WARNING - If you blow anything up, its your fault, even if we told you to
do it.
This article was found, in a donatory sort of way, by Gunnar Backman
whose whereabouts (for the sake of this article anyway)
are currently confidential. Not all the info you need to safely carry
out suggested tasks is included as this would take up inordinate space.
We are presuming you know all about Static Safety Precautions, RHoS compliance, and the
WEE directive.
Even so we strongly discourage the wearing of nylon
clothes when using a soldering iron. Do not play the guitar in the bath.
VG88
Picture & mod info courtesy Gunnar Backman
RMC VG-8 INPUT FILTER MODIFICATION
This VG-8 modification is designed to remove the subsonic
(very low frequency) content of the separate string signals
(produced primarily by piezo hex pickups). This helps the VG-8
perform more cleanly and without puffyness, especially when using
the detuning (pitch shifting) functions. There aren't any negative
side effects to this modification. Everything works the way it should,
only better. Subsonic filtering is like what you may have in your stereo
for the turntable preamp. It removes non-musical, mechanical vibrations
that are detected along with the desired sounds and relieves the signal
processing circuitry of a lot of useless, and sometimes unwanted, work.
The result is cleaner attack transients. This modification consists of
changing capacitor values in the analog "front-end" of the VG-8,
which attenuates subsonic frequencies which can otherwise enter freely
and upset the performance of the overdrive and pitch shifter in some cases.
By changing 12 electrolytic capacitors in the analog front end,
frequencies 25Hz and below are significantly reduced and the
low-frequency saturation symptoms go away. This can be accomplished
by a competent electronic repair technician. The VG-8 modification
consists of changing these electronic components and testing the
modified unit for proper function. - RMC / Richard McClish.
RMC don't perform this modification any more. We simply provide the
recipe. Please find it below. The VG-88 has similar circuitry and can be
modified in a similar manner. The component numbers are different
however, we are trying to locate this info, so be extra careful
if attempting this from scratch.
There are 12 capacitors to change in order to reduce the subsonic
sensitivity of the VG-8 and VG-88. This helps to significantly reduce
the objectionable thumping and/or momentary distortion which may occur
in the amplified sound when the string tension and/or the pickup to
string distance is changed abruptly.
The same problem typically arises with a GK-2 in proximity of a tremolo
bridge. Because the bridge moves up & down with changes in string
tension as you move the tremolo arm, this produces objectionable
subsonic signal components (which can be much larger than the useful
music signals) and adversely influence the A/D conversion in the VG-8.
The mod is a simple capacitor substitution which is of no consequence in
the audio spectrum. It simply helps to maintain the A/D converter in its
linear mode of operation.
The following caps refer to the VG8 only:
1) Replace C25, C29, C33, C37, C41 & C45 (all 10uF) with 0.22uF (metal
film, electrolytic or ceramic) capacitors.
2) Replace C201, C202, C203, C204, C205 & C206 (all 10uF) with 0.33uF
(metal film, electrolytic or ceramic) capacitors.
The Roland p.c.board has legend information which makes locating the
components easy. The work takes usually less than an hour to perform.
The parts are worth less than US$10.
You want capacitors somewhere between 16V and 50V (it's mostly a size
issue) and you want radial caps (with both leads at the same end like
the originals, since these capacitors are standing vertically on the
board).
You have a choice of either a standard aluminum electrolytic or a
tantalum electrolytic cap. They both work well. If you just look for
something small, it should work fine. This is not a critical application
and the portion of sound you're filtering is not in the audio spectrum.
Let a competent electronic technician handle the capacitor selection if
you have a problem deciding."
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