Repairing an Oltronix B602D Power Supply

Repairing an Oltronix B602D Power Supply

This is a repair and maintenance description of a 1970s linear bench power supply: the Oltronix LABPAC B602D.

I bought this unit from a scrap dealer and managed to negotiate a good price. It is a classic 1970s linear laboratory power supply with two channels: 0–30 V at 1.4 A and 0–60 V at 0.7 A. The channels are fully independent, with an isolation voltage to each other and to chassis of ±500 V DC (according to the manual). It even features a “digital” voltage display. Oltronix was a Swedish company that produced high-end laboratory equipment; parts of the company still exist today and are based in the Netherlands. 

There is also a similar three-channel model, the B603D, which includes an additional 6 V supply.

 

I opened the unit and cleaned out some dust before powering it on. “Take it apart,” as Dave Jones says. Internally, it looked reasonably good. On power-up, the 0–30 V channel worked fine, but the 0–60 V channel did not. I then proceeded with a more detailed teardown and troubleshooting.

 

Repair

Things that I observed and addressed:

- The transformer can be configured for 220 V or 240 V via a soldered jumper. I changed it to 240 V, since modern mains voltage is effectively closer to 240 V (nominally 230 V). 

- I removed the output transistors (Darlington NPN 2N6385) and tested them. All were functional, although one had roughly half the hFE of the others. This device is part of the relatively simple “pre-regulator” that reduces voltage to the main output transistor at lower output voltages, so the reduced gain was not an issue. I applied new thermal compound and remounted the devices.

- The pin headers on the regulator PCB and the connectors to the potentiometers were corroded. Mechanical cleaning restored operation of the 0–60 V channel, but the pins were in poor condition, so I desoldered and replaced them with new headers. 

- The current limit potentiometers were worn and caused erratic behavior. They are 500 Ω units with 6.3 mm shafts, which made sourcing replacements difficult. I eventually ordered Vishay 14910F0GJSX10501KA units. The shaft is slightly longer but can be shortened.

- The voltage adjustment potentiometers are high-quality multi-turn types mounted on a shaft connected to a mechanical counter. They appear to function well, and replacement would be difficult, so I left them in place.

- The “digital” voltage display had missing digits due to flaking paint on the two leftmost wheels, which are made of a rubber-like material. I repainted the digits with white paint, but the result is not particularly good.

- A user guide with schematics is available online in two versions, A and B. This unit corresponds to Version A. However, I found some issues in the documentation: in Version B, op-amp A30 has inputs 5 and 6 incorrectly shorted (I corrected this in the schematic). Additionally, diodes CR91–CR92 in this unit are wired as shown in the Version B schematic, even though the supply is otherwise Version A.

 

 

- The regulator circuitry is located on a small, removable PCB. The boards for the 30 V and 60 V channels are identical. In the schematic, this section is enclosed by a dashed line. A 10-pin header connects it to the main PCB. The circuit is relatively simple, using a 1N823 zener diode as a reference and five op-amps to handle voltage regulation, current limiting, output transistor drive, and LED indication for voltage/current mode.

- The ammeter on the 60 V channel was not working. The issue was traced to a poor connection in the Volt/Amp switch and a heavily corroded meter calibration trimmer. I replaced the trimmers and cleaned/lubricated the switch. If problems persist, the switch may need replacement.

- I did not replace any capacitors, as they tested well and showed no visible issues.

Schematics and user guide

I have uploaded a cleaned-up version of the user guide (PDF) at: https://johanh.net/Oltronix/Oltronix_b602d_b603d_laboratory_supply_1977.pdf (5.3 MB). 

The error in the Version B schematic has been corrected.

Observations about the design

- There is no dedicated voltage regulation for the regulator supply rail; it is approximately 32 V (negative to positive).

- The design is floating, allowing identical regulator circuits to be used for both the 30 V and 60 V channels. I verified that the PCBs are identical and interchangeable.

- The reference voltage is provided by a temperature-compensated 1N823 zener diode.

- In Version B of the schematic, the CR60 ZF12 zener was removed and replaced with a 1N4576 reference diode optimized for lower current. Some later Oltronix power supplies from the late 1970s use the same 1N4576. This may have been a cost reduction. In their 1984 703 series, the zener reference was replaced with a TL431 shunt reference, which is still widely used today. Temperature-compensated zeners are now relatively expensive, whereas TL431-type references are inexpensive and readily available.

Regulator PCB reverse engineering

I reverse-engineered the regulator PCB to verify consistency with the schematic. As noted earlier, it matches the Version A schematic, except for diodes CR91–CR92, which are connected as shown in Version B.

I recreated the design in KiCad in case I need to manufacture a replacement PCB or implement modifications. The KiCad project is available here: https://github.com/hevanaa/Oltronix_B602D_Power_Supply