Heathkit HP-23A Power Supply Restoration

HP-23A Power Supply Background

This HP-23A Power Supply was designed by Heathkit back in the 1960s to provide the necessary supply voltages for fixed station AC operation of Heathkit SB and HW series HF amateur transceivers. This HP-23B and HP-23C are variants of the HP-23A power supply and are similar except they have the following additional features:
- An internal circuit breaker instead of the fuses built into the AC plug.
- Switch selectable low voltages of 350 or 275 VDC.
- The ability to be wired for 240VAC or 120VAC, 50-60 Hz input voltage.
- Fixed instead of adjustable -130VDC Bias Supply.

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HP-23A Power Supply Specfications


The HP-23A Power Supply accepts an AC  input voltage of 120 VAC 50-60 Hz.  

It provides the following outputs:

Filament = Isolated 6  and 12 VAC, 5.5 Amp power supply.

High Voltage = +820 VDC, 250ma output generated by a full-wave voltage doubler.

Low Voltage = Internal jumper selectable, +350VDC or +275VDC, 100ma output generated by a half-wave voltage double.

Fixed Bias Voltage = -130VDC, 20ma generated by a half-wave rectifier.

Adjustable Bias Voltage = Adjustable, via Bias Control, up to -130VDC.

External Switching = Power to HP-23A can be controlled by an external Heathkit SB and HW series HF amateur transceivers or other Heathkit ham radio equipment.

Single Connection = All output power is transferred to external Heathkit SB and HW series HF amateur transceivers via a multiwire "Umbilical Cord" with 11 pin connectors at each end.

Link to HP-23A Power Supply Assembly Manual with schematic

Step 1 Bottom Cover Removal

Four screws hold the bottom cover of the HP-23A Power Supply in place. See areas circled in the picture below:

Below is a picture of the inside of the HP-23A Power Supply.

The HP-23A incorporates two glass cartridge fuses in the AC plug. See picture below:

Step 2 Initial Testing

The next step is to perform initial testing in order to determine any issues. You will need to jumper pins 9 and 10 on the output socket in order to energize the power supply. I have done so with a yellow alligator test lead. I use my Volt-Ohm-Meter to measure and record each no-load output voltage. I will compare these voltages with the voltages measured after the restoration has been completed. Be careful as you can contact potentially life threatening voltages with the bottom cover removed!

​Step 3 Electrolytic Capacitor Replacement

As electrolytic capacitors age, their electrolyte dries up causing their electrical capacity to drop and leakage current to increase. It is definitely a good idea to replace 50 year old electrolytic capacitors like the ones in the HP-23A Power Supply!

The High Voltage output section uses a full-wave voltage doubler while the Low Voltage output section uses a half-wave voltage doubler.  These power supplies employ four 125uF 450Volt Electrolytic Capacitors circled below:

Make a note of where the Phenolic and metal capacitor mounting wafers are placed. See areas circled in the picture below. It is important that they are installed in the same location otherwise the High Voltage and Low Voltage sections of the power supply will not work correctly!

Pictured below are the 125uF 450Volt capacitors from the High Voltage and Low Voltage section removed from the HP-23A chassis. We will need to salvage the Phenolic and metal capacitor mounting wafers to install on the replacement capacitors. 

With the capacitors removes, it is now a good time to clean the top of the HP-23A chassis. I use Q-tips moistened with Windex to get into hard to reach spots. See picture below:

It is easier to replace the Low Voltage and Fixed Bias capacitors, labeled C5, C6, and C7, in the schematic, with the external 125uF 450Volt  electrolytic capacitors removed. That will be the next course of action. 

You will need to remove the Bias Control potentiometer and the terminal strip next to it in order to replace the internal chassis capacitors used that are part of the Low Voltage and Fixed Bias power supply. See area circled in the picture below:

I replaced the 40uF 450Volt (C5) and the two 20uF 150Volt (C6, C7) electrolytic capacitors with modern 47uF 450Volt and 22uF 150Volt electrolytic capacitors, but did not mount them back into the chassis until the external 125uF 450Volt electrolytic capacitors were replaced. See picture below:

I replaced the 125uF 450Volt electrolytic capacitors (C1, C2, C3, and C4 in the schematic) with Cornell-Dubilier 150uF 450Volt (Manufacturer Part No:  381LX151M450K022) electrolytic capacitors. These Cornell-Dubilier capacitors are typically used in switching power supplies. These capacitors are much smaller than the originals. See picture below:

Below is how I prepared the metal capacitor mounting wafers to accept the new Cornell-Dubilier 150uF 450Volt electrolytic capacitors. I soldered a lug to the mounting wafer that will connect to the negative terminal for the capacitor. See below picture:

Below is a picture of the new Cornell-Dubilier 150uF 450Volt electrolytic capacitors installed on their capacitor mounting wafers. Several dabs of hot glue keep them in place on the wafers.

Two of the replacement 150uF 450Volt capacitors, labeled C1 and C2 in the schematic, require parallel 100K 2Watt Resistors across their leads. In addition, C1 needs a jumper wire over to the rectifier diodes located on the terminal strip. It is easier to solder these in place before install the capacitors in the chassis. Please see picture below:

Time to mount C1 and C2 back into the chassis and wire in place, pictured below:

And now mount C3 and C4, circled in red below, then wire in place, as pictured below:

At this point the adjustable Bias Control potentiometer and the terminal strip that connects electrolytic capacitors C5, C6, and C7 is still not mounted into the HP-23A chassis.  It is easier to wire the AC cord with these components not mounted to the chassis.

Step 4 AC Cord Wiring

I will be replacing the original two conductor AC cord, and non-polarized plug, with a three conductor AC cord and polarized plug that includes a ground pin. The original non-polarized plug had two 4 Amp fuses contained in the housing, see picture below:

I relocated the fuse holder inside the chassis, only one 4 Amp fuse is required, connected to the "Hot" side of the AC cord. See area circled in the picture below:

Time to finish up the wiring! I use two zip ties to act as a strain relief in order to prevent the AC cord from being pulled out of the chassis. The green wire from the AC cord connects directly to the chassis by solder lug. The black wire from the AC cord connects to one terminal of the fuse holder and a jumper wire connects to the other terminal of the fuse holder to the power switch. The AC cord white wire connects to the end terminal strip where the black wire of the transformer is terminated.
Finally, we can mount the Bias Control potentiometer and the terminal strip with C5, C6, and C7 back into the chassis. One lead of C5, C6, and C7 connect to the chassis by solder lug. See areas circled in the picture below:

Step 5 Final Testing

The next step is to perform final testing in order to determine any issues. You will need to jumper pins 9 and 10 on the output socket in order to energize the power supply. I have done so with a yellow alligator test lead. I use my Volt-Ohm-Meter to measure and record each no-load voltage, I will compare these voltages with the voltages measured before the restoration has been completed. You will notice that the High Voltage, Low Voltage, and Fixed Bias Voltage will be slightly higher than before the restoration, this is because the new capacitors are doing a better job of filtering the pulsating DC voltage from the rectifiers. Be careful as you can contact potentially life threatening voltages with the bottom cover removed!

Step 6 Detailing

Once the bottom cover is in place, I like to treat the chassis to a coat of wax so that it shines like the day it was built. I use Meguiar's Motorcycle Liquid Wax as I have it around for use on my motorcycles. Actually, any liquid car or motorcycle wax will do. First I unplug the HP-23A power supply, then I carefully apply two coats to the chassis, top cover (if equipped), external capacitors and transformer shells, carefully wiping all components with chamois in between each coat to remove any white residue. This really brings out the luster in the paint, see pictures below:

Step 7 Final step of putting the HP-23A Power Supply back into production!

My newly restored HP-23A is ready to be put back into production once the top mesh cover is installed. I use my HP-23A to power a vintage Heathkit HW-101 SSB Transceiver. See picture below: