service manuals.

The VDP requires only a +5V supply but the VRAM requires +5V, +12V and -5V. The TMS 4116 datasheet states that -5V must be applied before, or at the same time as, the other voltages and removed last. Failure to follow this this sequence will impact the long term reliability of the RAM. Similarly, a DC supply failure can cause one or more of the RAMs to fail prematurely.

Whether or not this is the cause, it does appear that VRAM failures are commonly found to be the source of the problem when investigating faulty MTX display output. 4116 RAMs are also used in many other computers and video games from the era, including the lower 16kb of RAM in the ZX Spectrum where they also seem to be a common cause of failure.

There is a wealth of information on fault finding 4116 RAM faults, on the Spectrum in particular, available on the web which can give useful pointers to identifying problems with the MTX VRAM.

The relative temperatures of the RAM chips can be an indicator of a faulty chip. In normal operation, after the machine has been switched on for a while, all of the VRAM should be warm to the touch (the VDP itself runs very warm/hot in normal use). An internal short in one of the RAM chips may lead to it running very hot when compared to the rest.

As I mention on my repairs overview page, if you have not referred to it already, you should really look at the MTX500/512 Service Manual, this document is obviously the best available guide for fault finding and repair of your MTX. Display faults are discussed under "Symptom 3 - Corrupt Video Display or Characters". The first and easiest thing to check is the supply voltages, particularly the -5V supply, to the VRAM.

Troubleshooting Serial Number 18439

Screen corruption getting worse as the machine warms up, this image is with the machine powered up when warm.

The general consensus of opinion on the Memotech MTX500 Facebook group was that this was probably a VRAM fault but the VDP and other components could not be ruled out at this stage.

A number of tests were suggested to see if the screen display could be used to give an indication of where the fault might lie.

This is the output when a "W" was printed in every character location.

It was noted that the screen shows a repeating pattern of 64 malformed characters, followed by 64 unknown characters.

The same display when the program is run with the machine still cold.

Display when the program stops - the "W" character positions appear fine, but empty space characters show "."s and the "Ready" text is corrupted.

At this point, (having just obtained a multi-meter), the power supply voltages on the board were checked an all appeared to be within expected values.

The Service Manual suggests using an oscilloscope to check the signal on Pin 14 of any of the VRAMs, but this will not be an option for folks without a 'scope. The Service Manual also describes how to disable each of the VRAMs in turn by earthing Pin 14 (via a 10-15 ohm resistor!) and checking whether the screen corruption reduces, the chip with the least screen corruption when it is being earthed is likely to be faulty and should be replaced.
Display when Pin 14 one of the VRAMs is earthed via a 10 ohm resistor. The "dot" pattern has disappeared, although there is still some corruption of the "Ready" text.

Most of the ICs on the MTX computer board, including the VRAMs are directly soldered to the board making replacement somewhat tricky. The easiest way is to snip the legs off the IC and de-solder the legs one-by-one.

View of the solder side of the board showing the location of the removed IC and the cleaned up holes - good job!

(This image is from the replacement of one of the VRAMs in S/N 16153)

View of the component side of the board, after the IC has been removed.

It is advisable to fit an IC socket in place of the existing VRAM and install a new IC in the socket. This makes future replacement easier and reduces the chance of damaging the new IC when installing it.

(Note: the small inverted PCB on the right hand side is the Video board, it can easily be removed if better access to the VRAMs is required.)

View of the solder side of the board showing the new socket in place - can you spot it? - Another good job!

New VRAM installed, machine powered up and hey presto !

 - It works !

All photos courtesy of David Kimberlin-Wyer

 

*A similar process as described above was followed to effect repairs to serial number 16153, this cured the screen corruption problem, but the fading colour problem still remained - by swapping over the Video boards between the two MTXs, David was able to confirm that it was a fault on the video board.

 

The fading colour could have been caused by a number of faults, including the power supply and the analogue components on the video board. The video board uses a LM1889N Video Modulator to condition the VDP output for use on a TV, on most MTX Video Boards, including Dave's faulty board, this chip is installed in an IC socket so it should have been easy to swap the modulator between the two boards in an attempt to isolate the fault. Unfortunately, on the "good" board, the modulator had been soldered to the board, but since the easiest, and relatively low cost (under£5) option, was to change the modulator, Dave bought one to try. Fortunately, this was indeed the problem and the colour fade problem has been resolved.

 

Some details taken from the Facebook Memotech MTX500 group with contributions from Andy Key, Inaki Castillo and others

 

 

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Page last modified 26/10/2014