Sinclair ZX80 / ZX81 / Z88 Forums

My lad's ZX80 build project has hit a bump in the road. He's done a really good assembly job and was justly proud. Turned it on (genuine Sinclair 1400 PSU), and... nothing. He's now suitably demoralised.
5V appeared at the correct locations, but there was no clock signal on the Z80. Which explains a lot.

From the schematic all the clock-y goodness seems to come from IC20. The IC fitted is a DM74LS86N, supplied by Silicon Ark (who seem to be UK and reputable). It tests fine on my TL866. The only wrinkle is that he has used a 6.5MHz crystal instead of the ceramic resonator (Grant Searle's page suggests this is fine).

For investigation, I've removed IC18 and IC9 so that the clock circuit is as isolated from downstream faults as much as possible.

Using an osciloscope, all I get on any of the pins of IC20 is a 50Hz sine wave (so at least I'm using the 'scope properly). No hint of 6.5MHz at any pin.
Continuity-wise, everything buzzes out fine. R27 measures as 1k, C15 is 330p.
Looking at IC20 voltages, I find:

1 = 0V 14 = 5V
2 = 1.13V 13 = 5V
3 = 0V 12 = 1.4V
4 = 1.85V 11 = 1.13V
5 = 4.37V 10 = 1.13V
6 = 0.1V 9 = 5V
7 = 0V 8 = 3.4V

2V across the crystal seems a teensy bit low?

Does any of this shed any light on the problem? Any suggestions where to look next?

Addendum: Having written all this, I've just noticed on Grant's page that he also used a load capacitance. Is this just C15, or will he have added an extra cap? If so, between which points?

A capacitor or capacitors used in association with a crystal are either in parallel with the crystal or from each crystal pin to 0V/GND.

Unless a series type crystal is being used, in which case the capacitor may be wired in series with it.

When it comes to gates used in an analogue mode, such as in an oscillator circuit, it's a 'black art' as the circuitry is relying on the exact same characteristics not just the logic function. So in some circuits a brand new fully functional chip from a different manufacturer may not work correctly.

For some crystal oscillator circuits, it may be possible to persuade the circuit to work by using different values for the resistors and capacitors.

Mark

Thanks.

Placing another capacitor in parallel with the crystal got it to "work", so long as I didn't much care about the frequency it generated. Closest I could reach was about 4.8MHz, with a 10pF cap. Then I ran out of anything smaller.

I've managed to buy the last ceramic resonator in stock on Retroleum, so I'll see if that makes the art any less dark.

Hokay…

Ceramic resonator arrived today. With a 10pF at C15, we can get 5.9MHz. Sadly that’s the smallest value I have, so can’t push it further.

Should 5.9 be close enough to spec for the system to work, albeit slightly underclocked?

As you might have guessed from the above question, there’s still nothing on the screen so tomorrow’s going to be a fun day with the logic probe, looking for signs of life elsewhere…

Would a second cap (presumably between the other pin of the resonator and ground) be of any benefit here?

C15 on my ZX80 is a 220pF ceramic (marked with the code "221"). R27 is 1kΩ as per the schematic diagram.

ALL the video timing signals are derived from the 6.5MHz clock and 5.9MHz is too far away from the needed frequency.

Mark

1024MAK wrote: ↑Thu Feb 13, 2025 11:22 pm C15 on my ZX80 is a 220pF ceramic (marked with the code "221"). R27 is 1kΩ as per the schematic diagram.

ALL the video timing signals are derived from the 6.5MHz clock and 5.9MHz is too far away from the needed frequency.

Mark

Bother!
Unfortunately, I’ve already discovered that increasing the value of C15 lowers the frequency, and i’ve gone as low as my stock of caps allows.
Hence the desperate hope that adding a cap on the orher end might be of benefit.

The schematic suggests 0-330pF. How do you add 0pF capacitance? Naivety suggests a wire link, but that doesn’t work at all.

There is no such thing as 0pF

Stray capacitance adds at least 1pF. What they mean by 0pF is leave it out (not fitted).

I would find it surprising if no capacitor at all was required.

What capacitors have you tried?

Also, in case you didn’t know, never attach or otherwise connect any test gear (*) to a crystal or resonator legs/pins or any associated capacitor, resistor chip or transistor. Instead test on a the output pin of the device that buffers the signal. In this case, it's IC20 pin 3 that you should be using.

* unless you are have some very, very expensive test gear.

Mark

So far:

10, 20, 30, 47, and 68pF

10 gives the highest frequency, then incrementally down to 3.something at 68.
With no capacitor, I was just getting mains noise.

You’ve previously taught me well about not measuring clock signals anywhere near the crystal or associated passives. The closest point I’ve been trying is IC20 pin 3.

The conponent Retroleum supplied looks to be a Murata resonator. I’ve managed to track down some CDA 6.5s and have a couple on order. Retroleum only had one in stock so I needed to buy a different one for mine and bought two because I suspected resonator manufacturer might contribute to the dark art.

Rather than risk accidentally damaging my son’s board, changing components, I’m going to build mine up and experiment on that. Might take me a day or two but I’ll report back when the other components arrive…

I've built enough to get the clock circuit working.

With the Murata resonator, the fastest I can achieve is 5.968MHz, and that's with no capacitor whatsoever fitted.

So, I've bitten the bullet and borrowed the resonator from my ZX81 kit. With a proper CDA 6.5, I get 6.546MHz.

Conclusion: The Murata resonator ain't up to the job

Managed to source the proper CDA 6.5 resonator.

RPTronics has over a hundred in stock, at 86p each inc vat. They're based in Europe so postage is about £6-7, and delivery takes a week or two, but they've got over 100 in stock if anyone else needs one...

With that fitted, I'm getting 6.498 MHz with a 10pF capacitor.