Sinclair ZX80 / ZX81 / Z88 Forums

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 previous work.

CPU, ROM and ULA removed. Guessing I start removing components unitl the short goes. I am thinking to start with RAM but any tips much appreciated.

Did you test the 7805 first?

Which power rail? Do you mean the (nominal) +9V input?

It could be something simple like an item of metal or a solder splash. If a visual inspection under a good light does not find the problem, then if you mean the +9V, remove the 7805 and TR9. Then retest.

Mark

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.

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

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

Well, you could try using the voltage gradient testing to try to narrow down the area where the fault is.

Every conductor (outside of superconductors) has a small resistance, including PCB tracks.

Hence if there is a short circuit, there will be a gradual voltage loss (drop) between the source of the supply and the short circuit. Beyond the short circuit the voltage should be level (same as the voltage at the short circuit).

The snag with a complex PCB is that there are multiple power tracks supplying various chips and resistors etc.

Also, you need test between the +5V and the 0V at the same place. Not use a common 0V point. The second problem is that poor connections with the meter test leads may send you crazy.

If you do use this technique, keep a note of your test results.

If it's a chip or other component that is the cause, a thermal IR camera may help.

Mark

Oh, and the voltage source has to be a constant steady voltage, otherwise you are wasting your time.

Mark

Followed the voltage gradient drop to C68, removed and voltage level improved but not completely. C68 thinks it is an inductor

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

After removing C68 the voltage kept dropping slowly.