The Philips PE1542 is triple channel bench power supply. It has the brown case that is typical of Philips instruments from the '80s. It is part of the widespread PE series and can be bought cheaply on Ebay. It has two 20 V, 1 A channels and one 7 V, 3 A channel. I can work in voltage or current controlled mode (no foldback). Analog panel meters either read output voltage or current depending on the state of a front-panel switch.
I fixed several ones and they tend to exhibit the following failure modes:
- Broken current sense resistors. When they open-circuit, typically they take the parallel trimpot in their downfall. The original sense resistors (0.47R or 0.22R) consist of a long brown glass tube with a fine wire (see picture below).
- Oxidized trimmers and potentiometers. The original devices are not hermetically sealed and tend to drift from they nominal value, upsetting the calibration and set ranges.
- Oxidized switches. In all the units I fixed, the V/I front panel switches were oxidized black up to the point were the contact resistance was in the kR range. This makes current readout impossible and voltage readout inaccurate.
One-off failure modes I have encountered:
- Blown output transistor or driver.
- Blown protection diode, the one in reverse across the output terminals. Physically resides on the heat sink together with the output transistor.
- In one unit, the X2 filter capacitor across line and neutral more or less exploded, releasing a horrible smell. Probable cause: high energy line transient.
The operation manual contains the schematic and calibration procedure. The operation is quite straightforward. The heart of each channel is a uA723 (LM723) voltage regulator. The chip has current sense/limit input pins. Voltage is sensed across the outputs. Current is sensed as the floating voltage drop across a resistor in the reference line, using a discrete differential amplifier (diffpair). The error amplifier and voltage/current loop handover (basically just a transistor that overrides the output of the error amp) is inside the uA723. One or more external pass transistor and their drivers allow for the high output currents. A zener diode in the drive lead of the pass transistors drops the external high voltage to within the limits of the chip. A negative auxilary supply rail allows the output voltage to go down to 0 V.
Top view. The output transistors of each channel is fitted on a dedicated heat sink. They dissipate up to 30 W worst case. Note the large blue ripple filter capacitors.
PCB board containing the bulk of the three channel's electronics. This one had all sense resistors replaced with moder high-power wire wound ceramic types. A few trimmer were replace. I ought to replace them all.
Guts of an oxidized switch. Luckily, they are easily cleansed with IPA.