[BC] Harris SX-1 Spur Problem

[Dan Dickey]

I agree with the previous suggestion about the loose terminals at the fuse 
holders.  Either the clips are not making good contact with the fuses 
(corrosion?) or the wires connecting to the fuse holders are not tight and 
corrosion free.

I don't have a schematic of this circuit handy and I don't specifically 
remember that I put two fuses in series but if I did it is probably for the 
following reason.  One fuse is a lower current slow reacting fuse.  The 
other is a higher current fast acting fuse.  This arrangement has the 
advantage of blowing quickly under high current loads without compromizing 
the lower current opening value.  If you just use a fast acting 4A fuse then 
it will have more nuisance openings.  If you just use a 10A fast fuse it 
will not blow at the lower 4A current that we wanted.   The arrangement you 
describe should protect semiconductors well without nuisance openings.  In 
your case that seems not to be working so I would look at the connections to 
find the source of the extra heat.

Best regards,
Dan

> Well, I've asked the question before Dan, but you're a new player in the
> game of SX transmitter questions!  The SX-5 I deal with blows fuses. 
> There
> are two fuses in series between the PDM amps and the PDM filter board---  
> one
> is mounted on the pull up board and is a 3AG 4 amp fuse, and the other is 
> a
> 10 amp KTK cartridge fuse, mounted in a holder on the front or back of the
> side panel.  The 10 amp fuses blow, but rarely the 4 amp fuses that are in
> series with the 10 amp fuse.  The PDM output levels are all real close to
> each other, and we've checked all the other operating parameters and 
> talked
> with Harris Field Service, to no avail.  The fuses that blow are always
> different--- sometimes on one PDM board, sometimes on another.  If you 
> shut
> down the transmitter and feel the 10 amp fuse holders some will be warmer
> than others--- sometimes almost too hot to touch.  Replacing the fuse 
> cures
> the problem for a month or so, until another 10 amp fuse blows.  I 
> wondered
> if the problem wasn't just heat that melted the fuse element so I rigged a
> muffin fan inside the back door over the power transformer to blow more 
> air
> on the holders mounted in back, and none of those have blown so far.
> So, why are there two fuses in series, and what's going on?  All other
> operating parameters of the transmitter, including the output as viewed on 
> a
> spectrum analyzer, look OK.  All power supply caps are new.
>
>
>
> Steve Brown
> Radio Rangers
> Minneapolis
>
>
>
>
>
>
>
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