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The Memotech MTX Series

Memotech MTX512-S2

Video Fault

 

System Description : MTX512-S2 Serial No.33003, 4000-04 computer board with 256k DRAM chips.

Problem Description : Faulty Video Display

Background

I bought an MTX512S2 with 80 Column board and SDX disk controller/drive. I was aware that it had a problem with the on-board video processor output, the symptoms described sounded typical of VRAM failure(s), so I was not overly concerned. The computer has an 80 Column board and SDX controller with a CP/M ROM, the system could successfully run CP/M and drive the 80 Column monitor output, indicating that the computer board itself was working.

The image above shows the status of the output from the VDP on booting the MTX without the SDX controller with CP/M ROM connected. Initially expecting a VRAM fault, I opened the case and inspection of the internals revealed a large amount of corrosion on the legs of various chips, including some of the VRAM and the TMS9929A VDP. I initially thought that the MTX may have been stored in a relatively harsh environment, such as a loft, where it might have been exposed to condensation etc. But that is not the cause of the problem - the 80 Column board and most of the MTX motherboard were in pretty good condition. On closer inspection, it is clear that the corrosion is confined to one area of the board - mainly around the VDP and the VRAM chip legs.

 

The VDP and VRAM, and the edges of some of the adjacent ICs, showing corrosion on the legs.

It appears that something "nasty" has been spilled onto the VDP and some of the VRAMs, there are marks on the chips themselves and a few other marks on the board which look like spill residue. Where it has hit the legs, it appears to have caused the corrosion.

The keyboard PCB in the upper half of the MTX shell shields the computer board PCB from spillages over the keyboard.

The UHF modulator is more corroded than is typical, it looks like the spillage has been through the rear of the case, over the modulator and onto the computer board and ICs.

The image at the top of the page shows how the video display looked the majority of the time after booting the MTX.

When pressing down gently on the VDP, the display on the screen often changed - sometimes as shown, and often improving markedly, suggesting that there may have been bad contact between the VDP chip's legs and the socket.

That would not have been a surprise, given the state of the legs and socket contacts!

I attempted to remove the VDP for closer inspection, when I did that, many of the legs disintegrated - leaving a number still in the socket.

The underside of the TMS 9929A VDP - missing a few legs

The socket was in a pretty bad way too, although the computer board itself looked OK

I cleaned up the board as best I could, removed the remnants of the old VDP from the socket, and "borrowed" the VDP from another MTX, but the symptoms were the same as before.

The screen corruption was unchanged and continued to vary significantly when pressure was applied to the VDP in its socket. The obvious next step was to replace the VDP socket.

Andy Key generously sent me a replacement VDP and 40 pin DIL socket to enable me to progress the diagnosis, and hopefully repair, of the system.

The first step was to remove the old socket, this was a little harder then removing an IC as you can not snip off the individual legs, but it was not too difficult with the aid of a solder sucker.

After cleaning up the board a little more, the new socket was soldered into place, and after checking for shorts between adjacent pins, the replacement VDP was installed.
On powering up the MTX, the video display was the same as the original fault, I had not expected that replacement of the VDP and its socket would cure the problem, but at least I had not made things worse.

At this point, the system correctly responded to <ctrl><G>, i.e., the computer produced the expected sound tone. If you look closely, you can make out what appears to be a very corrupted "Ready" prompt in the edit region.

Having a corrupted, but relatively stable, video display at least gave me the opportunity to carry out further tests as described in the MTX Service Manual. The manual advises that disabling each VRAM in turn by grounding pin 14 (via a 10 ohm resistor) can help identify faulty VRAMs.

I used test clips from my Logic Analyser to connect to pin 14 of each of the VRAMs and Pin 16 (ground) on one, then grounded each in turn using a short jumper wire/resistor.

I found that at least one VRAM in position 6F was definitely faulty, the display improved as shown.

The majority of the display area is now clear, although the "Ready" text is still corrupted.

The next step will be to remove the faulty VRAM, install a DIL socket and a replacement VRAM.

The second line of text is this display is a corrupt "Mistake" error message, this results when any BASIC command is entered.

This is particularly odd as the keyboard itself works fine in CP/M mode - perhaps there is a BASIC ROM fault too?

The best way to remove ICs soldered onto the board is to use a pair of fine nosed snips and cut though each leg in turn, taking care not to damage the PCB. Using a solder sucker, the individual legs can then be de-soldered and removed without using too much heat which could potentially damage the copper tracks on the PCB.

 

The holes should then be cleaned up to remove any excess solder before fitting a new IC socket to reduce the potential for damage to the new IC and making future removal easier.

David Kimberlin-Wyer kindly sent me a couple of compatible replacements (AMD AM9016) for the ITT 4116 RAM.

New socket installed and replacement VRAM fitted.

(Information on other compatible VRAM replacements).

After fitting a new socket and VRAM, the display improved as shown.

As you can see, the display is much improved but the "Ready" prompt is displaying "Peady". I don't think that that is a character display problem as typing "R" at the keyboard also results in a "P" which is interpreted by BASIC as a "P" rather than an "R".

In addition to any other faults that may be present, I think that there is also another faulty VRAM.

Using the same process as described above to disable each of the VRAMs again, when the RAM in position 7F was disabled, the display was as shown.

The next step will be to replace that VRAM too.

With a second VRAM replaced, the "Peady" message and the keyboard problems are unchanged. The keyboard still works fine in CP/M mode, but is unusable in MTX mode.

When it is displayed, the "Mistake" text is still corrupted but there are no more VRAMs where disabling them shows signs of improvement to the corrupted text, it may be a case of replacing the rest of the VRAMs and taking it from there.

In an attempt to identify the nature of the keyboard errors, I made a quick table to compare the typed key with the character echoed  to the screen; in all but one case, in each of the 20 or so errors, the ASCII value of the key reported is off by 2.

 

This corresponds to an error in the second bit of the data bus, i.e., bit "D1" appears to be stuck "low".

As noted earlier, this could have been caused by a ROM problem, to try and eliminate that possibility, I tried swapping all of the major socketed chips - ROMs, CPU, CTC and VDP - to no effect.

As the schematic shows, the VRAMs are not directly connected to the Z80 data bus - data transfer is between the VDP and the VRAMs. It is possible that a fault between the data bus and the VDP could be causing the errors - possibly a short between D1 and ground holding D1 "low".

I have ordered up some of the same type of DRAM as David gave me, from ebay.com - I will (hopefully) be able to make progress when they arrive.
OK - the replacement RAM has arrived, although I intended to replace all of the remaining RAMs, I wanted to do it one at a time to see if I could identify a particular RAM as being faulty.

I replaced the remaining RAMs in the following order :  4G, 5G, 6G, 7G, 5Fand 4F. On replacing each one in turn, I powered on the MTX and entered a single REM statement, in each case, the display shown was produced.

That is, until I got to the very last RAM in position 4F - on powering on the MTX - the system now operated correctly - I was able to correctly enter BASIC statements and the resultant display was correct. (The last VRAM replaced was the closest to the site of the spillage ingress.)

With apologies for the poor quality iPhone photo - herewith the display of a now healthy MTX512S2!

The replacement VDP in its new socket, along with the set of replacement VRAMs in their new sockets.

   .... and the parts they replaced  - the leftovers !
And finally, the MTX512S2 computer board back in its case along with the 80 column video board.

If you zoom in on the picture, you can see that the board looks in much better shape than before.

As you can imagine, by the time I got to the last VRAM, I was thinking that it was unlikely that a VRAM fault was the cause of the secondary problems described above.

Much to my relief though, replacing the last VRAM did indeed fix the problem. Whilst I am obviously very pleased with this outcome, I am struggling to understand how a faulty VRAM could cause erroneous key presses to be reported - particularly as the VRAM does not connect directly to the MTX data bus.

Update : I think I know why this was now - see here for details

 

 

 

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