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Author Topic: what a long wierd trip - HP 6296A  (Read 1877 times)
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Patrick J. / KD5OEI
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« on: September 20, 2009, 11:20:18 PM »

Picked an HP 6296A 0-60V/0-3A power supply out of a massive electronics junkpile yesterday. There were two guys there with shovels, literally shoveling the stuff into a 20FT trailer to go to the dump. The tag said "Bad regulation, large ripple, meter flaky, output low". So it was broken when I got it home, making only 30V regardless of load, and then having bad regulation. Someone had started to cut the cord before pitching it, but decided not to, and someone has been working on it. In the process of that, they blew a track off the circuit board.

BTW this thing is an analog power supply but there is no huge bank of pass transitors. It has a pair of 2N3689's acting as controlled rectifiers, and then there must be some kind of active filter pass transistor setup, as there is only one big TO-3 transistor on the heatsink. I finally figured out that the controlled rectifers were not being turned on soon enough. That was very weird.

The culprit was a small filter capacitor can, the filter for a separate small power supply inside the case that serves to drive and control the main rectifiers. Unfortunately the can had 4 pins and a tab. The circuit board would have taken hours to remove so I used a soldapult to suck out the molten "slobber", as certain people call it, around each pin, but then discovered they have solder pads on the top of the PC board, underneath the cap. Well no way to get to those. I suppose H/P has a special 5-prong soldering gun for this. I ended up walking each pin out a little at a time, in a circular manner, by pulling the cap to one side or another and heating the opposite pin, then letting it cool before releasing tension. My fear was that the cap would come right out, along with ribbons of PCB copper. But it did not. It was a 1500uF 40V cap, I put in two 1000uF 50V caps. Another trick HP did on this, is that if you remove that cap, you break a B- connection which is otherwise made by its pins. Dirty pool. Replacing the cap fixed this power supply. As for the meter, it was the switch that was flaky, just dirty. I used some of that red GC 1-1-1-trichlor based stuff and cleaned that up, now it works great and has <5mV ripple and noise at 60V/1.5A and less than 11mV ripple+noise at 60V/3A. The pilot lamp is missing and it's filthy and it might need other work to be perfect.

BTW there were 4 -cases- of that GC 1-1-1 I mentioned. I took two (appx 48 bottles) and my friend took 2. I also found GC radio cement solvent. I think it will loosen speaker cones or something, and some of the infamous "idler wheel non slip" stuff. One bottle of corona dope. So how about that, the chemicals to repair the set came with the free set.

The guy did not at first want me to take the power supply but when I asked him about the chemicals he complained that he was going to have to pay to dispose of them, so I told him I would take them if I could have the power supply and that was that. That GC contact cleaner is the best!

I suppose I forgot to look for the manual first.. It is on bama. But that made it more fun anyway.
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