Tuesday, September 8, 2015

Lab power supply repair

I have four Mc voice/Mc power NG1620-BL power supply units. These are the absolute cheapest lab supplies I've found with adjustable voltage and current (I bought mine from thiecom.de). They are not very high power with their 0-15 V and 0-2 A range, but they fit most of my needs. Also, as they are floating supplies I connect them up in series and parallel when needed.

Left running for several days, one morning I noticed one of them showed 9V on the meter when it should have been around 5V. First I suspected a meter fault, but I confirmed that the output voltage was indeed 9V. Good thing the device it was powering had its own regulator, so the supply didn't take anything with it. I've had the main filter capacitor fail in one of the other units, so I suspected it had gone bad in this one too. I replaced it, but the problem persisted.

The unit supplies 9V even when the voltage is adjusted all the way down. If the current limit is increased, the voltage increases also, independent of the voltage pot. Voltage rises to over 20V if the current limit is adjusted all the way up. There doesn't seem to be any obvious sign of damage anywhere, so I need to check things more carefully.

I couldn't find a schematic online so I decided to trace it out myself. I might not really need one for this fix, but I've wanted to have the schematic even before anything got broken. I first took photos of the top and bottom side of the PCB and then planned to superimpose them into the same image to see both the components and their connections. Luckily the PCB is one sided, as this helps a lot while tracing the circuit.

Figure 1. Top side of the PCB.
Figure 2. Bottom side of the PCB.

Figure 3. Top side image overlaid on the bottom side image.

The overlay turned out quite good. I didn't go out of my way to align everything. I just corrected for the perspective of the board in the photos and scaled the resulting images to the same size, and placed them on top of each other (of course mirroring the bottom side image). This was done using GIMP.

The board isn't the best construction quality, though it could be much worse. The soldering quality is quite good apart from some of the components having quite long leads (the burnt flux residue is from replacing the filter cap as I didn't clean the board before I took the photos). My main problem with the unit is however the dubious mains insulation. They have shrink wrap tube around the connections, but it has been applied sloppily and the connections are exposed in many places. The main chassis is properly earth grounded, but I'm not confident that the cover shell makes good contact as the parts are painted. Furthermore, the chassis has ventilation holes, which could easily pass a loose wire end inside in a messy workbench environment... I'll need to do something about this.

Figure 4. Raw traced schematic.
Figure 4 shows the schematic I traced out. I might still arrange it in a better way and re-draw this in Eagle. An important notice is that GND and the positive output terminal T+ are connected. The main electronics power supply thus always follows the output voltage and produces +12 and V- around that rail. The negative output terminal is basically what gets controlled.

The main voltage loop seems to be run by the LM741 (N1) and current limit is done using a single channel from the LM324 (N2). One extra channel from the LM324 is used to switch the secondary winding for the main power stage. Two channels of the LM324 (N2) are unconnected. There is something weird connected to the offset compensation pins of the LM741 (N1). Perhaps that motivates the use of an additional opamp instead of just using one extra channel from the LM324 (N2).

Before any probing around, I insulated all exposed mains wiring with hot glue. After that I started by checking the reference voltage, which was spot on. Then I checked the +12V rail, which again was spot on. Checking the V- rail I saw 25Vpp oscillation at mains frequency. This rail drives two shunt regulators to provide the negative voltages to the opamps, and is thus quite heavily loaded (not because of the opamps that is, but because of shunt regulation). My first guess was that the filter capacitor of that line (C2) might have gone bad. After removing the capacitor, a quick check with my multimeter revealed a significant loss of capacitance. I replaced the capacitor with a similar spec one, though with slightly larger physical size. Power on, and the device had come back to life.

Checking the failed capacitor with my ESR meter I got 6000 nF of capacitance and 18 ohms series resistance. That is significant. I also replaced the two other identical capacitors in the circuit (C1 and C3), although they both read fine on the meter (oddly enough, they were even better than the ones I used to replace them).

I probably wouldn't have needed the schematic for this, but having it at hand really allowed me to search for problems systematically and by publishing it here there is the remote possibility that others might find it useful as well.

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