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!