RE: 30M Rig Problems

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From: Bill Meahan (wwm@wa8tzg.mi.org)
Date: Tue Aug 29 1995 - 20:08:30 EDT


Larry East writes:
>The action of the drive control and high SWR into your antenna lead me to
>believe that the final is oscillating -- and at some frequency well removed
>from 30M. Do you have access to a frequency counter and/or 'scope?
>
>72 and good luck,
>Larry W1HUE/7
>

This was just one of the responses I received after I posted my little
tale of woe about my OHR Sprint (30M) tagged on as an ObQRP addendum to
a brief missive about the new Radio Shack "probe-style oscilloscope."

Most of the suggestions were related to things I've already tried or
made certain highly reasonable (but incorrect) assumptions about my
setup. Herewith some additional detail:

1. The R7 was newly erected over Thanksgiving Weekend 1994. All
connecting points were cleaned with 600-grit emery paper, liberally
coated with NoOx (on BOTH the OD of the smaller tube and the ID of the
larger one, and scuffed through the NoOx layer to ensure good contact
with the aluminum. By New Years, when I finished the Sprint, I noted
the odd SWR behavior on my new Christmas-gift SWR meter. Despite
several attempts to adjust things, I could not get the SWR down. Even
using the Collins as the exciter gave >3:1 SWR readings. By later that
weekend, cold weather finally made it to Southeastern Michigan and I
gave up. During a February warm spell, I obtained a new 30M trap from
Cushcraft and replaced the suspected bad unit. Things had improved as
far as the KWM2 was concerned, but still no dice on the OHR.

2. My setup is as follows (as best I can do ASCII art):

  ---------------- \|/
  | OHR Sprint | Patch cable |
  | |--< <-------- -------------- R7 |
  | | | | SWR Meter | coax 50' |
  ---------------- ---| |--< <----------------
                                   | | ------------
  ---------------- -------------- <--|Dummy load |
  | KWM2A | | |
  | |--< -------------
  | |
  ----------------
  
To switch between the rigs, the rig end of the patch cable is
disconnected from one rig and moved to the other. No other connections
are disturbed.

3. Suspecting a bad final transistor, I replaced the 2N3553 with an
MRF-237 I had on hand, being careful to use the old trick of bending
the middle lead backward to the "correct" position so the E-B-C
sequence matches the board (used to hot-rod old TR-2200 2M rigs for
More Power, as Tim Allen would say). Since the final transistor sits
up on an insulated spacer, careful routing of the base lead lets
everything come together nicely. No shorts. (I checked).

4. Unscrewing the antenna feed line and attaching the dummy load in its
place, the OHR works like a champ and the SWR bridge reads 1:1.

5. I've sextuple-checked the continuity of the final filter, measured
voltages and retouched every solder joint in the transmitter portion of
the board. I've *carefully* examined the board for solder bridges or
other problems numerous times. All off-board wires have been routed
for minimum possibility of unwanted feedback.

Now, if I only had a scope and a spectrum analyzer at my disposal like
I did 5 years ago in a different job assignment.....

TNX ES 72!

--
Bill Meahan WA8TZG                   wmeahan@wa8tzg.mi.org
Member of:  ARRL, IMRA, NorCal QRP (#407), G-QRP (#8468), MI-QRP (#M1458)
Hey, this is my OWN computer!  I can say what I want!
        cat: a purr bearing mammal


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