ZX81 rebuild

[1024MAK]

A sharp tip lies in wait...

[1024MAK]

If you have a 'scope or logic probe, check Z80 address lines A14 and A15.
Mark

Or go the whole hog and test every single pin on the Z80 CPU. See the link in my sig for the pin-out.

Most pins should be pulsing at various speeds.
Not normally used control inputs, or inactive inputs should be logic high (for example /RESET).
All the address and data pins should be pulsing if the CPU is running the normal ROM code and the RAM is okay.

Address line A14 tells you if the Z80 is attempting to access ROM (A14 low) or RAM (A14 high), but keep in mind that after every instruction fetch, there is a (DRAM) refresh cycle and the top half of the address (A8 to A15) comes from the current value in the I (interrupt vector) register. Address line A15 tells you if the CPU is accessing ROM or RAM during normal operation (A15 low) or reading data to generate the display (A15 high).

As you have some working Zeddies, compare the signals on a working one to the rebuilt one (don't forget to swap the power and ground leads over).

Mark

[Lardo Boffin]

Thanks! Which settings on the probe should I be using? I have the choice of TTL and CMOS.

[1024MAK]

For 5V logic use normally it's best to use TTL mode. Most 8 bit CPUs (made with NMOS technology and their CMOS replacements), along with their support chips normally use TTL compatible levels. As does most bipolar and NMOS ROM, EPROM, SRAM and DRAM chips that run at 5V.

74xxx, 74Sxxx, 74Lxxx, 74LSxxx, 74ALSxxx (these are all bipolar TTL technology) and 74HCTxxx (high-sped CMOS with TTL input switching levels) chips all use TTL levels.

So, in general, use TTL. The exception being if you are measuring inputs on 4000 series logic chips (which are CMOS technology), or on 74HCxxx logic (HC means high-speed CMOS).

The difference is in the detection window of the logic levels.

Mark

[Lardo Boffin]

For 5V logic use normally it's best to use TTL mode.

Mark

Having finally found some time...

I have tested pins 4 (A14) and 5 (A15) on a working zeddy - A14 has a slow-ish pulse, A15 has a fast pulse.

On my rebuild both are stuck high. None of the pins on the Z80 pulse. Some are high and some are low.

[Lardo Boffin]

Below are the resistors and capacitors I ordered from CPC (www.cpc.co.uk) if anyone fancies having a quick look to see if they are correct?

2C610EE1-96BB-4954-BDCF-35C01BAC8345.jpeg

(394.34 KiB) Downloaded 333 times

DC90F0FB-E8EE-4C32-901F-BA862F9A71E4.jpeg

(1.54 MiB) Downloaded 333 times

I have checked I labelled the bags correctly (I have) and checked that the resistors in the bags have colour bands that match the original description. I can check the placement of resistors but not capacitors as they are helpfully not marked...

Thanks!

[1024MAK]

Does the ULA have power to it's power pins (34 and 40)?
Does the ULA CPU clock out have a 3.25MHz signal (pin 14)? (Pulsing)
Does the Z80 CPU have a 3.25MHz signal On pin 6? (Pulsing)
Does the /RESET pin (pin 26) go from low to high (very quickly) after a power on?

If the Z80 has no clock signal on pin 6, or /RESET is low, then the CPU will just sit there doing nothing and some of the outputs will have random non-logical and invalid states.

Mark

[Lardo Boffin]

Does the ULA have power to it's power pins (34 and 40)?
Does the ULA CPU clock out have a 3.25MHz signal (pin 14)? (Pulsing)
Does the Z80 CPU have a 3.25MHz signal On pin 6? (Pulsing)
Does the /RESET pin (pin 26) go from low to high (very quickly) after a power on?

If the Z80 has no clock signal on pin 6, or /RESET is low, then the CPU will just sit there doing nothing and some of the outputs will have random non-logical and invalid states.

Mark

Thanks as ever.

The ULA has power to the pins.
Pin 14 of the ULA pulses.
Z80 Pin 6 stays low.
Z80 pin 26 goes from low to high quickly after power up.

[1024MAK]

IMG_5794.JPG

Clock circuit

(257.15 KiB) Downloaded 302 times

Check / test R5, R6, C7 and TR2.
R3 should not be fitted.

Mark

[1024MAK]

What type of transistor is fitted in TR2 position?

Mark