I Bought two of these HP/Agilent E3612A power supplies in "for parts only" condition off ebay. One just had a leaky electrolytic. The board was a mess, but after I cleaned it up and replaced all the elctrolytics it powered up and worked perfectly. This album is about the second unit, where the 1,000µF 250V bulk capacitor blew up, taking a good chunk of FR4 below it with it.
Cleaning with a distilled water and baking soda slurry and a medium/still bristled brush. This combination worked very well!
Here's the damage from the bulk cap "incident". One pad completely destroyed, a good chuck of FR4 gone, and plenty of leftover charred FR4.
I used files to remove the bulk of the damaged FR4. I don't trust the insulating properties of charred FR4. Doing this barely removed any copper too.
[imgheight=600]https://i.imgur.com/2pkKmQa.jpg[/img]
Cleaning the top-side with more baking soda slurry.
[imgheight=600]https://i.imgur.com/es2x4cE.jpg[/img]
Top / Bottom enclosure clamshell, you can see the bottom one had plenty of residue left over from the cap explosion.
Cleaned up, again with the baking soda slurry.
All new electrolytic caps installed, and the front panel re-soldered on.
Board cleaned up pretty well!
Here you can see how I re-connected the capacitor to the appropriate traces. I also used a dremel with a carbide burr to remove more charred FR4. I didn't go all the way through the board to preserve the copper traces on the other side.
For testing, I'm using bench power supplies to provide all the bias power to the unit. This lets me stay away from mains AC, and lets me safely and incrementally check all functionality. Hooking DC up to the output of each bias supply rectifier is very safe and easy.
Using the first E3612A I repaired to power up the LED display of the second. The circuit is just a transformer secondary rectifier, a bulk cap, and an L7905 linear regulator. The LDO is working just fine, 5V on the dot! The display is also at least properly reading zero too.
The main bias supply is a rectified center-tapped secondary that eventually provides ±12V with respect to the positive output. I used two supplies to provide ±15V to the DC output of the rectifier. Not pictured is the 0V banana cable plugged into the positive output of the PSU.
The ±12V rails look good, and the +5V reference is right on the dot!
This is the bulk cap bleeder resistor, and is a convenient place for me to apply main DC power. This voltage is usually 80V / 140V depending on the range selected. Using an external PSU lets me start at low voltage and keep a low current limit.
The first tests with main DC power weren't working right. The output of this op-amp weren't right, so I thought it was bad. Its an LF442, a very nice op-amp considering its from '97.
I replaced it with a MAX44246, probably overkill, but I had a bunch already soldered up on these SOIC-8 to DIP-8 adapters.
Replacing the op-amp didn't help anything! Turns out I wasn't thorough enough checking every pin, since the +12V traces had actually gone bad. Two vias on this +12V trace had corroded, and the corrosion pattern etched a nice little circle completely around each via. This essentially cut the trace into a few pieces, so I scraped the solder mask on either side of each via and made a little solder blob over each to re-connect everything.
Fixing the +12V worked, but then I discovered the gate-drive pullup resistor for the main power MOSFET's was completely disconnected from their gates. Again, a bad via was the culprit. I found another bad one right next tot he culprit.
scraping the soldermask clearly shows a gap between the trace and the via.
I found another through-via that was bad, but I couldn't fix it with a solder blob. I happened to have a 0.8mm drill handy, and re-drilled the via. I ran some 30AWG wire through it and re-connected everything. This board is so generously spaced I could have drilled a 1/8" hole, but I decided to give this a shot. It was pretty easy to hand-drill, good to know for future repair projects!
Two repaired vias.
It's alive! regulating properly with low-voltage main DC. CV and CC both work.
Working well with 120V on the main DC, the highest I can go with my Sorensen. There's a convenient little trim potentiometer that makes it easy to calibrate the voltage display.
Hooked it up to an electronic load and put it through its paces. Everything worked great!
I went to power it up with mains AC, and the front display didn't turn on, but I saw a CV light. I knew the display worked, and that it's LDO worked, so I checked the rectifier circuit. I checked for continuity between the two AC terminals of the full-bridge-rectifier, and it was open circuit. This should be a short through the secondary. The fuse was fine, so that meant I had more bad traces. It turned out to be another bad via, this time on the film filter cap. I replaced the cap, but lifted it a bit above the board so I could solder on both sides to re-connect them.
I decided to use a flashlight to check the rest of the board. I found another bad via! The right pad of another rectifier filter film capacitor was bad.
The positive DC output of the main DC rectifier was hanging on by a literal copper thread! This is the lowest current model at only 0.5A max average current, but still, had to beef this up.
I was paranoid about the other rectifier terminals since I couldn't see them. I de-soldered the rectifier to check the top side traces. Luckily they were all good. I replaced the rectifier, lifted it above the board plane, and soldered on both sides of the board to be safe. (not pictured) I did the same check with the main filter capacitor just to check, but it all looked good.
After that they're both alive and well! I will definitely be keeping a close eye on the upper unit, since who knows what other gremlins might be hiding. These units also have no OVP or secondary protections built in, so that's a little sketchy. I really want to make a little module for OVP and an output enable. I don't like cycling mains power every time I want to turn these on or off.
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