I just received a HP6632A System Power Supply which can be have for about 100-200$ from Ebay and which should make a quiet nice Lab supply. Lets open it and see what`s inside 🙂
Lots of dust everywhere. As to be expected from a fan cooled device with lots of airflow. The unit should look a lot better after some basic cleaning. More worryingly: One of the input diodes seems to have fallen apart! Fortunately I found one with close enough specs on one of my scrap boards.
Since I had to remove the PCB anyway I took the unit completely apart and cleaned everything out. I also tried to save and restore the 60x60mm 12V Papst fan which sounded like a turbine. But even after disassembly, cleaning, lubricating and balancing the fan unit the noise is still unacceptable. Sounds like the ball bearings have gone. In addition to that, the fan blades are quiet small and the fan runs at very high RPM`s so I will replace it with some slower running fan with larger blades.
Another option would be a speed controlled Fan. But the three pin connectors „FAN“ pinout is simply GND-12V-GND with no PWM signal output. The schematics also indicate that the unit only does over temperature detection and shutdown without actual temperature measurements. That`s annoying, but maybe I`ll hack something together in the future.
In order to remove just the PCB and repair the diode, quiet a lot has to be taken apart. First open the case, then remove the aluminium middle strut and unscrew the transformer. Plug out the transformer lead out on the board side – this way it`s much easier to correctly reconnect them later. No two power connectors have the same pin count. Make sure to disconnect everything and clearly lable the four front connectors, also try not to drop the transformer on the board or on the floor!!!
A quiet nice way to mount the PCB are the little standoffs, on which the hole board can slide out – after removing just three screws!!!
Tilt the board forward on the heatsink and carefully take it out of the casing. It`s a bit tricky but it should just fit.
Now you can unscrew the front panel (beware of scratches) by removing the two screws under the fake leather just behind the front cover on the sides. Now cleaning with pressured air and some mild soap is in order. Here`s a look at the solid, clean main case unit. BTW, as always, do not use alcohol, isopropanol or similar at any plastic or rubbery surfaces which could be damaged by the aggressive cleaning.
A look at the dusted off board. The main heat sink on the right of the board has been dusted off with air pressure and is quiet clean on the inside now.
The construction and layout is, as it is to be expected from such an expensive device – very nice. Some minor points are the excessive use of discrete diodes and transistors with individual heat sinks and the somewhat lose main heat sink when the fan plate is removed -The hole aluminium sink is then split in two and lose. Also, the capacitors seem to be only 85°C rated which is a bit disappointing because they are so close to warm or even hot components and might dry out at some point.
Since the „020“ front panel connection option is almost never installed, but HP has provided the necessary connectors on the front of the PCB as well as holes trough the plastic part of the front panel unit. I got some rather nice 5mm banana plug connectors which fit nicely into a 9mm hole. It`s very important for the connectors to be isolated on their mounting point since the front panel is made out of conductive aluminium and the outputs will short otherwise!!!
In order to drill the holes I would recommend starting with a 3mm drill from the back and then use a 5mm and after that the 9mm drill from the front. Also I would suggest to take the front panel off and protect the keypad and the electronics with some tape from the aluminium dust which could block & short out things later on. Fix the front panel down securely and use a vertical drill. Also, clean off the holes and remove any rough aluminum around the edges!
The third, smaller hole is intended for a 5mm acyl light guide and indicator LED (more in part II).
I used approx. 15cm long 15mm2 wire and crimped connectors (blue color code) on both sides. You could also solder them on to the connector but I like everything removable. Resistance check with 6.5digit multimeter and 4 wire test setup reveals about 0.01 Ohms between the original connectors on the back and the newly installed front plugs.
Don`t forget to install the sense wires (Sense+ to Out+ and Sense- to Out-) on the back. You could also connect cables on the backside sense ports and connect them to the front output ports, but since the resistance is that low I don`t think it is worth the trouble. If you worry about the lead resistance, I would rather connect a shielded & twisted pair cable and connect it directly at the sense terminals and your load as suggested by the original HP user manual to the 6632A.
Will soon continue with part II.