MTX Video RAM Modification

Courtesy of Martin Allcorn

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.

Failure of one or more of the ITT 4116 VRAMs is often 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 are some guidelines on identifying faulty VRAMs and a description of one repair performed on an MTX can be found on this page. That repair describes the replacement of a failed VRAM chip with a compatible 4116 type RAM which is likely to be of a similar age and subject to the same failure modes as the original chip.

Other vintage computer users have chosen to replace the failure prone 4116s in their machines with more reliable 4164 RAM. Although the 64K RAM is not a drop-in replacement for the 16K 4116, if you are replacing a failed VRAM, it dos appear to be a worthwhile upgrade. There are a number of options for doing this change on an MTX computer, including :-

• replacing the chip in situ, which would require some cutting of PCB traces and adding a jumper wire

• removing all of the existing VRAM chips and installing a custom designed daughter board

• modifying a single chip to install in a socket where the original RAM was installed.

Martin Allcorn has provided this helpful guide on how to perform this upgrade on a Memotech MTX. 

MTX VRAM Fix

As described above, the MTX uses a TI9929A Video Display Processor with its own memory. At the time the VDP was designed, dynamic ram chips needed 3 different supply voltages 12v, 5v and -5v in addition to a ground connection.

Later devices moved to a single 5v supply. This had the benefit of freeing up 2 pins allowing the 64k and 256k to retain the same package as the 16k one.

The 16k ram chip used by the VDP runs hot and is a common source of failure. A quick search of the net for “4116 ram” will reveal this isn’t unique to the MTX, it’s seems to be a “feature” of the device.

There’s also a common solution, replace the 4116 with a modified 4164.

The 4164 is a later device, and is used on the MTX512 for the main ram. I have a small supply of these courtesy of Dave, so I decided to give the 4164 a try. Looking at the pinouts of the 2 devices, there are only 3 differences between them.

•  Pin 1 is -5v on the 4116 and not connected on the 4164, I removed that pin entirely just in case.

• Pin 8 is 12v on 4116 and 5v on the 4164, this one needs to be bent up, and shortened, so that it no longer makes contact with the socket.

• Pin 9 is 5v on the 4116 and Address line A7 on the 4164. The VDP can only access ¼ of the new chip, so won’t use A7, it’s also not used for dynamic ram refresh so would need to be tied to either 0v or 5v to stop it floating. Since there’s 5v on the socket at this pin, it can be left alone.

One connection now needs to be made from pin 9 to the shortened pin 8 to supply power to the chip.

That’s it surgery complete. The pictures show the modified chip in place, and working.

I now need to decide whether to replace the remaining 4116’s or not. There are some definite advantages in removing all of the older chips.

Using the 1982 OKI data book as a reference their 16kx1 ram chip used 528mw while in operation. The 64kx1 chip on the other hand consumed 248mw, less than half. Multiply that by 8 devices and that’s 2w less power being drawn and a lot less heat being generated inside the case.

It would also mean -5v is no longer required and +12v would only be needed for the RS232 board.