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

If you adapt the PCB (cut some tracks and add some modification wires) it is possible to use EPROMs. But first, try removing all the ROM chips and see what happens to the data bus. If all the data bus lines float to logic high, the Z80 will execute RST 38h repeatedly. If all the data bus lines floa...

I managed to safely desolder all three 74LS244 chips so I fitted sockets and tried the machine with none in place... No change. Swapped them around, replaced one and then another with the new one... No change. So, I've now replaced/removed every chip on the data bus except 8B (74LS373) which acts as...

I've now got a replacement 74LS244 so I think that I'll chop out 2E as that disconnects the most logic from the data bus. As far as I can see the only other chips on the data bus are the 74LS244 (2A and 1D) running the keyboard, the ROMs and the 74LS373 handling the parallel port input (8B).

After doing some more 'scoping it looks like something is pulling the data lines to strange voltages, which changes with every reset using the two keys. Sometimes the machine "runs" with the data lines active. D5 always looks the best often D0 and D2 are good too. The others are variable. ...

They are surprisingly difficult to get hold of, at least the open ended ones with repositionable keys.

The edge-connector details are in the Memotech BASIC Tutor Reference and Operators Manual / Memotech Operator's Manual (on one of the pages that has the schematic diagrams) and also in the MTX 500/512 Service Manual and Technical Information manual (PDF page 23, paper page 20). Does that give all t...

I can cause a reset using the keyboard. The tone goes off and then comes back again. I have a cheap USB logic analyser but nothing I can use to plug it into. I'm thinking that the next step may be to buy an edge connector and wire-wrap board/pins and make a break-out board for all the bus signals so...

Every instruction cycle the Z80 will generate a DRAM refresh address on A0 to A15, the state of A0 to A6 counts up every instruction cycle (and wraps around), A7 reflects the state of bit 7 of the R register (which only changes if software changes it) and A8 to A14 reflect the state of the I (inter...

The /RESET line stays high.

The ROM were originally never removed. I've removed and reseated them, similarly the PAL (white dot).

ROM A is in the middle socket, ROM B in the upper socket and ROM C in the lower socket. This is a 4000-05 board.

I did get one interesting boot last night. Instead of a continuous tone I got a clicky-buzzing from the sound sub-system. 'Scoping the data bus showed it active and the speed of the buzzing changed (increasing when the probe was touched on one or two of the data lines). It was almost as if the sound...