Hello, new member, trouble building ZX80 :)

[zeddex]

Hello Mark, pin 20 looks nothing like compared to the real '80

Real

pin 20 (2).jpg (114.52 KiB) Viewed 2272 times

Clone

pin 20 (1).jpg (102.47 KiB) Viewed 2272 times

Not sure if the clock looks quite the same either. It didn't used to have that little "lower" edge - not sure if that's normal?

Real

pin 6 (2).jpg (102.21 KiB) Viewed 2272 times

Clone

pin 6 (1).jpg (102.6 KiB) Viewed 2272 times

I almost had a working clone previously, it's got to be a failed component or short... shurely

[1024MAK]

The clock on pin 6 looks fine. With the /IORQ pin high (and not pulsing low), you will not get a screen displayed.
Either the CPU is running garbage instead of the correct code from ROM, the RAM is defective, one of the control circuits that controls ROM or RAM has a fault, or the Z80 has not been correctly reset.
Or there is a connection problem somewhere. If you have not done any more soldering, it's less likely to be a short.

Mark

[zeddex]

Update - back to square one, but better than being at square -1!

• Reflowed all connections, cleaned thoroughly with Isopropyl Alcohol - no change.
• Removed & checked all logic with my TL866 EPROM programmer's tester - all passed (assuming the logic testing function is accurate.)
• Tried alternative Z80 - success!

Unbelievably, it seems I've killed yet another Z80. I guess somehow amongst the probing and general frustration I zapped it with static or too many volts. Luckily I have a reasonably large supply of Z80s...

So I still have mad flashing inverse characters but it does at least function now as it did in OP.

When I get time I will thoroughly follow the video circuit with my scope and compare them on seperate channels with the Real '80. The issue is still "resolved" by touching the capacitor leg with the oscilloscope probe.

Also, is C15 supposed to be 220pF or 47pF? My real one has 220pF but parts lists I've found say 47pF.

[1024MAK]

Mine has a 220pF for C15, as does another ZX80 that I am repairing.
In practice, it may be that the value is selected depending on the type of 6.5 MHz resonator fitted (X1).

CMOS Z80s will be at risk from static discharge. NMOS chips can also be killed by static.

Be very careful when probing about, if you short between two pins, the outcome is likely not to be good.

Mark

[MaFli]

Also, is C15 supposed to be 220pF or 47pF? My real one has 220pF but parts lists I've found say 47pF.

Mine is 220pF as well. And used that in my clone as well. Only the US version has a different value according to the ZX80 Assembly Manual. But if you see a steady clock you should be okay.

http://www.retroisle.com/sinclair/zx80/ ... mbly_7.php