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-12V short to GND on my Quadra 840AV logic board

gmipf

Member
I need some help regarding my Quadra 840AV. I have got this Macintosh stock. The behavior was at power on: Very dim Power LED, black screen and only FANs were spinning. 2 days later I have powered it on again and the power LED wasn't powering on any more but FANs were spinning. After longer power on time and some powering on and off I have heard the startup chime but still no video signal.

Then I have removed all the old electrolytic capacitors, cleaned off the legs and the capacitor leakage with vinegar and isopropal 99%, scratched off trace covers where a corrosion was visible. I had Wurth radial caps on hand and used these: 2x 10µF 35V and 13x 47µF 16V. There is no cmos battery leakage. The behavior was still the same after replacing the caps and I couldn't also get the chime any more. 

Then I have tested the power supply input according to this pinout:

840av_psu_pinout.gif

According to my multi meter -12V seems to be shorted to ground. I wanted to connect my power supply to it, but I don't know how. I have this one:

RND320-KD3005D: https://cdn-reichelt.de/documents/datenblatt/D400/RND320-KD3005D.pdf

Is it possible to just connect the black wire of my PSU to the -12V pin and the red positive wire to any GND pin of my logic board to get -12V? And what are your suggestions to the problem?

 
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beachycove

Well-known member
It has been a while, but as I recall, the area at the top around the main power plug on the logic board is prone to capacitor leakage, and needs to be cleaned up. Yon goo spreads over a large area. In my case, I had missed the spread on recapping and had problems, but a good clean worked wonders. Your troubles may have deeper roots, of course, but have a look at the area indicated, as the matter could be very simply dealt with.

There is an argument in here somewhere in favour of sticking the things in dishwashers....

 

gmipf

Member
I have read the pinout in reverse, not the -12V is short to GND, it is the 5V rail. The board is in the dishwasher now without soap. Should I put the board completely in isopropal 99% before drying?

 

Byrd

Well-known member
I would clean the board with IPA (gently use dipped cotton buds or soft toothbrush around ICs, traces) but not immerse the board in it.  Too much IPA can soften the resin coating on the PCB particularly on the underside which can become sticky  As you've discovered many caps reside around the 840AV's custom ICs - I'd have a good look at everything under magnification as you gently clean the board.

 

cruff

Well-known member
You should look for possible shorted tantalum capacitors too, but usually they tend to explode when they short.

 

gmipf

Member
I have washed the board through the dishwasher, removed all shorted/5V rail black and yellow tantalum caps. The short is still there. Between GND and 5V the multimeter shows between 16-17 and it beeps, I think the value is in mOhm? Should I start to remove those tiny ceramic caps too? They also sometimes seem to fail with a short. The PSU set to 5.4V the board draws between 2.2A and 2.3A. After a short while there is nothing too hot but if I wait longer the 4x 74F245 between the Nubus slots and the AT&T package are the hottest. I don't have any thermal camera. I just touched with my fingers.

 
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gmipf

Member
I think I will order the Flir One IR camera. I don't want to desolder every 5V rail ceramic cap. Any recommendations regarding thermal cameras?

 

cheesestraws

Well-known member
I got to play with a Flir One a little while ago, writing some software that interfaced with it.  It was impressive, honestly, and felt like good value for money.  I haven't played with any of the others, though.

 

gmipf

Member
So ordered the Flir One ver3 non pro usb-c variant. What kind of software did you wrote for it? Does it run on Android?

 

cheesestraws

Well-known member
Oh, I was writing software for a very specific purpose using one, doing analysis of soil samples, back when I was trying to go freelance :) .  It comes with software that just does the "camera" functionality.  And certainly when I was using it it would run on Android fine.  This was some considerable time ago, though...

 

gmipf

Member
I have read that the non pro variant have low ir resolution and is on finer pitch pretty blurry. The Pro variant is really good it seems but 400€ is too much for my budget. I hope it will be usefull.

 
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cheesestraws

Well-known member
Yes.  If you want IR resolution things get very pricy very quickly.  The composite IR and visual doesn't work quite as well on a phone as it does in the standalone cameras, because the distance between the visual and IR cameras isn't controlled in the same way.

 

gmipf

Member
So I have some update to this issue. I could aquire another dead 840av, I have measured its 5V to GND and this time it was again 16Ohm. Therefore I came to the conclusion, that the problem is NOT a short, despite my multimeter is beeping. On my first board then I resoldered all 5v rail tantalum and electrolytic caps and cleaned the mainboard off and it is chiming now! The odd thing is, at the same time my speaker from the first case was also dead. So propably it was working after the first recap but I didn't noticed. The VGA adapter is also finicky, sometimes I need to remove and reseat it. I'm not sure if it is 100% working, but at least I get a picture now.

PS: Why isn't it possible to edit my topic post?
 

dougg3

Well-known member
I missed this post at first and you've already figured it out, but I just wanted to reassure you about the 16 ohm multimeter beeping. You're absolutely correct. Continuity mode can fool you into thinking there's a short circuit when there's not. It's a common thing that tricks people. Each multimeter is different. For example, mine (a UNI-T UT60A-CN) beeps as long as the resistance is below 100 ohms. It's normal for motherboards to show a really low resistance on a voltage rail, especially if that rail is going to the CPU. If I'm confirming that something is a short circuit, I'll put it into resistance mode without the beeper and just look at the resistance reading. If it's down in the 0.1 or 0.2 ohms range it's usually a short.
 
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