Sinclair ZX80 / ZX81 / Z88 Forums

SHZ88 wrote: ↑Thu Jun 25, 2020 9:18 pm The image does appear to change, whenever I press on the PCB or "fiddle" with the power input.

Check the voltage during "fiddeling" with your meter. Is it stable? Sometimes the power connector has dry or broken joint. Use a magnifier glass.
Heinz

What is the voltage at the emitter? If it is zero then the transistor is faulty or the ground connection of R is dry. You should be carefully with this wiring without any caps, a short at vid-out could kill the ULA.

Pin27 has to be connected with /WR. If you just use a pull up resistor, then a writing to the RAM is impossible. My board looks as follows (no further wires on the component side, but a cut for pin 26. Pin26 has to be isolated from +5V and connected with D3 or D5 on your board. (You can permute D1, ...

I had the same problem: No tape loading sound and no BEEP. There was even no signal at pin 28 of the ULA. The reason was a faulty ULA.

Check, if the oscillator frequency (at Pin 6 CPU) is stable at 6.5MHz. Could you replace the ULA?

mrtinb wrote: ↑Wed Aug 08, 2018 6:21 am Again if anyone has a scan of a component-less issue 3 pcb, that would help as well.

Hi Martin, as a compensation for the trouble I have made I send you the wanted scan.
Heinz

Sorry, Martin,
what I said was nonsense! I have mixed it up with the ZX Spectrum scheme! You are right: The collector of Tr2 should show the clock signal, i.e. it should pulsing (or 2-3V on your multimeter in DC). The same at CPU(pin6).

Hello Martin, you should not unsolder the socket if there is no serious reason to do so! Look to the scheme again: The amplifier of the clock is Tr3 (not Tr2) for Iss3 board. So, you should check the join between Tr3(collector) and CPU (pin 6). Tr2 is mixing the FBAS signal and has nothing to do wit...

But would a "bad" transistor make the signal disappear after C7/R6? I will try decreasing the Y values to see if the signal is low. Yes, it would in case the Tr2 has an inner short from emitter to basis. Replace that transistor by a new one. The amplitude at the mentioned point should be at least 0...

Check R6 for the right value with a meter. It should have 2.2kOhm and it is located between the ULA and the CPU for Iss3 boards.