Re: MFJ-9020 Blows Fuse Over SWR?

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From: Gregory Lapin (lapin@casbah.acns.nwu.edu)
Date: Wed Mar 16 1994 - 15:12:34 EST


> My MFJ-9020 popped a fuse yesterday during normal operation and I
> wonder...
>
> 1. if anyone else has experienced similar behavior, and
>
> 2. does my hypothesis about the cause hold water?
>
> The 9020 is 3 weeks old. I had been chatting with a station in
> VA for about half an hour. At one point, just as I began to
> transmit, the internal PC trace fuse link blew. (It is nominally
> there to prevent damage in case of reversed power supply
> polarity. Clearly that was not the case here.) I repaired the
> fuse and everything works fine again.
>
> The antenna I'm using presents about 3:1 SWR in the 20m CW band.
> (Don't flame me about using an impure antenna. I'm temporarily
> using our club station antenna which is normally corrected by an
> internal antenna tuner on the club's TS-440S. My own antenna
> will be pristine at 14.060.)
>
> Here's my theory:
> The high SWR causes abnormally high power supply currents in the
> xcvr which caused the fuse to wear out after a dozen hours or so.
>
> If my theory has too many holes, I will have to search for an
> intermittent component failure as the cause. CORRECTION: MFJ
> will have to search for the failure; I will ask for a
> replacement.
>
> Any opinions? Please respond to me directly. I will post a
> summary of the replies to the responders if I get any.
>
> Regards,
>
> -- Mike, WB4ZKA
>
> ==============================================================
> Mike Pulley Intel Corporation
> Phoenix, Arizona (USA) Mike_J_Pulley@ccm.hf.intel.com
> Callsign: WB4ZKA
> QRP rig: MFJ-9020 20m CW, Ten-Tec PM2-B
> QRP ant: Butternut vertical at ground level
> QRP interests: Operating (WAS or bust!)
> ==============================================================

It is likely that the finals overheated due to the high SWR. As the
temperature went up, your current draw went up with it and eventually got
high enough to blow the fuse.

Greg Lapin KD9AZ
glapin@nwu.edu


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