Sinclair ZX80 / ZX81 / Z88 Forums

C61 seems to be missing? Looks like someone's cut it off with small legs still soldered in holes. It turns on, signals rubbish which is why I'm doing a recap but all pictures I've seen online of the 4a some have the capacitor and some don't??

Am I missing something

Thanks

Hello siiidave and welcome

Are you sure it’s not C47 that has been removed rather than C61?

C61 is a small axial ceramic capacitor used for decoupling and should be fitted (although if missing it’s not a show stopper). Whereas C47 is removed as part of an official Sinclair modification of the DC/DC converter circuitry.

Maybe posting up a photo would help...

Mark

yes you are right... c47... saw pictures of other 4a boards and some had and some didnt... so if it was a mod when I recap do I just leave it off??everything else looks factory fresh so ive ordered a refurb kit and a membrane which should sort the old caps/diodes/resistors and get keyboard fully working...... then I can actually test it lol given the previous owner just left it at the local dump I thought id give it a new home in my collection.

Yes leave C47 off. It was used to smooth the -5V rail but has been replaced by a different capacitor fitted in another position. Hence is no longer needed.

Because of the modification, I strongly recommend you take careful notes of the existing capacitors and where they are fitted. Including the polarity. Including taking photos of them. This makes it so much easier if there is any confusion when fitting the new capacitors

Mark

Will do. Will update once I resolder

Thanks

Right... Done the capacitors and pic has improved a lot... However no colour and a static noise.... What's the next parts to replace the regulator is getting hot so will replace that with the low heat modern equivalent.... Can't remember the name but for the colour etc... What would you recommend next

Thanks

Right... Done the capacitors and pic has improved a lot... However no colour and a static noise.... What's the next parts to replace the regulator is getting hot so will replace that with the low heat modern equivalent.... Can't remember the name but for the colour etc... What would you recommend next

Thanks

Is the keyboard working?

As the copyright screen is black text on a white background, it gives no indication on If the system can display colours.

Are you getting some of the colours or none of the colours? If the latter, so you get various shades of grey instead?

How is your display connected, RF/UHF or composite video?

The static noise, please describe exactly what you are not happy with.

The 7805 voltage regulator is supposed to run hot. That's why it's fitted with a metal heatsink. The heatsink helps transfer the heat from the 7805 to the air.

There are four brands/makes of DC/DC converters (switching regulators). Including Recom, Traco Power and MuRata.

Mark

Hi keyboard is working.... New membrane.

Just black and white... I'm used to amstrad which had the inherent blue screen. How do I test colour output??

It's on the original uhf so the vertical ghosting I believe is due to the modern TV but I've tried to fine tune and the audio is a constant buzz like on a dead TV channel.

All the restores I've watched online keep recommending to replace the regulator with a modern equivalent just to reduce the heat.

Will need to find a manual.... Keyboard is confusing as hell.... Press c and it types continuous..... Must be a button I need to press to try and write a simple code lol

Thanks for the help...

OK... Found instructions on how to write a basic program.... We have colour and sound.

Think that's all I need to test can put it back together and then see if I can get a game to load.

Wish me luck lol