Tuesday, September 7, 2010

A problem with my Watson W-25AM power supply

My Watson W-25AM power supply has a serious problem. When I switch it on everything seems fine. The voltage needle reads 13.8v and the lights are on and everything seems fine and dandy (see photo below). The PSU will power my FT-2500M and my FT-897 while both are in receive mode.

However, everything goes awry when I key up on either radio. The voltage needle suddenly drops and when I release the microphone key it springs back up again but fluctuates for a few seconds before eventually returning to 13.8v position. This happens even when I turn the voltage knob on the PSU up full.

The more power I try to draw, the further down the voltage needle drops and, of course, I do not wish to damage my radios so I cannot keep the mic keyed. I tested it also with a 5w 2m handheld and the voltage didn't drop at all. But anything more (25w from the FT-2500M or 100w on HF or 6m on the FT-897) and the voltage drops like a lead balloon.

I have been told the problem might have been caused by modulation of my AC through RF from my longwire.

What I would like to know from the experienced hams and electronics experts out there is - should I find a deep hole to drop the PSU into, or can it be repaired? (Also, of course, can it be repaired cheaply and/or easily?!!)

If you have any advice that can help me, you can either comment on this blog post below or send me an email to hamradioireland (AT) gmail (DOT) com. Thanks.


  1. Hi Anthony, it looks like a heavy current draw is causing the voltage drop - A 100w radio draws about 20 amps on FM, what is the current draw on the supplies ammeter when you key up ?

  2. Paul, I was using SSB on HF and giving a whistle. This problem happens even with just one radio connected. The current draw varies according to the RF power selected on the radio. The PSU was working fine and one day my 2m FM rig started to switch off as I was talking and then I noticed the voltage dropping.

  3. Hi Anthony, this is either a RFI problem or you got a earthloop somewere. Did you connect everything with separate wires to your ground connection? So if you did, try to remove them one for one to see if it makes any difference. I did have the same problem you can read it here. Although I don't have meters on my supply my radio switched of at a certain amount of power (SSB). Good luck, 73, Bas

  4. Bas, the power supply is doing this in the shack of another ham who took it away to look at it. It is broken I think. It was not connected to the ground system. I have the FT-1000MP, FT-897 and the MFJ-941E tuner all connected to ground but they are connected to each other too. The Watson PSU was never connected to ground. I had it away from the shack last night and the problem was still happening.

  5. Ok Anthony, I hope the problem can be fixed. I don't know how this supply is regulated, but from the size I think it is not a switching one. So I think then one of the final transistors broke.
    I guess the other ham has experience with it. Good luck...
    73, Bas

  6. Hi check capacitor C6 on PCB board.
    It is connected to 5th and 6th leg of lm324. this part of op-amp is looking after current limit level.
    for test you can remove diode D3 but the current limiter will no longer work. Incrising value of this cap should sort your problem.
    Just in case don't test this with radio use car head lamp bulb as load.

  7. I am looking for the black screw thing that holds a wire on to the post of my W-25AM. Any idea where I can get one?


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