A sort of short! - Annoying.

[adsa]

I wonder if you guys can help. When I connect a PSU to my Capcom Trojan PCB the PSU dies immediately. When I measure Ohms across +5V and GND I get 111Ohms. If I use a diode test then I get 63mv across 5V and GND.
I have started to take components out of the top PCB and wondered if there is an easier way to diagnose? Any experience guys?

Cheers

 

[Darksoft]

When you say "dies" you mean that there is a shortcut big enough to make the PSU shutdown for security reasons, right?

I would start by providing first 12V only, then 5V only, etc. and see which line is giving you trouble. That's a start.

I'm no expert at repairing PCBs that are not mine, but I would first check for chips or components getting too hot.

 

[adsa]

Hi Darksoft

Thanks for replying. Power supply shuts down so no chance of anything getting hot. 12V is fine. Nothing wrong that side. Only the 5V side.

Cheers

 

[Hammy]

There's 5 ways to go ( in order of desperation )


1: Meter from your short to all chip legs, sometimes a chip will have multiple legs grounded out, trojan has schematics i think so will be easy to tell if the short is also on other lines.

2: Continue to remove chips as you are, ram / processors etc are the most likely but have seen standard stuff like 157's, 174's and 161's etc shorted out.

3: Cut tracks to try and isolate the row / column of chips, the power traces are laid out in a grid.

4: There's a device called a ToneOhm.. i never used one, but it's something to look at before causing more possible issues.

5 <last resort> : Remove all valuable parts and Power it up on a unrestricted bench supply, slowly crank it up until you see smoke, use nose to find the problem.

 

[Darksoft]

@Hammy great response.

I though about proposing #3 in your list but I find it a bit destructive

Definitely #5 is the quickest one

 

[adsa]

Awesome. Thanks for your reply.

 

[Hammy]

Yeah obviously don't wait for smoke if you go for the kill or cure method, check for heat, and slowly slowly crank it up... if the short is inside a board layer it can get very messy.

 

[Apocalypse]

I wonder if you guys can help. When I connect a PSU to my Capcom Trojan PCB the PSU dies immediately. When I measure Ohms across +5V and GND I get 111Ohms. If I use a diode test then I get 63mv across 5V and GND.
I have started to take components out of the top PCB and wondered if there is an easier way to diagnose? Any experience guys?

Cheers

First of all, you're using a Capcom Classic adapter right?

 

[adsa]

I wonder if you guys can help. When I connect a PSU to my Capcom Trojan PCB the PSU dies immediately. When I measure Ohms across +5V and GND I get 111Ohms. If I use a diode test then I get 63mv across 5V and GND.
I have started to take components out of the top PCB and wondered if there is an easier way to diagnose? Any experience guys?

Cheers

First of all, you're using a Capcom Classic adapter right?

Oh Yes. It could get messy otherwise...

 

[Apocalypse]

If you measure 111 ohms between ground and +5V there's no way a milliohm-meter could help you since that value is normal (when a short is generally only few ohms).
I'd simply go for the smoke method after I made sure there's no visible pins shorted.

 

[adsa]

Cheers. I can understand where your coming from. I will have a think about it. Thank you everyone. Much appreciated for your input.