Here at Zerotronics, we've got a new sideline operation: modifying Realistic (Radio Shack) Stereo Reverb Systems. We've been acquiring them (on eBay, of course), modding them and reselling them (on eBay, of course). A person who bought one left some interesting feedback: "Super happy. Super, super happy. You will be too." Thanks, we appreciate it!
Technically, these things are not truly reverbs. They're really analog delay lines with feedback. The circuit uses an MN3007 BBD (bucket brigade delay) chip driven by an MN3101 clock chip; it provides up to 100 msec of delay time.
Here is the block diagram of the "reverb" before modification. The mono input is mixed into the left and right inputs. Then the left and right (plus mono) inputs are combined to form an input to the delay line. The output of the delay line is mixed (via the depth control) with the dry left and right (plus mono) inputs to form the wet (reverberated) left and right outputs. In addition, the delay line output is mixed back into the delay line input to provide a repeating and decaying echo. The feedback level is fixed at 55% (approximately 5 dB of attenuation). The stereo "expander" is just a switchable inverter that reverses the polarity of delayed signal that is mixed into the right output.
So, if you have one of these and you'd like it to be more useful, we'll be happy to perform the following modifications for the paltry sum of $50 plus shipping...
1) The three noisy 4558 op amps will be removed and replaced by socketed 4580's (the same op amps that are used in many modern mixers).
2a) The front panel "mic mixing" control (actually the gain control for the microphone preamp) will be replaced by a new gain pot located on the rear panel. The preamp has a very high input impedance and works well with an electric guitar; it can be pleasantly crunchy with the gain cranked up.
2b) The original front panel "mic mixing" control will be re-purposed to act as a decay (feedback) control that adjusts the amount of delayed signal that is fed back into the delay line. With the control at minimum (zero) there is no feedback and the output is just the delayed input. With the control at or near maximum, the unit is capable of breaking into nasty self-resonance.
3) Two small toggle switches will be installed on the rear panel that allow for selective muting of the dry left and right input signals that are normally mixed into the outputs. This allows the unit to be used as a true send/return effect, rather than only as an insert effect, as well as creating other possibilities. For example, by muting only the right channel and setting the decay control to zero, the right input and output can be connected through a volume or filter pedal to create an external feedback loop.
4) The original silver knobs will be replaced by dark soft-touch (rubberized) knobs with pointers that are easy to see in dim lighting. Transparent decals will be applied to the front panel and the rear edge of the top to indicate the new control functions.
All modifications are performed using genuine Radio Shack solder...