Sinclair ZX80 / ZX81 / Z88 Forums

Multimeter set for 200Ω. Between TR2 collector and R5 leg 1 - no reaction Between TR2 collector and R5 leg 2 - no reaction Between TR2 collector and CPU pin 6 - no reaction Strange thing: This was working when I bought it 10 years ago. I have a small SMD SOIC 555 from a "Try soldering SMD"-kit. Does...

That's almost impossible, as the Z80 is soldered without socket, and most traces goes beneath it.

Edit:
I'll get a desolder gun in a few weeks. That might help.

I've ordered MPS2369s but they only show up a month from now.

So I seem to have a choise.

Try replacing R6 with a higher value.
or
Try replacing the resistor with a resistor that might work.

Andy Rea wrote: ↑Mon Aug 06, 2018 11:22 pm my ula should provide a fairly clean square wave near to 3.2v p - p

Measuring with a multimeter I get 2,27V

On Andys ULA pin 35 OSC, I get 3,4 when multimeter is set for 200mV.

Measuring pin 6 on CPU is still 5V.

With multimeter on 200mV setting I get the values on TR2: Base 3,3 Emitter 2,5 Collector 2,2. That's a puzzle to me.

You mention I should try a higher value for R6. I have the following values: 2K7 3K 3K3 4K7 5K1 6K8.

Could you please confirm where you are taking these voltages. Is the voltage on the transistors collector, not the same as the voltage on pin 6 of the Z80? You've got a point there. The output of one of the transistor legs should be the input of pin 6 on the CPU. Therefor they should have the same ...

Well it seems CPU pin 6 should be 5V with or without the wire. It is.

The difference is before the transistor was: 0V, 0.45V, 0V.

With the wire the transistor is: 0V, 0V, 0.24V.

The clock (CLK) output from the ULA (pin 14) goes via resistor R6 and capacitor C7 to transistor TR2. Resistor R3 is not normally fitted. Transistor TR2 should be switching on and off in response to the clock signal on it's base (input) terminal. https://trello-attachments.s3.amazonaws.com/598af660...

Yeah!

I saw that on eBay, and I'm glad it was sold to one I know.