Sinclair ZX80 / ZX81 / Z88 Forums

Hi all.

I managed to find an old ZX Speccy 48k issue 3 in my mothers loft the other week and wanted to see if it could be resurected.

I conducted a resistance test on the 7805 regulator and it was returning around 83k ohms instead of the 100k+ ohms as advised by a video online. Swapped out the regulator for a brand new one and still getting the same result.

Any ideas? It is confusing me a tad.

Thanks

Is this across the input (+9V rail), or the output (+5V rail)?

Secondly, these measurements are only ‘ball park’ (appropriate) figures as your multimeter will be measuring everything else that is connected to the power rail....

Have you tried powering it up?

Mark

Thanks for thanks for the reply.

What I forgot to mention in the original post (burning kids tea at the time) was that I did a recap as a few caps looked a bit rubbish and swapped out the TR5. Also did the composite mod.

I tested this on the 9v input leg of the regulator. Also tried powering it up with a brand new PSU from TFW8B. Checked the voltage on the PSU that reads just under 9v and tested the power socket. But at the regulator, I get 0v on both 9v input and 5v output. Switched it off straight away just incase something goes wibble.

Cheers again.

There are only PCB tracks between the DC power input connector and the 7805 voltage regulator.

Are you definitely getting voltage on the PCB tracks where the DC power input connector is soldered on?

Mark

Test with black probe on the back of the socket and red on the 9v track leading to the regulator reads 8.96v and its now the same on the 9v input pin. But get 8.78v on the 5v output pin. But putting the black probe on the regulator center pin and the red on the left pin, I get 0v.

Am I doing this correctly?

Just an update.

Checked the pad at the back of the socket and it didn't have sufficiant connection to the grounded part of the board. Bit of soldering and Now the regulator reads the proper voltages on the regulator. Woop. Now doing further tests.

Somewhat of a result as I now get a white boarder with red and black columns with flicker on lower half of screen. When I do the lower ram piggyback test with a single chip, IC12 goes from white border with red and black columns to white boarder with solid red box. I'm guessing IC12 is a tad knackered?