Help in resurrecting my ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: Help in resurrecting my ZX81

Post by 1024MAK »

There is still a problem with the Z80 CPU clock. As I said above, without a clock input, the CPU does nothing and it's other pins will be in an undefined and invalid state.

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. When it is on, the collector terminal, which is connected to pin 6 of the Z80 CPU should go low (logic zero, less than 0.8V). When transistor TR2 is off, resistor R5 will 'pull-up' the voltage on CPU pin 6 to a logic high (near to the +5V supply voltage).
ZX81 CPU clock circuit
ZX81 CPU clock circuit
IMG_7532.JPG (31 KiB) Viewed 4151 times
So with the power disconnected, assuming you have a multimeter, test the resistance of R6. It should be about 2.2k ohms. If you have a diode test range, connect the red (positive) probe to the junction of R6 / C7 / TR2 base terminal. Connect the black (negative) probe to the 0V / GND rail. The meter should indicate around 0.6V.

Mark
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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb »

If I understand you correctly: Pin 6 on Z80 should pulse together with pin 14 on ULA.

Thanks.. I'll check it when I get home.
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Andy Rea
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Re: Help in resurrecting my ZX81

Post by Andy Rea »

your in good hands witrh Mark.. but yeah if you got no clock at pin 6 of the CPU you'll be going nowhere fast...
so definately check out the TR2 circuit to start with...
what's that Smell.... smells like fresh flux and solder fumes...
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1024MAK
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Re: Help in resurrecting my ZX81

Post by 1024MAK »

mrtinb wrote: Mon Jul 30, 2018 2:03 pm If I understand you correctly: Pin 6 on Z80 should pulse together with pin 14 on ULA.

Thanks.. I'll check it when I get home.
Yes, when ULA pin 14 is high, the Z80 CPU pin 6 should be low. And when ULA pin 14 is low, the Z80 CPU pin 6 should be high. Both should be pulsing at the same rate (3.25MHz).

Mark
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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb »

1024MAK wrote: Mon Jul 30, 2018 12:33 pm 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.
Image

There is Pulse High Low on pin 14 of ULA when measured with digital probe. This is shown as 2.26V with a multimeter. After R6 there is 0.45V with a multimeter and the digital probe shows Low as it is probably Pulse High Low with 0V and 1V. Both outputs of TR2 is 0V. I assume TR2 should be replaced.

TR2 is a ZTX313 in both the schematic and on the pcb. I need a replacement. I don't have a ZTX313 but I have listed below the transistors I have. Could anyone of them be used
  • 2N3904
  • 2N3906
  • 2N5401
  • 2N5551
  • A42 A331
  • A733
  • A1015
  • C945
  • C1815
  • KSP92-706
  • S8050
  • S8550
  • S9012
  • S9013
  • S9014
  • S9015
  • S9018
Martin
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1024MAK
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Re: Help in resurrecting my ZX81

Post by 1024MAK »

Before you change anything, try temporarily putting a wire between the junction of R6, R3, TR2 base and 0V/GND. That is, in position R3. Yes, I do mean short out the unused R3 position. This (if done correctly, so double check that your wire is in the correct place before switching on) will force the base of TR2 to 0V. It should then switch off. You should then get a logic high on pin 6 of the CPU (the clock input). If you measure this with a multimeter, it should show the same voltage as the +5V supply rail. Once you have done this test, remove the temporary wire.

If this works as described, then TR2 is okay. The problem then is that it is not getting time to switch off. You see, at high frequencies (here we are talking 3.25MHz), the time for a transistor to switch off becomes significant. Alas, you need a good oscilloscope to see this problem. Logic probes, digital analysers and multimeters are not much help.

It may be that Andy's replacement ULA module produces a stronger clock signal. So one solution may be to fit a higher value resistor in position R6. A higher value resistor (once you have experimented and have it working) will not drive transistor TR2 so hard. Remember transistors are analogue devices. The less hard you drive them, the quicker they will switch off when the input signal drops below 0.6V.

Mark
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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb »

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.
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1024MAK
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Re: Help in resurrecting my ZX81

Post by 1024MAK »

mrtinb wrote: Sun Aug 05, 2018 10:19 pm 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.
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?

Mark
ZX81 Variations
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mrtinb
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Re: Help in resurrecting my ZX81

Post by mrtinb »

1024MAK wrote: Mon Aug 06, 2018 8:23 am 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 value.

I'm sorry, I'll measure again.
Martin
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McKlaud
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Re: Help in resurrecting my ZX81

Post by McKlaud »

Hi Martin,

I've attached some graphs taken from a health Zeddy. I hope they would help.
Attachments
ULA - RC oscillator
ULA - RC oscillator
ULA - CLK output
ULA - CLK output
Z80 - /CLK input
Z80 - /CLK input
Claudius
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