Sinclair ZX80 / ZX81 / Z88 Forums

1024MAK wrote: ↑Fri May 17, 2024 7:02 pm
If it's a chip or other component that is the cause, a thermal IR camera may help.

Mark

Sounds like an excuse to buy a new toy

1024MAK wrote: ↑Fri May 17, 2024 7:09 pm Oh, and the voltage source has to be a constant steady voltage, otherwise you are wasting your time.

Mark

How right you are

Followed the voltage gradient drop and, nearly, voltage level improved but not completely. C68 is an odd looking inductor

1024MAK wrote: ↑Fri May 17, 2024 2:26 pm Shorts on the +5V rail are rather tricky to find, because almost everything is connected to it...

How good is your multimeter?
How many digits and what is the maximum count?
What is the lowest DC voltage range?

Mark

4 digit, 4000 counts auto range. I think
mV

1024MAK wrote: ↑Fri May 17, 2024 12:49 pm Which power rail? Do you mean the (nominal) +9V input?

Mark

+5v, so on the PCB

Will inspect the PCB. Thanks.

Hi, I liked my friend's Spectrum +2 so much I decided to get my own but this one is not in a good shape. There is a dead short across the power rails. I have set the bench power supply limit at 600ma for now. Nothing appears to be getting hot. PCB looks OK, bit dirty, and no obvious signs of any pre...

1024MAK wrote: ↑Sun May 12, 2024 2:22 pm The joystick chip is believed to be the same as the chip in the Sinclair Interface 2. Hence it's not believed to be essential.

Mark

In my excitement of passing the RAM tests, I forgot to put the ROM back in

Bit of progress. :D 20240512_114244_exported_6245.jpg The clue was a stuck key on bit 1 but also bit 4. Both of these bits from part of the joystick interface. So I removed the joystick controller IC and all RAM tests now pass and no stuck keys. Not booting yet but guess that is because I need a joy...

In summary. ZX Spectrum +2 (grey) displays typical RAM failure pattern on power on. Diagnostics ROM indicates IC31 bit 1 faulty. Removed IC31 and tested off board, and passed. RAM is good. Socketed IC31 and replaced with a new 4164 but faster speed. Problem still persists. Other actions taken but st...

If the fault is or has been intermittent, poor IC socket contacts, dry jointed solder joints, and PCB track faults are still possibilities. Mark Indeed, that is what I am afraid of. But the board is clean and in good visual condition and no previous work has been completed. As a precaution I reflow...