View Full Version : Grundy Newbrain help needed!

August 21st, 2011, 10:29 PM

The newbrain is not starting, i have replaced all the electrolytic capacitors on the mainboard => did not help
I even replaced the lower 16k memorychips => did not help

Using an oscilloscope, i found the following:

measured on the ZX80 at powerup:
the INT line is static LOW and not changing
at powerup the datalines start to work for about 2 seconds and then D3 stays High (as if the signal is shrinked to 0.5V and "added" to 5V)
BUSAK and BUSREQ are Low instead of pulsing
WR is not pulsing

I do have the shematics, but i am no expert in electronics, just a bit of basic knowledge picked up repairing my Vintage Computers ...

Any Help is Welcome!

August 22nd, 2011, 12:03 AM
Firstly to say I have no idea about the specifics of this machine.

However, I wondered whether there is a 'power good' signal from the PSU, and if there is whether that is working. Also is the output from it stable enough, i.e. free of ripple?

August 22nd, 2011, 06:14 AM
Hi pearce_jj,

Thanks for your quick reply, the newbrain has an external PSU (+5, -12 and +12V) and an internal conversion to "stable" +5, -5, and +12V.
I have checked all the lines and they are good and without ripple.

Greetings TOM:-)

September 3rd, 2011, 03:23 PM
Good luck, I hope you get it running. Where did you get the schematic?

September 3rd, 2011, 03:56 PM
I think this is Tom's Blow-by-Blow on another forum (http://www.newbrain.info/component/option,com_agora/Itemid,62/task,viewtopic/pid,93#p93/).

It might save some time.