Spectrum Computing

No worries

Mark

@]-[ I suggest you start a new topic. Having multiple repairs in the same topic often leads to confusion.

Mark

The extra transistor (TR6 for issue 3 and later boards) for issue two boards was normally placed on top of the processor (where it is known as the ‘spider’), not the ULA , or on the PCB between capacitor C47 and IC19. Although it is occasionally found in other locations. Although it's correcting a d...

dfzx wrote: ↑Thu Feb 01, 2024 9:53 am Is that what's going on here?

Yes. It's known as the dead bug modification.

Mark

Officially, for 74LS series logic chips, it's strongly recommended to tie unused inputs to the +5V supply rail via a 1kΩ resistor. However, most 74LS chip inputs will float to a high logic level if left unconnected (due to the design of the internal circuitry). Floating high here is equivalent to fi...

That explains why the chip fitted in position IC26 had been modified. OKI 32K DRAM needs a different DRAM address (row/column) compared to TI 32K DRAM. This is what the OKI / TI DRAM links select. The H / L links select either the upper (high) or lower half of the DRAM (32K DRAM are 64K chips where ...

They did improve the timing of the RAM accesses for later boards and ULAs, but that should not affect a program normally. Of course, it may be that when combined with the slight differences in timing when a machine has 'warmed up', it may affect border effects or other timing sensitive ULA video tri...

Mark

The schematic looks reasonable to me. The I/O ports as depicted in the schematic are: I/O Ref Function address 1F Y0 ADC 3F Y1 4 bit switch inputs and 4 bit relay outputs 5F Y2 8 bit input port and 8 bit output port 7F Y3 DCP Expansion Connector 9F Y4 DCP Expansion Connector Mark

If the diodes have the same colour bands (yellow, brown, yellow, grey) as the diodes on the bottom right on this ZX81 board, then they are 1N4148 diodes.

Mark

It's possible that it is just a single fault now. Maybe a poor connection somewhere.

But yes, have a break and then come back to it.

Mark

Ventilation

Although note that on the +2B the case does not have the floppy cut-out or the connector. See the photo here

Mark

I presume the 10% one is actually working as a 48K machine?

Mark

Different ULAs do affect loading. The state of the Ear socket contacts also has an effect (how clean they are and if they have enough springiness to make good contact). If the electrolyic capacitors are in a poor condition, the increased noise on the internal DC supplies will also have an effect. Ma...

When CPU is in reset, it's not driving address lines, so they are floating. Floating inputs change between 0 and 1 randomly. Actually, when the address bus is not being actively driven, depending on what is connected to the address bus (board issue, type of chips fitted, any expansions fitted), the...

Both the plastic case and the metal faceplate must be totally clean of any previous glue or adhesive. So clean until all traces have been removed. Use really good quality double sided tape. And apply it around all the edges where you can fit it. But before sticking it down, do a "dry" fit ...

Good that you now have it running

Well done.

Mark

Yes, a Z80 held in reset will not refresh the DRAM. This does not matter for the 'lower' DRAM, as only the ULA refreshes this. But would affect the 'upper' DRAM. And yes, if an external device puts an address on the address bus within the address range of the 'lower' DRAM, there will be a an address...

A broken track would only cause some of the keys to become unresponsive, the rest would still work.

Unless it's a control line to the ULA.

Mark

That could be the ULA at fault. Did it respond to the keyboard before?

Mark

What make of 'upper' DRAM chips are fitted? And which issue board is it? On issue two boards, for some makes of DRAM, this logic chip had to be modified so that the make of DRAM that was fitted would work correctly. Some issue 3 boards also used this system even though the wire jumpers on it could b...

Contention occurs only when the ULA (or gate array) detects a memory address on the address bus that is in the 'lower' RAM range OR an I/O operation that accesses or which may access the ULA (or gate array). Once the Z80 has reset, the address on the address bus will be 0x0000 which points to ROM. A...

Yes, cutting the data in (pin 2) and connecting it to 0V for the DRAM chip that provides D0 or D1 should be enough. BTW I got a bit carried away in my earlier post, I had 0x02 in my head (as that is what the code counts down from), so it's the chips that handles data bit D0 and D1 that are relevent,...

Which 'upper' DRAM chip have you removed?

If it's the chip that handles data bit D0, D1 or D2, that's enough for the ROM memory check to fail the 'upper' RAM, the machine should then try to operate as a 16K system.

Mark

You could try disabling the ‘upper’ (or expansion) DRAM. On this forum, the topic about this is here. Note however, that this disable test assumes that the DRAM chips respect the control inputs.

Mark