Quadra 950 PSU "crackling"

[Byrd]

Hi,

I'm currently restoring a Quadra 950 which I gather has not seen use for many years.  It has an Apple PPC card in the PDS slot and Apple network card.  I took to stripping down the case (which is a dream to work on!), cleaning the plastics, blasting out any dust and cleaning the motherboard with electrical contact cleaner.  It was pretty grimy to begin with but physically came up well.  Of note there was some white crusty stuff around the PDS/PPC card connector, which came away easily and being a Quadra 9x0 board it has tantalum capacitors.

With the key set to the "on" position, Quadra plugged into mains power, I can hear an unpleasant, fairly quiet "crackling/ticking" sound coming from the PSU before I've even turned it on.  There is also a low-pitched hum inside the PSU at the same time.  I can't see any arcing inside the PSU casing, nor is there any smell.  But I'd put my bets on should I attempt to fire this up something will go BOOM.

For someone with basic electronic skills (but good at soldering!), what sort of things should I be looking at inside the PSU to replace first - transformer, capacitors ... ?  It seems the beastly Quadra 9x0 PSU is a reliable unit from searches I've made online, with no particular weak points.

Thanks

JB

 

[Unknown_K]

What are you using for RAM? A long time ago for kicks I used a bunch of 30 pin RAM extenders (4x1 per slot) and filled the unit up with RAM and noticed a weird sound coming when it was running. Figured I was overloading something with so many SIMMs installed and went back to normal RAM no issues.

 

[CelGen]

I'm just going from my SGI Origin 200 which is a totally different beast but in that case at least the crackling was apparently normal when I ran in 240v mode. I guess the one thing you could look into is the line filter capacitors? They might have a very high resistance short and there's arcing going on inside them that isn't enough for the condition to race away yet??

 

[Byrd]

Thanks all for your tips - I've replaced line filter capacitors in Apple II PSUs before, so might crack open the 9x0 PSU and check these out along with the capacitors.  I'm so tempted to just fire it up - the crackling sound kind of settles after a few minutes (but still "cracks" every 10 seconds instead of the initial "crack crack crack" when first plugged into 240V).  But I'd consider that a foolish move knowing it hasn't been touched for possibly a decade in storage.

It has unknown RAM capacity as I've yet to fire it up, the banks are full but all modules look of low capacity (1 - 4MB sticks?)

JB

 

[Juror22]

One of my SE/30's had an AB that did that (the periodic ticking).  For that one, it was just a bad solder joint.  I had it opened up and I could see where it had what looked like missing solder in one area.

I added some additional solder to the joint, re-flowed the solder in that area and it fixed it right up.  Good luck sorting yours out.

 

[olePigeon]

Look for scorch marks when you open it up.  Could be something arching.  I've seen that in a power supply.

 

[Byrd]

An update to this, today I took apart the Q950's PSU, it's difficult to pull apart completely due to two separate PCBs which don't come apart terribly easily; the casing opens like a clamshell but not fully.  The "crackling" was related to a poor ground wire attachment, tightened up.  I can't see any obvious signs of damage, apart from years of thick black dust, most of which I could clean out.

The main transformer MP-130B (at location T5) appears to be the culprit for giving off a louder than normal humming sound.  With everything looking OK and not "crackling", I think I'm going to try my luck and fire it up - wish me luck!