Subject: Improving Audio in the PRO-2005/6 Scanners
Tom Wheeler, N0GSG
Johnson County Radio Amateurs Club
The Realistic PRO-2005/6 scanners are considered by listeners to be
one of the finest scanning receivers available. These units cover
25-1300 MHz in AM, narrow-FM, and wide-FM modes with only a single
break in coverage from 520-760 MHz. All this was packaged for under $400,
(and under $300 when they were finally discontinued by Radio Shack.)
There is one major fault with these receivers:The audio fidelity
is very poor. The designers did a great job in the RF sections of
the receiver, but must have slept through the audio design!
Scanners are not supposed to be hi-fi receivers; however, their audio
performance should be at least as good as a common amateur transceiver.
The author compared the audio of the PRO-2006 with that of the Kenwood
TM-2550 transceiver. Even when the '2006 was connected to a good-
quality external speaker, the audio was miserable. The culprit:Several
coupling and bypass capacitors in the audio section have been mis-
calculated by the designers.
Making the component changes suggested below will dramatically increase
the audio quality of the PRO-2005 and 2006 scanners. These two units
are virtually identical (The main difference is that the LCD backlight
can not be dimmed on the '2006. It can only be be turned ON or OFF.)
Be aware that alterations to your scanner might void any warranty left,
and if not carefully done, can be hazardous to its health. Any work
on these units should be done at a static-free workstation, if possible.
THE AUTHOR ASSUMES NO LIABILITY FOR THE INFORMATION CONTAINED HEREIN!
All the suggested changes are on the left-rear corner of the LINEAR circuit
board, which is located on top of the scanner. These changes can be made
without removing the board, but it is much easier to remove it. Be careful
when re-installing the board; there is a hidden connector on the bottom
of the LINEAR board that must be lined up with the PLL board underneath.
The following changes will improve the speaker and headphone audio. If
you're using an external speaker, you'll be very pleasantly surprised by
the results:
Component DesignatorOld ValueNew ValueComment
--------------------------------------------------------------------------
C226 100 uF 1000 uF Helps bass response
C211 6.8 nF 0.1 uF " "
C206 47 nF 0.47 uF " "
C207 56 nF 0.47 uF " "
C204 10 nF 4.7 uF RF/AF bypass. Original
part is surface-mounted on
PCB. DO NOT REMOVE ORIGINAL.
New C204 is soldered on TOP
of IC5. Watch polarity:
Cap (+) to pin 3 of IC5
Cap (-) to pin 11 of IC5
C208 4.7 nF 0 uF REMOVE C208. This helps to
correct faulty deemphasis
in WFM mode. (WFM will sound
a lot crisper after this
change.)
These changes modify the buffer amplifier that drives the TAPE OUT jack.
If you don't use this jack, these changes may be omitted. Even if you
don't do them, TAPE OUT quality will dramatically improve:
Component DesignatorOld ValueNew ValueComment
--------------------------------------------------------------------------
C212 6.8 nF 1 uF Helps bass response. Watch
polarity. (+) lead goes
AWAY from Q23 base lead. Q23
is surface-mounted.
C213 47 nF 4.7 uF Helps bass response. Watch
polarity. (+) lead goes
towards Q23 collector.
Most of the changes noted above correct weak coupling between stages at
low audio frequencies. Many of the original values produced audio that
rolled off sharply below 1 KHz -- perhaps they were afraid of amplifying
hum and wanted to mask it. The new values will provide fairly "flat"
audio response down to 100 Hz, with no trace of hum or switching thumps.
In addition, PL tones (which are in the range of 50 - 250 Hz) now pass
quite easily through the audio section, which will help those of us who are
using PL decoders and displays with this unit. In particular, AM sounds
much better with the improved low-end response; every beat-note produced by
interfering signals in the 11-meter band is clearly audible!
Radio Shack created a real winner in the PRO-2005/6 receivers. Carefully
performed, these modifications will unleash its potential audio performance.
Happy listening!
SOURCE: The QRZ Windows Ham Radio CDROM |
