Thanks for the info shockwave.
I'm happy to have a place to share these things with others. When someone buys a radio with extra bells or whistles, we don't expect one of them to detract from the radios main function of sounding as good as possible.
Now that I see a schematic of this echo board, it's easy to spot its deficiencies. The first active part the audio goes through is an op amp that has a negative feedback loop designed to knock out treble. Then they have two more dedicated low pass filters deeper in the audio chain marked LPF1 and LPF2.
That pretty much rules out anything over 2500 cycles before rolloff is noticed in the high end. Then there are more than a dozen coupling caps to eat up the low end bass along the line. The circuit chops off the low end and the high end....but why?
It all comes down to the time base delay required for an echo and the quality of the parts used. The faster the time base or frequency is, the more bandwidth the circuit can pass and still provide a clean delay.
The issue is the faster the time base is, the more important it becomes to use higher quality parts or things like slew rates in op amps will be the weak link in the chain. Quality was sacrificed in favor of limiting the bandwidth.
The key thing to note in this circuit is the "ECHO ON" line through J7. It just turns 5 volts on or off to the mode control. When the echo is off, the mode control simply disables the time delay to stop any echo. All of the low pass filters and high pass coupling caps remain in the audio line!