Friday, February 2, 2018

Noisy power supply -- Heathkit HP-13

Resurrecting a Heathkit HP-13

As seen in my previous post I've taken the dark plunge into the world of tube radios.  I want to start restoring my Heathkit HW-101, but before I can do that I must be able to power it.  As I previously wrote, I taken the road less travelled and got an old (circa 1965) HP-13 mobile power supply because I thought it would be nice to power the radio from a 12v source rather than mains. 

Unrestored HP-13 with broken solder joints and trashed capacitors

Well there seems to be a reason these don't seem very popular... I'll get to that.

I found a very nice article from RDF-Electronics regarding modernizing the HP-13 power supply.  That document includes the components and part numbers for everything you will need to restore your HP-13.  My parts arrived from Digi-key and I began snipping and desoldering the old component off the PC board.  The replacement electrolytic capacitors are all much smaller and have radial rather than axial leads.  That makes placing things on the board require a bit more creativity.  

HP-13 Schematic

The only truly problematic components to replace was replacing the twin positive axial lead electrolytic (C11) with two electrolytic capacitors (space issue) and replacing the C1 and C12 due to the tricky wiring around the transistors. 

The orange cap on the left under the Q1 (C1) was tough to replace with a much smaller radial lead capacitor

The article suggested replacing the original 100 uF/50 V capacitor with a 4,700 uF/35V to better control ripple.  The article goes into quite a bit of detail concerning his testing of ripple using an oscilloscope.  Finding room for that big cap and it's accompanying filter disc required a bit of creativity as seen below, where it's laying on its side between two of the rectifier caps.

I will glue all the caps to the board before I put the power supply into service.

New capacitors and diodes

I replaced all the electrolytic caps and diodes.  The diodes might have been ok but they are blocking over 300 volts each and a single failure would let the smoke out for sure.

Out with the old

In with the new
All the resistors except one 100k 2 watt were ok.  So I only replaced that resistor.  The 1.6kv disc caps even measured ok.

The internals look a bit different now with the new caps standing up where the old, larger axial lead components laid flat.

Ready for testing


After performing resistance and continuity checks I buttoned it up for the test.

The Noise

I had no idea how audibly noisy this power supply would be.  The switching that occurs in the transformer creates a very loud whine.  I understand why hams would install these in the trunk.  There's no way you'd be able to stand this for long if it was sitting next to your station.

I have a longish intro in the video.  Skip towards the end to hear it powered up... turn your volume down when you see the "hearing protection" sign come up in the video.


This was a good learning experience.  I learned about high voltage transformers and got some practice restoring older equipment.  I practiced electrical safety and didn't kill myself, so I'm pleased about that.

I now have a power supply I could use from a sturdy 12v source if I needed it, BUT due to the noise in operation I'm going to look into restoring a HP-23 which runs off house mains (AC) and is mostly silent. 

I'll keep this on the shelf waiting for a day that I need to run the HW-101 mobile. 

That's all for now

Richard AA4OO


  1. I really enjoyed exploring your site

  2. Hi Richard,

    This is a great re-build. I have an HP-13, and that whine is loud. I went a slightly less advised route. I powered mine up as-is, and ran the radio with it. It probably hasn't been used since my uncle used it in the 70s. Worked great, I figured I got lucky, and used it for about 2 hours with my SB-101, then the radio just faded out. All voltage outputs gone, only the 12V bias from the battery remained. High pitch wine gone.

    Something quit, don't know what. Should have re-built first. Opened it up, nothing except the 12V anywhere, and had a heck of a smell in there.

    Leads me to my question. How do you feel about selling your re-built HP13?
    I enjoy operating a portable station at family Sunday night bbqs, it would be fun to be able to do that with a 12V battery.

    A "no" is no problem for me, just figured it doesn't hurt to ask. I'll just re-build mine, if need be, however I find that my laziness told me to ask about yours first.

    1. you have the scematic, so i would first Check the 2 transistors and all capacitors, then the resistors ..then the diodes.
      you can do, it is just a matter of time and effort;)
      .... replace the transistors is the worst case, because Everything else is easy :)
      if you need to replace transistors you will face thome difficulties!
      you have to studdy the datasheet exactly!!!! and you have to m8dyfy circruit to match the new trans!!