Hi Gang,
Several folks have asked from my RIT mod circuit.... so I've
decided to use the bandwidth again to repost the circuit. I've
made one change to the original post... the base resistors are
now 10K instead of 2.2K.
Jerry KR5L
----------
From: At Ease Designs[SMTP:jerryh@webzone.net]
Sent: Tuesday, January 28, 1997 7:52 AM
To: 'Doug Hendricks'; 'qrp-l@Lehigh.EDU'
Subject: 38S RIT Mod /Chirp Remover (Long)
Hi Gang,
Several of you have asked for a schematic of the RIT mod I made
to my low power version of the 38S. Here are the symptoms
that this mod corrected in MY UNIT (your milage may vary) such
are the vagrancies of electronics. I was experiencing considerable
chirp at 300mw level into a 50 ohm dummy load. Also, I noticed
when you turned the RIT circuit on, it would shift the transmit
frequency by 1.5 khz --- not neat feature!!
I removed diodes D301 and D301 in the RIT circuit. and replaced
them with two 2N3904 transistor switches. The culprit (in my
opinion --- subject to the scrutiny of better minds) is the -R line
does not go all the way to 0 volts --- mine drops to 50milivolts.
The -T line however goes all the way down to 0 volts with some
RF hash riding on it. The 50 milivolt differential will cause the
RIT to tune about 1 - 2 khz when switched on.... causing the
shift in transmit frequency. With the RIT turned off, the -R and
-T lines are "wire ORed" together and the chirp is caused by
the small difference in the ground reference point seen by the
VXO circuit. Ori suggests using matched diodes for D301 and
D302 --- I felt the 50mv reference difference would be eliminated
or reduced by using saturated transistor switches....it worked
in my case.
Anyway here is the circuit ......
| -D301 Anode Hole | - D302 Anode Hole
| |
C C
2N3904 } B--- B--- } 2N3904
E | E |
| | | |
| 10 K | 10K
GROUND | GROUND |
| |
D302 D301
CATHODE HOLE CATHODE HOLE
(BANDED END) (BANDED END)
I made a little homebrew pc board and piggy backed it on top of
the D301 and D302 mounting holes. You will notice I reversed
the keying logic -R and -T because the 2N3904's invert the
logical sense of the original circuit. I hand picked my transistors
from my junk box by selecting ones with the highest hfe (gain)
the ones I used had gains in excess of 200. I just wanted to make
sure the transistors were driven to saturation.
This mod also works in the 5 watt version of the 38S.
Let me know if this mod helped. Also, if anyone has a better
(read easier) solution, please share it with the rest of us.
73's
Jerry Henshaw
KR5L / QRP
jerryh@webzone.net
ARCI 9165, QRP-L 847, NORCAL 1999
49er, ARK 20, Wilderness Sierra, (Soon 38 Special)
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]