Sinclair ZX80 / ZX81 / Z88 Forums

1024MAK wrote: ↑Wed May 26, 2021 10:50 am So, the resistance tests on the address lines have not shown up any short circuits or suspiciously low resistances. That points to the output stage of A11 address line being faulty.

Okay, my suggestion, if you have a known good spare Z80A, is this:

Rather than try to desolder a 40 pin chip, instead we sacrifice it....
Mark

So if I understand correctly the Z80 would be generating a very weak A11 signal.
I know it's probably a silly idea but what would happen if I pull up pin 1 and use a transistor to amplify it like in the simple composite mod? I don't even know if it would work...Surely the z80 is dying and won't last too long and I'll end up having to cut the other 39 pins....

Well, it certainly looks like the pin driver for the Z80A is damaged. It is still possible that another chip connected to the address bus is dragging the voltage down. But the resistance test did not show up anything. Because we are dealing with semiconductors that may have been damaged, none of these tests are completely definitive. As it is possible that unintended diode junctions have been formed inside a damaged chip. These can be tricky to detect, and can cause rather random symptoms.

Well, you could try isolating the pin for A11. And then buffering it. Things to keep in mind: the address line has to be driven high enough (greater than 2V, normally specified as 2.7V minimum) for a logic high (1) so that all receiving chips see a minimum of 2V. And it has to drive the address line below 0.8V (normally specified as a maximum of 0.5V) for a logic low (0). And the switching speed must be fast enough so that the logic level is stable before the multiplexer chip presents the address to the DRAM chips, and similar for the ROM.

Also, if one part of the Z80A has been damaged, there is a possibility that something else has been damaged.

It may be easier to skip all that and just temporary connect a 560Ω or similar value resistor between A11 and +5V and then see what the signal looks like on your ‘scope...

Mark

560 Ohm resistor didn't change anything.. ( I used a preset) lowering that value started to rise the base line at about 220 Ohm but not the pulses peaks (nor the oveshooting)
I'll start to desolder, carefully...

If you desolder and lift pin 1 on the Z80A (for A11), or cut pin 1 in such a way that you can solder it back together again, then try to use a resistor to pull up the PCB track for A11, what does that do the the PCB track for A11?

Mark

1024MAK wrote: ↑Thu May 27, 2021 12:33 am If you desolder and lift pin 1 on the Z80A (for A11), or cut pin 1 in such a way that you can solder it back together again, then try to use a resistor to pull up the PCB track for A11, what does that do the the PCB track for A11?

Mark

How do I use a resistor to pull up the line? Like before? From 5v to A11?

bola_dor wrote: ↑Thu May 27, 2021 12:49 am How do I use a resistor to pull up the line? Like before? From 5v to A11?

Sorry, yes. Pull-up meaning, to pull the voltage level up towards, or to the positive supply line.

But connect the resistor from a +5V point to the PCB pad for address line A11. NOT to the Z80A pin 1. The idea here being to see what effect this has with the Z80A isolated from the PCB track. With the resistor providing (hopefully) a logic high (1) level on the A11 track.

If the PCB track for A11 goes to a logic high (1) voltage (above 2V), then the Z80A is definitely defective.
If however the PCB track for A11 stays low (less than 2V), then something else is dragging the voltage down.

Mark

1024MAK wrote: ↑Thu May 27, 2021 1:20 am

If the PCB track for A11 goes to a logic high (1) voltage (above 2V), then the Z80A is definitely defective.
If however the PCB track for A11 stays low (less than 2V), then something else is dragging the voltage down.

Mark

Thas exactly what happened.. a nice 5v flat line..

Reviewing the board. A11 line is crossing behind R62, which I lifted up and connected to a 12v source regulated to 50mA Max when dealing with LM1889 supplie. Perhaps at some point the 12v inadvertently reached A11. The supply was connected to the same GND in the board.. .

Starting to desolder the Z80A..

At last.. it worked!! What a relief to see that copyright again .
I desoldered the CPU I broke only two lines but it came of easily.. I think I overdid it with the desoldering braid. Luckily I could fix it easily.

A11 line restored... Now all that remains is to restore the keyboard, change the rest of the electrolytics and try to improve the jailbars without breaking anything in the process.

Thank you a lot Mark, and any one else that contributed and gave me some advice...
I'll go to rest happier now