Another random find from my usual weekly surplus store visit.
0-100V linear power supply. Not bad, I am somehow a big fan of high voltage power supplies coz it helps with testing stuff, like tube circuits. 100 V/0.75 A is not gonna fetch me much, but hey that is the best excuse I have at this point in time.
Now, this was in really really bad shape. The pic above is after a day+ worth of cleanup.
It was absolutely dirty, screws missing, front panel pots were not secured, and fuse and holder was missing,
Several screws were missing. Long story short, it looked like a carcass from a failed repair attempt.
Anyways, a day and a half down the line, it was all in shape.
Here is the high level map of the PCB.
As always visual observation is the first step, I noticed someone removed the main caps and soldered them back, Maybe to test them.
I removed all the main caps and tested them. All of them turned out to be with low ESR, very low leakage, and within tolerance from a capacitance perspective, except C12 and C20.There was something funny with the leads of C12 and C20 though. It was not a mechanically firm mount and I noticed one ground lead was broken for both caps. Maybe the previous repair attempt was done without de-soldering all leads and that cause it.
As basic housekeeping I removed both the main pass transistors tested them.
While testing them I found Q7 as good, but Q6 was bad. Luckily I had a source for replacement and I could get a replacement Q6, the HP transistor 1854-0250. Seated them back with a fresh coat of heatsink compound. This is very important if you are working on old equipment. The heatsink paste or thermal compound can get dry and causes component overheating/failure due to inefficient heat transfer to the heatsink.
Inserted the fuse holder and mounted all pots properly. It also had bad solder joints for the front panel pots, which was also an easy fix.
I decided to pull out C12 and C20. Tested them and found them to be okay. Just before reinserting them, I noticed something funny.
In the PCB design, they are using the capacitor ground or can/case as a jumper. i.e if you remove the capacitor the circuit will break.
Upon further research, found that the way it breaks open the circuit is disastrous and that is the reason behind Q7 failure. Note the pic below.
The yellow line is the point where the capacitor case/can is used as a jumper. Now in my case, for C20 (Top in the picture below) the left side tab of the capacitor can/case broken. This means connection C20 to A9 was broken!
In the picture above, I have already jumped them using the black wire.
However, on C12 it is more severe. This is the point where the bias supply -ve is linked to rest of the circuitry.
I was not so confident about Q10 and measure it, and as expected it was blown too.
I didn’t have an exact replacement, but I used a 2N3906.
So two blown components. I was not sure how it is gonna behave but decided to go ahead and power up. However, to my surprise, the unit came back up perfectly fine!
So now I know what went wrong. Someone tried to restore it, broke the capacitor leads, which blew Q6 and Q10.
So if you are recaping this unit, be careful!
Few Tips for working on the unit.
1. There are three sections. Bias supply, Reference supply, and the actual supply for output. Ensure the bias and reference are working fine and voltage values are within tolerance.
2. Make sure the rear connections on the terminal block are connected for as per normal use scenario. Remember, HP shows +ve as grounded in their default normal use. I removed that link and made it floating power supply.
I removed the link between ground and +
3. The main bleed resistor, Q7 bias resistor, and the current sensing resistor – They all get really hot. It is normal.
4. There are indicators on the PCB and schematic for various test points
Some of the voltages are also marked.
5. Remember to use +S as common for all measurements.
6. Use the terminal block in the rear side to isolate different blocks and troubleshoot one section at a time.
Remove the jumpers in there and start investigating one section at a time.
So it is a happy power supply now. I am going to do basic adjustment/calibration on it and put it as part of my bench.