LC575 shorts - don't understand why...

[solidpro]

Hi Everyone

I was working on a LC575 logic board and foolishly I didn't actually test it before I started replacing all the surface mounted caps. Anyway I finished my tidy job, put it back in the unit and it doesn't power up. I put in a color classic board and it does.

So, I noticed that I had put 1 cap on the wrong way around and replaced it.

However upon more testing it seems various pads all short against pins 3 and 4 on the main edge connector (in the pic below, if you're going left to right, I mean pins 3 & 4 of the bigger pins:



Both pads on C15, C8 & C29 all short against each other and these edge connector pins, which I don't think can be right?

I've checked for shorts on all the other caps and they're all good. I removed all the new tantalum caps entirely, hoping one would reveal a change and nothing does.

So I have no idea now how to work out why these pads are shorted together. Any ideas?!


Thanks

 

[3lectr1cPPC]

I believe that those pins are responsible for power delivery from the PSU. I couldn't locate the pinout for that edge connector in a quick search, but at least one of those pins, if not both, are going to be ground pins. This means that C15, C8, and C29 are all shorted to ground, which isn't exactly great. Not sure where they're intended to connect to as I don't own a 575 but hopefully this can at least give you an idea of what's going on.
Possible that you blew something out when you installed that cap the wrong way round, but I'm not sure...
Was there leakage before from the old caps? If so, do the normal expected checks for broken traces and other connection issues.

 

[solidpro]

Oh no. I thought they might be shorted to ground but they don't appear to short to things like the back connectors and there isn't any other obvious ground point to test against. I suspect an IC has gone bad but that's way beyond my pay grade. I guess I'll go over it again to see if anything stands out but I may have created an almighty mess for myself.

There was very slightly crustiness under some of the caps but it hasn't done any damage at all. I considered myself extremely lucky to have a complete LC575 in box and just wanted to make it safer to store. Hmmm.

 

[solidpro]

Right. I took off this:



And all the shorts disappear. I put it back in and all the shorts come back...

 

[3lectr1cPPC]

That’s a RAM module. Would you happen to have another to test with?

 

[solidpro]

Ok wow, I fixed it. I kinda lost track of everything that was going on but it seems there were two issues. C15 was a problem. I removed all the caps and replaced 3 times - I'm amazed nothing got hurt along the way. It didn't help I kept working back to C15 as the last cap I removed but for some reason everytime I mounted a tantalum to it, it shorted. So I went back and did every other cap one-by-one testing the shorts as I went and all went back fine. Then on C15 I mounted a through-hole cap to the pads instead and it didn't short. Put it back in the machine (without the RAM stick) and it booted!

I ran out of time - it's getting late here in the UK, so I stopped. I will try with the RAM tomortrow and I likely have some spares somewhere to see if it's the stick or the board.... so relieved I at least have a working board again....,

 

[3lectr1cPPC]

Excellent! It’s possible that the replacement cap you fitted there itself is defective. I guess it could happen. Either that or some issue with the pad.

 

[solidpro]

Thanks for the nudge. I might not have gone back tonight without any replies!

 

[3lectr1cPPC]

No problem
Have a good night.

 

[solidpro]

I went back and re-inserted the RAM stick and it shorts again. I didn't yet have the time to dig out my box of RAM sticks but I've assumed the module is bad and thrown it in the bin!

 

[3lectr1cPPC]

I wouldn’t do that yet - it’s possible something related to the RAM only shorts when you’ve got the stick inserted. I’d keep it until you’ve done more testing.

 

[solidpro]

I retrieved it out of the bins!

 

[3lectr1cPPC]

Good

 

[Pushpull76]

I retrieved it out of the bins!

Vcc pins are 10, 30 and 59.
Vss (gnd) are 1, 39 and 72.
Simply check with a tester if there is a short without putting the ram in the slot, it's too early to put baby in the corner