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 Designator Old Value New Value Comment -------------------------------------------------------------------------- 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 Designator Old Value New Value Comment -------------------------------------------------------------------------- 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!