"Dead" ZX81 Issue 1

[iturbez]

Hello all!

I'm trying to bring back to life a ZX81 Issue 1.

Symptoms: No picture on TV, no matter if I use the modulator signal or composite video mod.

Measurements:
5 Volts on all Vcc pins of chips (OK)
RESET (Pin 26 of Z80): 0V at power-off. It goes to 4.20V at powering on.
CERAM OSC (Pin 36 of ULA): Nothing (measured with digital scope).

May I "see" a square wave coming from ceramic oscillator at this pin?
If this resonator is dead, what actual parts can be used?

Thanks in advance.
Regards.

[PokeMon]

Could be the resonator.
But isn't the resonator connected to pin 35 (not pin 36) ?
This is in my schematic and in real on my board.
Maybe check C6 (100pF) and R4 (18k).

You could find resonators here:
http://www.sellmyretro.com/offer/details/3469

And the wave form is not square, more or less like sinus curve.

[1024MAK]

I don't normally test on the resonator circuit, as sometimes the capacitance of the probes upsets the circuit.
I prefer to test on the clock output pin of the ULA, and the clock input pin on the Z80 CPU.

I have found one faulty ULA that produces a white picture, but the Z80 CPU does nothing as there is no clock output from the ULA

Mark

[iturbez]

But isn't the resonator connected to pin 35 (not pin 36) ?

Yes! You're right. I've tested on pin 35 (not 36)...

I prefer to test on the clock output pin of the ULA, and the clock input pin on the Z80 CPU.

Well... I will test on pin 14 of the ULA and pin 6 on Z80... Maybe tomorrow...

Thanks a lot!

[PokeMon]

I don't normally test on the resonator circuit, as sometimes the capacitance of the probes upsets the circuit.
I prefer to test on the clock output pin of the ULA, and the clock input pin on the Z80 CPU.
Mark

Well this should not happen if you use a 10:1 probe.
The input capacitance of the scope (about 25-40pF) is eliminated if adjusted.
This can be done easily with an in-build digital square wave generator available in nearly every oscilloscope and a small screw driver.

10X probes (also called 10:1 probes, divider probes, or attenuating probes) have a resistor and capacitor (in parallel) inserted into the probe. Figure 4-36 shows the circuit for the 10X probe connected to a high-impedance input of an oscilloscope. If R1C1 = R2C2, then this circuit has the amazing result that the effect of both capacitors exactly cancel. In practice, this condition may not be met exactly but can be approximated. The capacitor is usually made adjustable and can be tweaked for a near perfect match. Under these conditions, the relationship of Vs to VIN is

See:
http://zone.ni.com/devzone/cda/ph/p/id/17

scope.gif (9.6 KiB) Viewed 5789 times

[1024MAK]

@ PokeMon, I agree that it is better to use a 10:1 'scope probe (and yes I have some).
But I also have some 1:1 'scope leads, and often use these as they have 4mm jack plugs so I can attach either croc clips or hook/grab probes.

Mark

[PokeMon]

Yes I have a set with croco clips, too.
But I normally don't use it - maybe sometimes to connect GND but you have to ignore the noise on the signals.

[stians]

Hi guys, thought I'd show my interest to this thread. I, too, have a dead ZX81 I'm trying to revive these days. Hope this does not come off as a thread hijack, but rather encourage more members to enlighten us "newbs" further.

I acquired the ZX81 ("issue one") a while back, knowing it was "faulty". From what I've read, the ULA is a fragile IC and is most likely the reason why it's not working.

Here's what I have found so far, maybe some of you can help.
- The first thing I did (before measuring anything) was to replace the Z80. I have many, and if this was it, the fix would be quick. No success.
- I've measured the ULA and the Z80, pins are either high or low. No activity on the data- or address bus.
- CLK (pin 6) on Z80 gets nothing, but the 6.5MHz signal is generated from the ceramic oscillator to the ULA.
- Replaced the voltage regulator, in case it was fried, no success.
- Chips heat up but no activity anywhere.

As I said I'm fairly sure the ULA is gone, but I thought I'd check with you guys before thinking of alternative plans. The FPGA replacement is an option.

[1024MAK]

Here's what I have found so far, maybe some of you can help.
- The first thing I did (before measuring anything) was to replace the Z80. I have many, and if this was it, the fix would be quick. No success.
- I've measured the ULA and the Z80, pins are either high or low. No activity on the data- or address bus.
- CLK (pin 6) on Z80 gets nothing, but the 6.5MHz signal is generated from the ceramic oscillator to the ULA.
- Replaced the voltage regulator, in case it was fried, no success.
- Chips heat up but no activity anywhere.

That sounds just like one of my faulty issue 1 ZX81's. This board has one of the very early ULA chips. I now have a replacement ULA, but have not yet found time to try it, as on the early issue 1 boards, Sinclair soldered the ULA . It's no problem for me to unsolder it, but I have other working ZX81's.

I have also investigated problems with some other ZX81's sold as "spares or repairs" that I got from a certain auction site. Most had expired membranes (but to fair, this was visible in some of the photos of some of the err, “lots”). One had a faulty Sinclair ULA, one had a faulty Z80 CPU, and one had the ROM incorrectly fitted in the socket. I already had some spare Z80 CPU’s and I bought a ULA2 from a member of the Sinclair ZX World forum . I used a spare membrane or a Memotech keyboard to test them (forget which). Now all of them work (but some are still waiting for me to fit replacement membranes).

Also the RAM or ROM may be faulty, and TR2 may be faulty, all of which will produce a white screen with no K.

The quickest way, is to have a working ZX81 that has all the chips socketed, so you can swap the chips from the faulty Zeddy to work out what is good and what is bad. I have a ZX81 that has the ULA, the Z80 and the ROM socketed, so that's handy. Myself, I've not had the on board 1K RAM chip(s) fail. If you suspect RAM, and have a 16k RAM pack, plugging in the RAM pack disables the on board RAM (although if the on board RAM chip(s) have developed short circuit faults on their address or data bus pins, this will not help). Keep in mind that with a RAM pack, it takes the Zeddy slightly longer to test the memory, so there is a pause before the K appears.

Good luck

Mark

[1024MAK]

The output from the 7805 voltage regulator should be between 4.75V and 5.25V. The input to the 7805 needs to be between 7.5V and 12V. It can be higher, but then the heat-sink gets rather hot . If not using a Sinclair PSU, check, test and double check the polarity. The tip is positive and the ring is negative.

By the way, if the Z80 is receiving a valid clock signal, and there is no bus request signal (not used, so pulled high on a ZX81), you should get the refresh address on A0 to A6 even if the CPU is not running a valid program. This counts up from zero to 127, then starts the cycle again. A7 should be zero, but can be set under program control to a 0 or a 1.

Mark