Sinclair ZX80 / ZX81 / Z88 Forums

The connection 12 V a.c. is at the collector of TR4, there it is -12V a.c.. In the schematic it is only "separated" not "unrelated" and the "-" is lost in your separated schematic part. What's about the -5V at D19? Possible that you have to replace this 5.1V Zenerdiode - best check all diodes in thi...

On the 128K it is possible that TR4 (ZTX650 better ZTX651) and TR5 (ZTX213 or BC557) make this kind of fault. C111 and C112 are in this 12V (a.c.) rail, so I think the -12V / +12V isn't stable enough ...

Best is to cut. Paint is only useful to repair stripes wich will be not mechanically stressed.

As you described the fault in my humble opinion there is something with IC 14 (LM1889) or the parts around this: X2 (may age and therefore be out of tolerance), R43-47 (R43 -3K- and 44 -5K1- must have low tolerance -2%), C37-39, C65, C40, C72 (16pF 5%), C52-53, R76-R77. Sometimes it is only a little...

Your tester is useful for 4164 and 41256 RAMs - for 4164 you have to set the jumper as printed on the pcb! As I told, you can use 4164 types with 150ns access time and 128 row refresh. This includes parts from the following manufacturers: Hitachi, Intel, Mitsubishi, Mostek, Motorola, NEC, OKI, Panas...

Perfect - in my third post I said: "Also the groundpin of the 7805 looks "cold" on the solder side."

Contended video RAM is IC 6 - 13. Uncontended 'upper' RAM IC 15 - 22. Note from the service-manual: All RAM chips should have 150ns access time and 128 row refresh. Because the 128K uses "real" 4164-chips (the 48K used "broken" 4164-chips which where declared as 4132-types) there is no hint to use o...

I think there had must be a difference result by disconnecting C 76 (47nF) from pin 3 of the LM1889. If there is no change it or the LM1889 could be damaged. But first do the tests wich hellraiseruk suggested and X2 (4.433619MHz) is still suspicious. I think you'll find the solution soon.

"I have never tried the capacitor approach - never needed to before." - yes, you'll have no problem with most TV's. But it is a security option to prevent interferences/spikes between the power supplies. What happens if you disconnect the cap soldered on pin 3 of the LM1889? Do you get a better pict...

As I can see you take the video signal directly from emitter TR 2 ( connector 15A at the expansion bus) without an capacitor between. I for myself prefer the solution with an 100µF cap. Also I made the mod with an transistor, but this produces sometimes poor pictures an some TV's. In your case I wou...