Spectrum 2+A blinking power issue

[dfzx]

With a regulated bench power supply that has current limiting...

Thanks for the excellent and comprehensive explanation.

[ziggaboogi]

So the new power supply came and after verifying voltages I did try powering on the unit again.

Still has the blinking red light issue which tells me I probably did fry something on the inside. This power supply also has an LED installed which also blinks when the unit blinks. I'm leaning towards that I may have fried a chip and something is bridging that isn't supposed to which is making the PSUs reset.

I'll be doing some more testing but I did notice a burning circuit type smell near IC 12 and 13. Would it be worth ordering replacements for those two while I dig further around the board?

[1024MAK]

So it’s likely that the PSU is shutting down when the output current exceeds its ‘trip’ level. Then it automatically restarts.

If any of the chips, transistors or diodes are getting too hot to touch, then they are damaged. They should be removed.

Either desolder them (if you have the correct desoldering equipment and are good at removing chips from double sided plated through boards), otherwise use electronic snips to cut the pins of the suspect chip as near the plastic body as you can. Then you can remove one pin at a time, which is much easier and reduces the likelihood of damage to the PCB tracks and the plated through holes.

Don’t replace any chips until the supply voltage is a nice stable 4.75V to 5.25V.

Mark

[ziggaboogi]

Thanks for the advice Mark.

So I ended up removing both IC 12 and 13 with my desoldering gun as the smell still made me suspicious and for now I've just put in sockets as i wait for the replacement chips to arrive.

The good news is that now the power supply is nice and stable and giving just a little over 5V!

Also now the system power LED is no longer blinking nor is the PSU LED. Here's hoping that those chips were "sacrificial" in a sense and that them causing the PSU to keep tripping saved all the other components from frying.

Anyways once I get the replacements for IC 12 and 13 I'll try hooking it up and doing a full test on it and come back with an update.

[Ast A. Moore]

You can certainly run your Speccy without IC12 or IC13. Those are just RS-232 driver/receiver and are only needed if you want to use the AUX/MIDI sockets.

[ziggaboogi]

So I did try connecting it to a screen and have found a few things.

The screen is in fact getting the RGB video but it's just a black screen for now.

The four DRAM chips of IC 3 ~ 6 are getting very hot.

After checking out the transistors, it looks like TR201 has a short as it's NPN but it's seeing a voltage drop of zero on one side.

I've ordered replacements for TR201 and will keep looking around the board for other possible faults.

[ziggaboogi]

SUCCESS!

So after changing the four DRAM chips it booted right up!

I was even able to get the tape loader to work on a copy of Wolfan!

So in total due to my wiring mistake I had broke the following

IC12, IC 13, 4 x DRAM and a transistor.

The lesson learned here is to not just triple check your wiring work, but to also triple check the diagram itself!

Thanks again to all the advice on here! I am extremely fortunate that nothing hard to replace had been destroyed due to my wiring mistake.

[1024MAK]

Well done on getting it working. I’m sure you are both relieved and delighted that you did not give up!

Have fun now

Mark