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Polyphony Magazine Cover - July - August 1981

Modifying the GR-500 Guitar Synthesizer

by Gary Cowtan

Polyphony Magazine, March - April 1981


This article will describe how GR-500 owners can add a number of modifications to improve the straight guitar sound. The main problem seems to be the humbucking pickups, which being low impedance, don't have the characteristic resonance (2 - 3 KHz) of a higher impedance Gibson or DiMarzio pickups. Due to the low output level of this pickup, there is a preamp built into the guitar body under the rear cover. The schematic below shows the GR-500's standard circuit:

The IC is one half of a 4558 (dual 741) op amp. I replaced the standard humbucker with a DiMarzio P.A.F. which I modified with a coil tap. The high output from this pickup over loaded the preamp, so I changed the 330k resistor to 47k to prevent distortion. You can leave the 100 pF capacitor as is, since this rolls of the gain starting at about 30 KHz - way above the response of the pickup.

I next removed the so-called 3-way "EQ" switch and replaced it with a DPDT switch for the pickup coil tap. I then replaced the .047 uF capacitor connected to the tone control pot (which is 500k, so it can be left alone without having to worry about loading down the high impedance pickup) with a .022 uF type, which gave a more useful range with the P.A.F. pickup. With regards to the electronics, I removed the 4558 preamp IC located nearest the end of the instrument, soldered in an 8 pin socket, and plugged in a TL072, which is pin compatible (Nationals LF153 will also work - Ed.). I used a socket, because IC technology is changing so fast that next year there might be something better available and I would want to be able to try it out without doing any more soldering. I had considered changing all seven 4558s in the guitar to TL072s, but my local supplier ran out of them and won't get any more for six months!

It is also a good idea to add J separate output jack for the guitar pickup. When holding the guitar in a playing position, there is room for this jack just above the multipin connector. Use a closed circuit type to switch the active circuitry out of the signal path when a plug is inserted. This facilitates using an electronic tuner backstage, without having to take the synthesizer console into the dressing room - roadies don't seem to dig that. It also, of course, allows you to plug the guitar into an amplifier in the conventional manner. This will, however, mean rewiring the volume control (for the guitar sound) unless you use a foot pedal volume control, as I do. Normally, the guitar volume control is wired after the preamp; for this application, it would need to be connected between the humbucker and tone pot.

Another tip involves the triggering response of the synthesizer section, since it can be adjusted more than Roland's instruction book would have you believe. You may find, as I did, that after setting the height of the hex synthesizer pickup and the threshold control on the console, different strings still trigger differently. Well, alongside each of the aforementioned 4558 ICs is a trimmer the sets the triggering threshold of each individual string. The IC closest to the neck is for the low E string, with the next one down for the A string, then the D string, and so on. One word of warning, if you don't stick to one brand and gauge of strings, you will be forever resetting these pots! Speaking of strings, many of the triggering problems associated with guitar synthesizers are due to dead strings: they must be changed VERY often. Otherwise, the hex pickup has a hard time trying to determine what note is actually being played; also, an adequate signal level leads to better sustain characteristics.

Two more tips, more of a mechanical nature, but which aid the triggering and the straight guitar tone, replace the bridge with an all brass, heavy-duty construction replacement such as the Might Mite Tunamatic, and replace the nut with a brass type as well. This gives the instrument itself more sustain, and makes life easier for the pitch to voltage converter and envelope generators. I have found that Gibson equa strings (.O09 - .038) give a good uniform response but you have to change them often. (note: do NOT do this if you want to use the 'infinite sustain', this change will burn the circuit out).

Finally, a few words about the synthesizer sound. Basically, the GR-500 is a one-oscillator synthesizer, which can tend to sound rather sterile. You can, however, mix the "Solo Melody" with the "Polyensemble" (which is really a hex-fuzz) and de-tune the former slightly, to give a fatter sound. If you then run the whole thing through a stereo setup with a stereo chorus unit such as the Roland "Dimension D" or a stereo flanger such as the Dynacord TAN 19 (pricey, but good); it livens up your sound no end!

I hope these tips help you get more enjoyment out of the GR-500.

Copyright March - April 1981 Polyphony Magazine

View the original article Polyphony Magazine - March - April 1981