Let's Try This...
As written by KC9BZX, and grossly plagiarized by me from mods.dk;
The Radio Shack Pro-97 headphone jack only allows approximately 30% of the receivers audio through it, to protect your hearing when using an earpiece. This however, makes it impossible to use an eternal speaker with the unit, as the audio signal is greatly reduced through the headphone jack and may sound distorted when the volume is turned up.
To obtain crisp, clean, loud audio using an extension speaker, follow these simple directions:
Remove batery cover and battery case from unit.
Remove 4 black screws from back of radio case and remove entire cover casing from back of radio.
Gently remove main circuit board by gently pulling upward until the board "unplugs" from front board.
Turn board over, board will remain attached to front by black and white speaker wire. For simplicity, this can also be unplugged.
Create a jumper using a small bead of solder between the small metal tab on the headphone jack and the grounded plate above it.
Reassemble radio.
This will give you full audio from the headphone jack, permitting you to use a external speaker on the unit.
NOTE: Be cautious when using an earpiece after performing this modification, as the modification will now permit the full amount of audio to be transmitted to the earpiece, possible harming your hearing.
This mod works on the PSR-500, and probably the PSR-300 and PRO-164 as well. Of course, you can probably kiss the warranty goodbye. This mod is done at your own risk. Be careful!