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Dummy Load for Astron RS35-M

Discussion in 'Radio Circuits, Repair & Performance' started by KD3LT, Jul 26, 2020.

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  1. KD3LT

    KD3LT Ham Member QRZ Page

    Blew mine up a while back: no DC output, line fuse blown. Since I have no workbench here, I took it to an electronics repair place, and sent him links to the Astron repair and schematics site.

    He found one of the output transistors shorted and replaced it, and then replaced the IC723 regulator, and pronounced it fixed.

    Sure enough, it has idle power back, but when I turned my TS-430 to transmit (into a dummy load) and slowly cranked up the carrier, the TS-430 went crazy. Looked like the current folded back on the Astron, too.

    I checked the TS-430 with my back-up Astron and it's fine: power cranks up smoothly to where it should be.

    So I need a variable DC dummy load for the Astron so that I can load it incrementally without risking my equipment.

    Would a string of 60 watt incandescents do the trick?
  2. WB7OKU

    WB7OKU Ham Member QRZ Page

    You may not need a dummy load. I killed my Astron last week. A coax connector shorted the transistor case to ground. I replaced both transistors with Ebay specials. False economy. The power supply did the same thing yours is doing when I keyed the transceiver. Turns out one of the transistors was defective. I got new ones from Astron directly. It is working great right now. So I suggest that you pull the output transistors and check them. Both of the originals were probably bad and should have been replaced.
    K0UO likes this.
  3. KD3LT

    KD3LT Ham Member QRZ Page

    Thanks. Glad to hear somebody’s been down this path—although my RS35 has four output transistors rather than two. But in any event, I seem to recall reading on the Astron repair website that the output transistors should all be replaced at the same time rather than piecemeal, which is what my guy did.
  4. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Household 60W incandescents are designed for 120V, and may not be so suitable for 13.8V. It won't hurt to try, but it may take many in parallel to push a 35A supply toward its limit.

    Automotive incandescents are well-suited. Old sealed beams are good, and not so expensive. Be careful with small, bright halogen bulbs, though. They can explode if contaminated with a little bit of oil from fingers, so they shouldn't be used without a protective housing.

    Power resistors are another option. Submerge them in mineral oil for more power-handling capacity.

    Whatever you use, put several small loads in parallel, switching components in and out to vary the load.
  5. W9GB

    W9GB Ham Member QRZ Page

    120 VAC incandescent bulbs, NO.

    13.8 Volts is Astron Power Supply Output, SO You use Automotive Headlights (12-14 Volts).
    K0UO likes this.
  6. WA0YDE

    WA0YDE Ham Member QRZ Page

    What I use is a cheap Harbor Freight inverter. Connect the output to one or more 120v lamp sockets. Use bulb(s) to vary the supply current drain. Auto bulbs work OK, but you have to add them in parallel gradually for high current drain.
    K0UO, KB0MNM and N3PM like this.
  7. W9WQA

    W9WQA Ham Member QRZ Page

    old headlites usually have one good filament left. save them...
    WA9SVD likes this.
  8. G4COE

    G4COE Ham Member QRZ Page

    Avoid halogen lamps....! They have a very low 'cold resistance' and will trip *some* power supplies there is a knack round this but not worth it... the trick is to insert a low value resistance then short it out after you power up - I ran a bank of 12V 35 Watters, too darn dangerous due to blinding light and the heat, so what to do...?

    Well I thought of the kettle, they get peevish when there's no water in, so I got a bunch of low value resistors, twisted em' all together and pop em' in a pan of cold water my home brew power supply drew 35 Amps for a good 5 minutes.... by the time the water was boiling.

    Try this with no water they it'll pop like a flash gun, it's surprising what a wire wound resistor will take when cooled. To replace my halogen light bank I got a 10 Watt ceramic resistor.... think it was 150 ohms or something like that then I stripped some low value resistors down for their wire, twisted about four or five strands together then wrapped them round the high value resistor and used screw terminals out of a electrical connector strip - we call the chocolate blocks this side of the pond. I didn't bother to calculate the resistance required, I found it by trial until I reached the required current, the dummy load has seen many a power supply and still working strong.

    Far, far better and safer than a bank of 6 12V 35 watt halogens..... and cooler, no need for a preheat either.

    Do note:
    Many a commercial power supplies are not built to run full output continuously.

  9. KA5IPF

    KA5IPF Ham Member QRZ Page

    DC Dummy Load. Anybody interested? IMG_3241 (Medium).jpeg IMG_3242 (Medium).jpeg IMG_3243 (Medium).jpeg
    KB0MNM and KA9JLM like this.
  10. KD3LT

    KD3LT Ham Member QRZ Page

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