38S Problem Resolved

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From: Jim Bennett (jbennett@ebmud.com)
Date: Sun Jan 26 1997 - 00:26:38 EST


Fellow NC38S builders:

Yesterday I posted a note to the group asking for assistance in
resolving
a problem I had with my NC38S after doing the 5 watt power mod. I got
quite a few replies, and I want to thank each and every one of you,
especially Ori, AC6AN and Paul, NA5N. As I promised Paul, I'm posting
the
result so that others don't do what I did to my rig.

The 5w mod calls for two mono caps @ .1mf (three caps if you are not
doing
the suggested additional mod from Mike, K1MG) plus some other parts. I
did
the mods as prescribed. After I powered it up, it received for less than
a
minute before blowing the fuse in my 13.8vdc supply. For a full
description see my prior post titled: "38S Need Help (long)".

The bottom line was that Mr. know-it-all (me) was out of .1mf caps and
picked up some at the store - but did I get "mono" caps? Nooooo. I
thought
tantalum caps would work just fine. I didn't realize that they are
electrolytic caps and have a plus side and a minus side. Hmmmm...what
WAS
that black stripe doing on one side of the cap? -:(

Evidently I got one of them in the circuit with its positive side where
it wasn't supposed to be. It lasted about 40 seconds and shorted
internally, providing me with a nice path for my 8vdc to ground. I was
lucky; no other components bit the dust - just the fuse and the cap.
Well,
actually I did totally mangle the IRF510 getting it off the board to
test
it!

So - I strongly suggest NOT using a tantalum capacitor where a mono cap
is called for. Unless you want to learn a lot more about your circuit
and how to spend lots of time de-soldering components.

My 38S is now on the air, full power, no more fuse blowing. Alright, now
I'll do the TiCK mod. What could possible go wrong..........?

 
*----------------------------------------------------*
| Jim Bennett / W6JHB |
| jbennett@ebmud.com |
| Martinez, CA |
| |
| NWQRP #431 QRP-l #596 ARRL Life Member |
| Dumb-ass of the year, 1997 |
*----------------------------------------------------*


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