Sinclair ZX80 / ZX81 / Z88 Forums

OK - you use a 28C256 instead of an 27256. They are not pin-compatible! The WE-connection on pin 27 at the 28C256 is not used - in the Harlequin on pin 27 must be the A14 signal which the 27256 has here. You can fix it: cut pin 27 at the 28C256 and make a connection from pin 1 (there is the A14 at t...

Something with the ROM? I think you have to burn the 16K ZX48K ROM into each bank of the EPROM (#00000, #04000, #08000 ...), so more copies into an 27C256 than an 27C128.
On the other hand - are all the IC-pins perfectly fittet in the sockets? All soldering points checked?

Yes - to reassemble is a good idea. Are the values for C2,C4 (27pF) and Y1 (28.375MHz) correct? I'm asking because it is very difficult to find "blue" 27pF and also the 28.375MHz crystal types. With the latest pictures it seems there is a interference which produces this signal. So the areas around ...

It is right, that the TL712 isn't for video out signal - but it is only the first remarkable part which differs from the original. It is therefore possible that other deviations from the original assembly can lead to this error. Again, the question: Does the error also occur via the normal video out...

On an old Harlequin 2D, which I've assembled, there is also the TL712, wich looks different to your picture ... ... with composite video out

Is there the same distortion with video out (CVBS)? Your TL712 looks a little bit strange - maybe that's the fault.

I've found this information (if you haven't found it too)..
https://spectrumcomputing.co.uk/zxsr.php?id=1000458

"...Powering up, or pressing the reset button, automatically loads the operating system into memory."

From the instruction for Spectrum+ upgrade:
If the word STOP does not appear on the screen, the following modification will be necessary:
locate R68 on the circuit-board. Solder a 22K resistor across it, being extremely careful to avoid solder splashes and short circuits.

Ok - the first thought of mine was that there is a faulty RAM. But your discharging experiments with C111 und C112 confused me so that I focused it to the inverter stage TR4/TR5. As 1024MAK described IC 33/34 didn't make any sense for video signal - they are for the RS232 function. Because your powe...