B&K PR-28A

The B&K PR-28A is a High Voltage multimeter probe by B&K. I've bought mine in March 2026 on Marutsu.

I'm using it to measure whether a CRT is still charged or not. Last year, I've bought two used Macintosh 512K computers that I'm trying to repair. One of them mostly doesn't work, the other one mostly works. With the former, I don't hear the Macintosh startup sound, but it does emit some high-pitch sounds. The latter turns on and boots into Macintosh System 3.2. The internal floppy drive has some reliability issues and I want to apply grease to all moving parts in the floppy drive.

Now, the first thing to do when disassembling a Macintosh 512K is to make sure that there's no charge remaining in the CRT. I assumed that the PR-28A probe could help me with discharging, since it's made to connect to high voltage circuits. Attaching the probe's crocodile clip grounding to the CRT chassis and sticking the probe under the anode suction cup showed a 0.3 V charge on my Fluke 115 digital multimeter. The PR-28A scales any voltage measurement by a factor of 1:1000. 0.3 V then means that there is an electric potential of 300 V between the CRT chassis and CRT anode. I won't explain how the discharge procedure works here, but I was curious why the PR-28A probe won't let me discharge the CRT as well.

Understanding probe and multimeter

I wanted to write down some measurements that I've performed between the B&K PR-28A probe, my Fluke 115, and a cheap FNRSI multimeter and oscilloscope.

Fluke 115 measuring FNRSI voltage, FNRSI measuring resistance

Here, I'm directly connecting the Fluke 115 and FNRSI COM/V terminals.

• Fluke (Voltage DC): ~0.610 V
• FNRSI (Resistance): ~11.06 MΩ

Fluke 115 measuring FNRSI resistance, FNRSI measuring voltage

Again, the Fluke 115 and FNRSI terminals connect directly. The voltage went up every few seconds, as well as the resistance. When the values settle, you get the resistance and voltage listed here.

• Fluke (Resistance): ~11.10 MΩ (up to ~20 MΩ)
• FNIRS (Voltage DC): ~0.525 V

Fluke 115 off, FNRSI measuring resistance

Again, the Fluke 115 and FNRSI connect directly. I've turned the Fluke off.

• FNIRS (Resistance): 26.10 MΩ

FNRSI off, Fluke 115 measuring resistance

The Fluke 115 and FNRSI connect directly. This time I've turned the FNRSI off and the Fluke 115 measures its internal resistance.

• Fluke (Resistance): 6.71 MΩ

Fluke 115 measuring PR-28A resistance

Here, the PR-28A connects to the Fluke 115 through the COM and V terminals.

• Fluke (Resistance): 1.167 MΩ

FNRSI measuring PR-28A resistance

Here, the PR-28A connects to the Fluke 115 through the COM and V terminals.

• FNRSI (Resistance): 1.166 MΩ

PR-28A resistance

The PR-28A probe's manual lists two facts:

• The probe contains a voltage divider that divides incoming voltages by a factor of 1:1000. If you measure 30 V, the probes are measuring a 30,000 V potential.
• The probe's internal resistance is 1000 MΩ or 1 GΩ.

I was surprised at first that my multimeters measure a resistance with nothing attached to the PR-28A. Confirming how a voltage divider works, I could see that on the Fluke side, there's a ~1.167 MΩ resistor. The total resistance is 1000 MΩ. Then, the other resistor in the voltage divider circuit should have$R=1000\cdot 10^6-1\cdot 10^6=999\cdot 10^6$

The other half of the voltage divider has a resistance of 999 MΩ. Today I've learned how you can use voltage dividers to safely measure high voltages with multimeters.