Help! (MTX512 Problem)

[AndyGarton]

Hi all, I have an MTX 512 with a REMEMOrizer 2 which has been working fine for several years. Rather stupidly, and prompted by a recalcitrant keyboard that I wanted to clean, I decided to tear down the machine and give it a good all round clean. You guessed it - once it was back together it doesn’t work. With the REMEMOrizer attached I get a flashing white cursor on the VGA output, nothing on the composite (or a black/blank screen anyway). If I reset via the keyboard (the two blank keys), the cursor flashes purple, and the composite screen flashes, so it seems as though the machine is somewhat alive and can at least see the reset keys.

I tried removing the REMEMOrizer (and thus added back to the OS/BASIC ROM that I’d originally removed, leaving the new GAL as per the instructions), and basically get the same behaviour - the composite out is black (actually it might be some other colour, I’m using a black and white monitor at the moment), but flashes when I do a keyboard reset.

I’ve had the machine apart again, checked for anything obviously wrong (although I’m not expert enough to know!), put it back together, same issue.

(I have tried another working MTX with the same PSU, cables and display, and it works fine. I also swapped in the working keyboard, no change.)

Thanks in advance for any help.

[AndyGarton]

Other other thing I forgot to mention - the first time I tried it after putting it back together, the keyboard didn’t do anything, and I found I’d put the connector back on off by one pin. Is it possible/likely this killed it?

[Dave]

Misconnecting the keyboard won't have caused any problems.

Do you have the audio connected when you are testing? It's quite handy.

I'd continue to test without REMEMORizer at the moment. With audio connected, do you hear anything? A constant tone perhaps? That's the usual symptom of a failure of the ROM startup code to execute, very often caused by bad RAM, but just about anything can do it. If there is no sound, the ROM may well have executed enough code to silence the sound chip. Just in case the system has booted, but has no video output, hit <CTRL><G> and listen for a "ding" (bell) on the audio output.

First thing I'd suggest is re-seating the socketed chips just in case you disturbed them when you were in the machine :

CPU
CTC
VDP
GAL
ROMs

regards
Dave

[AndyGarton]

Thanks Dave, I wasn’t using sound, but having dug out a cable, I do get a constant tone. Any thoughts on best way forward from here? You mentioned bad RAM, but I believe the REMEMOrizer supplies “replacement” RAM the way I have it set up, so that seems unlikely? I have since tried swapping the CPU, VDP and ROMs from my good machine, none of which made any difference. I haven’t yet tried the GAL or CTC though (I have reseated them).

[AndyGarton]

Now swapped the GAL and CTC and no difference.

[1024MAK]

A REMEMOrizer can replace the internal RAM in a MTX. Or alternatively, the machine can continue to use the original on-board RAM, be that 32K bytes or 64K bytes.

Differently programmed PAL / GAL chips are used depending on which of the above configurations are used. The correct PAL / GAL chip must be fitted.

The important point being, that if the original internal on-board RAM, is faulty, then no matter what PAL / GAL chip is fitted, without the REMEMOrizer, the Z80 will crash due to not being able to ‘see’ any working RAM.

Similarly, if a PAL / GAL chip is fitted that does not use the original internal on-board RAM, and without the REMEMOrizer fitted, again, the system will crash.

Did you clean the edge-connectors? If yes, describe what you did.

Mark

[AndyGarton]

Thanks, that’s beginning to wake up my memory banks - I can’t find any old emails on the subject unfortunately, but I’ve got a feeling at least one of my machines *did* have bad RAM, and Andy Key provided a suitable GAL for it with the REMEMOrizer. One of my GALS is marked 2_0, the other 2_32, I think (they’re a bit faded now). So it could well be the current problem machine does have bad RAM, and always has had. However, that doesn’t explain why the REMEMOrizer isn’t booting with it (it does work fine on my working machine). I have both of the jumpers on the REMEMOrizer set high (which I think means replace both the ROMs and RAM?).

(I have tried both GALs in this problem machine, plus the original PAL. All give the black screen of death (and if the REMEMOrizer is connected, flashing cursor on VGA.(

[AndyGarton]

Forgot about the edge connectors - I did clean them, but only very lightly with IPA and cotton buds.

[Martin A]

Checking Andy's site

2_32 was for a 2 rom MTX500, so 2_00 is likely for a 2 rom MTX with busted ram.
The A rom only on the 2 Rom board needed removing
Both jumpers "high" would be the correct setting for replacing the roms and ram.

The MTX memory test is pretty much the only thing thats done before the the VDP is set up, both of those are completed before the sound chip is initialised.

So the black screen means the MTX crashed before completing the screen setup, the constant tone from the sound chip confirms that.

Incidentally, the tone from the sound chip is an indication that the crystal oscillator is running, and that you have a reasonably good 5v power supply.

If you're running the 2_00 GAL and using the RAM on the ReMemorizor, and the ReMemorizor is working on another system, then suggests that the ReMemorizor ram chip itself is OK. So it's sounds like a bad connection between the MTX and the ReMemorizor, or less likely, but not impossible a bad GAL they're pretty robust though I have managed to break one .

A bad rom image could cause the same lockup, because paged roms are initialised in the memory test. But, if you've tested the ReMemorizor, then that shouldn't be the problem.

NB you can run an MTX with no CTC if there's any chance that's the issue, you just won't have any interrupts, so no VDP cursor flash, no break key!

[Martin A]

Something I just remmebered, the colour of the VGA cursor changes with the system activity. Having just looked at Andy's VHDL to refresh my memory.

Green is linked to the reset
Red is linked to MISO, so will flicker if there's disc activity
Blue is linked to the ROMS pin.

So if the cursor is purple which is red and blue mixed, that's just an indication that the system is in reset, there is no SDcard activity and ROMS is high. It would then go white after you release the reset keys and the aditional internal reset delay expires.