Is my ZX81 ULA dead?

[briantwhite]

Hi all
I recently obtained a nearly-complete ZX81 kit that appears to be in the original packaging and bags. The only thing missing was the PC board. All the chips were in the original black anti-static foam.
I decided to try breadboarding the parts to check them out before working on a PC board. My first test was the clock generator in the ULA. I connected +5V to pin 40 and GND to pin 34 and wired up X1, R4, and C6 to pin 35. Although the ULA drew some current from the 5V supply, I could not see any signal at pin 14 (clock out) - it was stuck at logic 1. I checked the photos of the circuit board around X1 and confirmed that the center pin is grounded and C6 is connected to one of the other pins - no matter which ones I tried, I could not get any oscillation. Pin 35 is near 0V and pin 14 near +5V. Other pins are at logic 1 or 0 so the ULA isn't totally dead.
I also tried driving the CERAM OSC pin with a 6.5MHz square wave through C6. Even at amplitudes of 5V P-P (I was scared to go higher), which resulted in a few hundred mV of signal at pin 35, nothing came out of pin 14.
I also tried connecting all the control lines from a powered Z80 to the ULA (even though the oscillator circuit should be independent of these) with no luck.
It seems that the ULA's oscillator is dead. It's hard to imagine how - as the chip has sat in anti-static foam for almost 40 years and some pins are at logic 1.
My question is this: is it likely that the ULA is dead? Is there anything else you can think of to explain this? Is there anything else you can suggest that I try to get it going?
thanks for your help
Brian

[retrofan8081]

Just guessing but maybe you should peg reset at 5v?

[1024MAK]

There is no reset pin on the ULA...

The oscillator pin (35) is not a logic level signal pin. If you connect any test instrument/test gear to it, it will affect the signal.

If using a bread board, keep in mind that stray capacitance may cause unwanted effects. The oscillator is rather sensitive.

Some production ZX81/TS1000 boards have an extra resistor added to the oscillator circuit. We don’t know why.

I don’t know if your ULA is faulty. The real test would be to try it in a real Sinclair ZX81/TS1000 board.

Silicon chips can degrade even if not used. So it is possible that your ULA chip died even though it has spent the last thirty odd years in storage.

The ULA can fail in a number of ways, but the clock / oscillator section does appear to be more prone to problems. As does the TV/video output pin.

Mark

[briantwhite]

Mark
Thanks. Rats - I think the verdict is 'dead'.
I will put it all away and, perhaps, someday, I can either get one of those substitute ULAs or build it out of individual logic chips.
thanks again
Brian