*** M E M O R U M ***

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.

That looks excellent, and a lot of painstaking work.

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.

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.

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

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.

Definitely the correct voltages to the Z80.

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

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

stephen_usher wrote: ↑28 Apr 2019 19:49 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

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.