Sinclair ZX80 / ZX81 / Z88 Forums

Thanks for posting the NE555 based charge-pump circuit for generating the +12V supply. I have a question, I can’t make sense of the circuit around Q1. The circuitry for the base makes sense, it’s monitoring the voltage on the +12V supply. So I presume it’s intended function is to control the NE555 ...

Auto tune is often mostly harmless but also completely useless for getting a modern TV tuned in. Where possible use manual tune and fine tune. Unfortunately some TVs will only lock on to signals that are at, or very near to one of the ‘correct’ TV channel frequencies. Because the modulator in a ZX81...

Don’t forget that the FRAMES counter only advances by one each video frame. For a 50Hz video system (Europe), that’s every 20ms. Also FRAMES only advances if the machine is generating video.

Mark

What test gear do you have?

Mark

Well a vertically rolling picture normally means that the display sync. circuitry can’t lock on to the vertical (field) sync. signal. So it’s possible that the problem is simply the voltage level from the ZX81 being too high. You could try fitting one or two 1N4148 diodes between the ZX81 and the vi...

Thanks for your reply bola_dor

Don’t worry, I’ll work it out eventually... but not today

Mark

Is the lower RAM made up of eight x MCM4517 DRAM chips? Thanks for posting the NE555 based charge-pump circuit for generating the +12V supply. I have a question, I can’t make sense of the circuit around Q1. The circuitry for the base makes sense, it’s monitoring the voltage on the +12V supply. So I ...

Hi Chris I think that interface is actually a parallel ‘user’ port rather than a RS232 interface. The main chip (INS8255N), is a PIA (see the attached datasheet), there are no voltage level shifters for the line to the 25 way D connector and there is no crystal for the baud rate clock generation. Ma...

If you enter the 128K or 48K editor, does this make any difference to the symptoms?

Also if possible can you use your RGB to SCART lead, at least until the fault/problem is found.

On that subject, have you tried it on another display?

Mark

What was the RF output like before the mod.?

Mark