I have another stupid question

Discussion in '"Boat Anchor" & Classic Equipment' started by AA4OO, Mar 10, 2018.

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  1. AA4OO

    AA4OO XML Subscriber QRZ Page

    Ok, I know there are no stupid questions, only stupid people, but here goes...

    I rebuilt both a HP-23 and a HP-13 power supply for my Heathkit HW-101

    The HP-23 has 12vAC output for the filament while the HP-13 doesn't. I assume I'm just to use the 12vdc that powers the HP-13 also for the filament voltage. I looked at the schematic but I'm missing where that 12vAC is rectified to DC.

    How does it work with either 12vdc or 12vAC?
  2. N2EY

    N2EY Ham Member QRZ Page

    Tube heaters don't care if they are powered by DC or AC.

    Fun fact: Directly heated tubes have filaments. Indirectly heated tubes have heaters.

    All the tubes in the HW-101 are indirectly heated.

    73 de Jim, N2EY
    WA7PRC, N3AB and AA4OO like this.
  3. KE4OH

    KE4OH Ham Member QRZ Page

    The original purpose of the HP-13 was to power Heath transceivers installed in a motor vehicle. The heaters were supplied directly by the vehicle electrical system, nominally 12.6v. My guess is that you are planning to try out the HP-13 using a bench power supply rather than a car battery.
    N2EY likes this.
  4. AA4OO

    AA4OO XML Subscriber QRZ Page

    Late last year, while on Christmas break, I had too much time on my hands which is always a dangerous thing.

    I thought I'd like to learn about tube radios by getting a restoring a HW-101. I needed to power it first. I saw that it could be powered from either a HP-13 via 12v or HP-23 from mains. Not knowing any better, I thought I'd start with a HP-13 so I could run the HW-101 either in the shack or from a picnic table powered by a car battery. I wasn't aware of the 15-amp@12v receive requirement.

    So, I first got an old HP-13 and replaced all the electrolytic caps and out of spec resistors. Testing it with my bench 12v supply at 13.5v outputs the appropriate LV, HV and bias but makes a noise that belongs in a Gitmo torture chamber.

    Torture begins at the 4 minute mark...

    So, I then bought a HP-23B and restored that using all new components and a PCB from oldheathkitparts.com. I'm using the HP-23B with my HW-101 now, but I hate to see the HP-13 go to waste so I was planning on building an enclosure with long internal ducting for the cooling fan, to reduce the audible noise from the beast and have it as a backup. When I started wiring up the Jones plug to a chassis 11-pin Amphenol connector I got to wondering about the filament voltage being DC rather than AC as in the HP-23B which is why I asked the question

    That's the story.
    WA7PRC likes this.
  5. KE4OH

    KE4OH Ham Member QRZ Page

    Not sure that the HP-13 is a doorstop. They aren't exactly rare, but not that common either. I suspect that a good one would fetch a decent price on eBay or at a hamfest. Surely there are Heathkit boffins out there who want the mobile power supply.
    WA7PRC and N2EY like this.
  6. K6BSU

    K6BSU Ham Member QRZ Page

    The DC-DC converter apparently uses an oscillator frequency low enough to be in the audible range. The noise comes from the transformer actually vibrating (magnetostriction). A possible fix would be to remove the transformer and soak it in a can of varnish for a couple days. Dry it, and repeat the varnish. Chances are, that will lock the core and windings and reduce the noise.
    WA7PRC likes this.
  7. KX4OM

    KX4OM Ham Member QRZ Page

    I just watched the video. By any chance, did you remove the 2W 270-ohm resistors and check them (or replace them)? They could be aged high, which might have an effect on how the switching is balanced between the two transistors. Those resistors are one of the two differences I saw between the HP-13 and the HP-13A schematics. In the A, they were decreased to 210 ohms. I wonder if the intent was to move the switching frequency above the range of normal hearing. The other apparent difference is the 450v 20UF caps were replaced with 500v 20uF. I mentioned that once before. Mouser doesn't have them, but modern electrolytics are much better than ones of that generation, so the tighter tolerance may make them okay.

    I found my two HP-13s in the vast electronics storage closet in the basement yesterday. "Vast" because there is so much junque in it, not its physical dimensions. Somewhere there is a Galaxy mobile supply as well, because I found a cable for it.

    By the way, the June 1970 issue of Ham Radio has an article on testing and repairing switching power supplies showing the HP-13 as an example. The diagram is not that of the HP-13, however.

    Ted, KX4OM
    Last edited: Mar 13, 2018 at 10:39 PM
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  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    After my latest transformer rewind, I had some magnetostrictive noise. A local transformer shop was kind enough to vacuum-pressure impregnated it with a polyester resin (and then bake it) free gratis.

    Another idea is to increase the switching rate somewhat higher than the OE ≈1500 Hz. If necessary, use faster rectifiers.
  9. AA4OO

    AA4OO XML Subscriber QRZ Page

    The 270 ohm 2w tested ok so I left them. I could try 210 ohm resistors on those transistors but they may have changed the transistor part as well with the HP-13A. As far as I can tell from my research the transistors are a mystery part, so if you blow them your out of luck.

    I spoke to a local ham who used to run a HW-101 mobile with the HP-13 PS and he said it did make a heck of a lot of racket but he hasn't been over to sample mine, so I don't know if the noise from mine is typical. I work at place with hardware engineers (I'm a software guy) and one suggested that the power filter caps themselves may be resonating but I'm fairly certain the majority of the noise comes from the area around each transistor.
  10. AA4OO

    AA4OO XML Subscriber QRZ Page

    Here's a photo of the transformer... it appears to be well sealed (although dusty on top). I don't see any crazing in the epoxy... What do you think?

    Before component updates...

    After component updates

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