Sinclair ZX80 / ZX81 / Z88 Forums

Hope you had a great new year. Back with updates on these two (4a, and 4s) repairs. The Voltage at the base of TR5 is 4.26V I've checked connectivity across the address lines and voltage/grounds of the 4116 sockets and they all seem ok So I added a socket and tested the 4116 one at a time to hunt fo...

Latest update. 4116's now all removed Voltage at the regulator, 4.96V Voltages at 4116: +5v - 4.96v +12v - 10.6v -5v - -4.74v Familiar high pitch whine now, and max consumption of ~600mA, so progress is made I'll have to order up some 16 pin sockets - presumably I can put 1 in at a time until I find...

So where i'm at: All modifications to DC-DC converter are done All electrolytics have been recapped Have done some checking with a multimeter and are happy the DC circuit now looks as per the diagam Have tested resistors in-circuit Have replaced TR4/TR5 and tested them again outside of circuit Only ...

The modification instructions on your blog state, for two steps: Replace R54 with D19 (anode to ground) 5.1V Zener Replace R55 with D18 (anode to C46) 1N4148 But the diagram you provide for modifications, it suggests D19 should have the cathode to ground and D18 the cathode towards C46 Could you con...

Thanks for the resources - the diagram on your blog helped me track down the issue on a 4S board I had and got it working with the help of a new TR4 and a little resoldering - that spectrum now awaits a keyboard membrane before I can make a working unit to play with. I've managed to trace out the co...

Hi folks, I'm trying to recover an Issue 4a spectrum that I put aside years ago for when I wanted to start repairing/enjoying my childhood computer collection once again. This particular board I got in the early days of ebay for spare parts, and judging by it's state it looked like someone had tried...