Resurrecting an MTX500.

[Martin A]

The wiring on the diagnostic rom board is complete, as is a candidate version of the GAL.

The next step is to get a test rom written that will run without using the main ram, but will test it.

I've got some video routines working in emulation, once I've got a bit more I'll put together a test image with the 3 system roms and give it a try on a real MTX.

[stephen_usher]

That looks excellent, and a lot of painstaking work.

[stephen_usher]

Right, I'm back from my holiday.

This morning I unsoldered the ROM C socket and I can't see anything wrong down there. I was probably mistaken about the short. I must admit that I didn't have a lot of time to look at this before I left.

The fuse hasn't blown, which means not a great deal of current flowed. The +5V line is wobbling all over the place, so I'm going to order a complete set of capacitors, regulators etc. which make up the power circuit.

[stephen_usher]

OK, so re-soldering some of the power supply components (dry joints?) has got me a stable 5V rail again.

During testing of this there was a sudden spattering and arcing under the blue "test socket" so I powered off immediately. (I was no-where near it at the time, checking the output from the regulators, and there were no leads on that side of the board.)

Again, I de-soldered that socket and I could see nothing.

Now, when I power up, the CPU doesn't run. Swapping the CPU, memory, 74LS157s makes no difference and I don't seem to have a RAS or CAS on the RAM either. I'm making things worse rather than better. At least the video processor is working and I can see lots of nice video RAM access.

Both of the crystals are working and I can see a normal clock.

I wonder what's been fried this time. I really wish that I'd not used the solder sucker during my initial re-soldering efforts now as it seems to have spattered tiny bits of solder everywhere, even though I can't see it.

[Dave]

See the drawing on this page : http://primrosebank.net/computers/mtx/c ... m_mtx3.htm

(or in the manual), to see how RAS and CAS are generated, it might give you a few pointers

regards
Dave

[stephen_usher]

It's probably due to the CPU not outputing RFSH and MREQ. It's merely sitting there doing nothing at all. It has power and 4MHz clock as far as I can tell, but I'll double check the former.

[stephen_usher]

Definitely the correct voltages to the Z80.

However I've just noticed that chips 7C and &d are scorchingly hot!

[1024MAK]

The ‘sparks’ and arcing can also be caused by cracked / fractured tracks and dry joints. Does any of the non-functional circuitry have missing +5V supplies, or are lacking good 0V/ground connections?

The requirements for a Z80 to run are:

• +5V and 0V power
• /RESET having gone from low to high
• A valid clock signal
• /BUSRQ high

Then, unless it executes a HALT instruction it should run even if it is executing nonsense. Regardless, /REFSH and /MREQ should be active.

Mark

[1024MAK]

Definitely the correct voltages to the Z80.

However I've just noticed that chips 7C and &d are scorchingly hot!

That’s not good

Mark

[stephen_usher]

Looks like somehow I have a dead short on A0 for a start.

I can't SEE any dodgy tracks or any aberrant solder.

Other than when the solder sucker bashed the PCB I'm always as gentle as possible and try not to twist it.