B&K Precision 1601 Power Supply Repair

A dead 0–50 volt bench supply that kept popping its fuse, and the troubleshooting that brought it back to life.

The finished B&K Precision 1601 regulated DC power supply on a cutting mat, with its two meters and output terminals, in front of an oscilloscope — The finished supply, back in working order

Another operator gave me a B&K Precision Model 1601, a 0–50 volt DC current-limiting variable power supply that wasn't working. It kept popping the glass fuse. I ran out of glass fuses while troubleshooting, so I replaced the fuse with a manually resettable circuit breaker. This is how the rest of the repair went.

0–50 Vvariable output, current limited

3.2 mVripple on 2 V at a 2 A load

0.16 %ripple, measured on the scope

1. The fuse keeps blowing

The 1601 opened up on a workbench with its wiring, transformer and heat sink exposed, surrounded by tools — The supply opened up for troubleshooting

The rear panel of the supply with a red resettable circuit breaker button mounted beside a warning label — The resettable circuit breaker I fitted in place of the glass fuse

2. The transistor checks out; the rectifier doesn't

Following a video on YouTube, I tested the power transistor and it checked out OK. I moved on to the bridge rectifier, removed it and followed another video to test it. It tested bad: the diodes in the rectifier were no good. I ordered a replacement rectifier, and a new transistor insulator, because the original one was damaged when I removed the transistor to test it.

The power transistor mounted on the supply's aluminum heat sink, seen from inside the chassis — The power transistor on its heat sink

The power transistor with its damaged clear insulator and a red test clip attached — The damaged insulator, during testing

A removed BR-61 bridge rectifier held by a test clip, with its leads bent out — The old bridge rectifier, which tested bad

Four bridge rectifiers laid out on a cutting mat: two shown from the side and two from the underside — The four-pack of replacement rectifiers: two side profiles and two underside shots

The new bridge rectifier mounted in the chassis among red, yellow and orange wires — The new bridge rectifier installed

3. The filter capacitor

With the new rectifier installed, I found that the filter capacitor was no good. To confirm it, I removed the capacitor completely, and the power supply came to life and gave variable voltage. I was fully confident (famous last words) that this was the last piece of the puzzle. I started putting back what I could: the rectifier, the resettable circuit interrupt in a newly drilled hole, and a lot of the wiring I had disconnected for troubleshooting. I also replaced other caps as needed.

The new capacitor arrived and went in. The old one had screw terminals and the new one has solder leads, so it doesn't look pretty. I installed the transistor with a new mica insulator and fresh conductive paste, and powered it up again. No smoke and nothing blew, which was a good sign.

Two large electrolytic capacitors standing on a small white box: a new black one with solder leads on the left and an old silver one with screw terminals on the right — The new filter capacitor (left) beside the old one, with its screw terminals (right)

The inside of the supply from above, showing the large capacitor, the circuit board and the wiring — Inside the supply during the rehab

The inside of the supply next to a soldering iron in its stand, showing the transformer and wiring, in front of an oscilloscope — Soldering in the new parts

A syringe of high thermal conductivity paste on a workbench beside the power transistor, with the heat sink and the supply's wiring above — Fresh conductive paste, with the transistor out of its heat sink

The power transistor mounted on the heat sink, with a can of contact cleaner beside it — The transistor back on the heat sink

The supply on the workbench with its cover off, a multimeter and soldering supplies around it — On the bench, ready to be cleaned up and calibrated

4. Testing the other capacitors

I tested all the other capacitors. One 4.7 µF cap was questionable: its ESR was slightly high, which indicates it's breaking down, so that one was replaced. All the others seemed good.

I learned a few things along the way. You can only test electrolytic capacitors for ESR, because they are the type that breaks down (it's worth searching for ESR, there is lots of information on it). With a $25 tester you can only really check ceramic and film capacitors for capacitance. Thanks to KF7PSM for suggesting the LCR-TC1 tester.

A small handheld component tester with a capacitor in its socket, reading a capacitance value, on a workbench — Checking capacitors with the LCR-TC1 tester

The component tester with a capacitor clipped in and its screen showing capacitor measurements — Testing a capacitor

The component tester measuring a yellow film capacitor held by a test clip — Another capacitor on the tester

5. A new reset button

The supply's front panel has a button to set the current limit or to reset an overload, and I designed and printed a new one. I designed it in Tinkercad and printed it in PETG on my Flashforge. It took a few iterations to get it right.

The front panel of the B&K 1601 with the current range knob and the button to set the current limit — The front panel

A computer screen showing a small box-shaped part on the Tinkercad workplane — The reset button, designed in Tinkercad

Three small gray 3D-printed buttons of different sizes in front of the supply's front panel, with one fitted into the panel — The printed test buttons, and one fitted

6. Surge protection

Another operator suggested adding a varistor across the 120 volt AC input to the transformer, for protection against spikes. It's the red disc in the photo.

Close-up inside the supply showing a red disc varistor wired near the transformer among colored wires — The varistor (the red disc) across the 120 V AC input

7. On the oscilloscope

I put the power supply on the oscilloscope. Using a 1 ohm resistor to make a 2 amp load, I checked for ripple and noise. The output voltage was very clean: about 3.2 mV of ripple on 2 volts, or about 0.16 %.

A Tektronix oscilloscope beside the open power supply, with a probe clipped to the output terminals — The scope set up on the supply's output

A Tektronix oscilloscope showing a small noisy trace on its screen — The trace at a 2 amp load

An oscilloscope screen grab labeled 2 amp load, cursor 3.20 millivolts peak to peak between the yellow cursor lines — Screen grab: 3.20 mV peak-to-peak at a 2 A load

Parts

Manually resettable circuit breaker (in place of the glass fuse)
Replacement bridge rectifier (a four-pack)
New mica transistor insulator and conductive paste
New filter capacitor, a replacement 4.7 µF capacitor, and other capacitors as needed
A varistor across the 120 V AC input
A 3D-printed reset button (Tinkercad, PETG)
Tools: an LCR-TC1 component tester, an oscilloscope and a 1 ohm resistor

The result

It was fun, and I learned a lot fixing this. I also got used to working with an oscilloscope, among other things. I now use this supply on the bench, including for my 50 µA current source.