Hello, new member, trouble building ZX80 :)

[zeddex]

Hi,

In an effort to improve my understanding of my favourite computers, i decided to build a ZX80 as a companion to my real one. Board came from Martin's copy of rev 1.1 of Grant Searles foils.

I'm not terrible at soldering (most of my projects worked first time, I'm still very amateur though) and I don't believe there are any shorts. I've carefully inspected it all over and tested for resistance / continuity.

RAM works, tested in real ZX80. Ditto the ROM and CPU. Oscillator nicked from a scrap ZX81 and does work OK. All logic is LS type and mostly brand new.

I'm not terribly technical, better at following instructions, so this issue has me stumped.

All the logic matches the part numbers on my real '80 and 5V is getting to them. I randomly polled various logic pins and capacitors and mostly the waveforms look similar apart from all those which are flatlining. In particular apart from the "heartbeat" of the clock, it seems the RAM, ROM, CPU and video output are doing mostly nothing.

I have so far noted the following:

• Pin 6 of the Z80 has a not-quite-square clock signal close to the real one (almost... Around 4.8V peak to peak) , but all the address / data lines appear to be doing nothing. .
• The reset pin on the Z80 is stuck low (am I right it should be high?)
• Video output - no activity on the scope. Does produce a totally blank screen on the TV, (compared to the analogue snow when switched off), but I guess that's just the UM1233 getting some power.

As I say, I really am a bit of a thicko when it comes to debugging. I've looked at the schematics but I really don't know what would cause this behaviour (clock but nothing else.)

Sorry for the long-winded post! Hello again to anyone still awake Would love to hear some suggestions...

[PokeMon]

Reset must be high or better will rise slowly from low to high in a few hundred milliseconds to get the power stabilized. If reset stays low the CPU is hold in the reset state and does nothing. Maybe a short around reset ... or an accidently set jumper on a reset pin header like somebody did on finishing the ZXmore.

[1024MAK]

In the ZX80, the Z80 CPU is a major part of the video system, so if the CPU is not running, there will be no video picture.
The reset circuit is fairly simple. Resistor R21 (220k) goes from CPU pin 26 to the +5V supply. And capacitor C10 (1uF) goes from CPU pin 26 to the 0V (GND) rail (this being the negative lead of the capacitor).

IMG_4693.JPG

Photo of the reset components on a ZX80

(549.34 KiB) Downloaded 290 times

Mark

[zeddex]

Hi, thanks for the replies and helpful pics.

I'm embarrassed to say this, but the problem was the Z80. I've never had one die, and it definitely was working... but I replaced with another Z80 and suddenly it leapt into life! With a faint K on my screen

The picture is not great, though. Admittedly this is on an LCD with brightness cranked to max, but my Real ZX80 displays OK, with clearly black text on the grey-ish background. On my clone, it's the grey-ish background with sort of white text, with traces of darkness around it.

Text appears sort of white, with regular flicking, and the inverted K doesn't look very inverted. Again though, this TV is not really a reliable indicator, so I will withold judgement until I either mod for composite or test on an old CRT TV.

The video signal on the scope looks fairly close to the Real ZX80 but the on-screen flickering is reflected in the waveform.

Hopefully if I order one of the composite boards I can eliminate this issue.

Thanks once again though - knowing how reset circuit should be, and readings gave a strong indication that something was amiss around the CPU after I'd eliminated the other components! Tragic I didn't try a CPU swap first though

[1024MAK]

Good hear that you got it going

The DC levels in the video circuit will affect the contrast. Check the resistors here.

Mark

[zeddex]

Hello,

It was a great feeling seeing it actually doing something!

Now I have it hooked it up to a little B/W caravan CRT TV, the picture is far more stable, but now the issue is that the K is not inverted. It's a perfectly clear K, but obviously not right

If I type something else which should be inverted, such as getting the "?LS" message, you get a faint trace of an inverted character but it just flickers.

It can run a simple 10 PRINT "HELLO" type program no problem... which part(s) are responsible for the inverted characters?

[zeddex]

I can answer my own question regarding the strange flickering inverted characters!

It's something to do with IC18.

I swapped it for another brand new 74LS74AN, the image worked fine for a while. Then I powered it off, left for a while, powered back on and the flickering started again. Bummer. Can't be far from a fix though!

[1024MAK]

Possible missed solder joint, or dry solder joint?

Mark

[zeddex]

Hi Mark, you may be right, as I have a further update!

I was comparing IC18 between my Real ZX80 and clone; What I've noticed is that if I check pin 9, with the probe grounded to the Modulator, the flicking completely disappears and it all looks normal.

I'm pleased to have (hopefully) narrowed it down, but as mentioned in my original post I am a real thicko - why does it behave this way and what I can do to resolve it? The solder joint looks fine as-is and seems to have continuity. I will of course re-flow it just in case.

The other thing I must say is that the capacitors I bought were very cheap from eBay and although they do read the correct values, they are tiny. I assumed their size was due to them being more modern but I'm wondering now if they're causing issues...

[1024MAK]

Modern electrolytic and ceramic capacitors are smaller. Party due to improved manufacturing, and partly due to modern types (ceramic) being available with lower voltages.

Buying from eBay is fine if you buy from a known good supplier. But if you don't have experience, it is possible to buy possibly poor quality components.

Any test equipment when connected will slightly alter the circuit. Another possibility is the physical movement. The dry joint may not be on the pin that the probe touched, but a nearby joint, or somewhere back or further on in the circuit.

Mark