18 9710 VCA
000216
2) With the controls still set as on the preceding page check the front panel ADSR LED which
should be cycling between fully on and fully off over a period of less than a second.
If the LED
is "stuck" in a dark or lit condition, quickly feel around on the circuit board to see if any parts
are hot or very warm. If you find any, disconnect the power and carefully examine the circuitry
around the hot part looking for solder bridges, backward capacitors or other polarized parts. If
nothing seemed warm but the LED is still not cycling between dark and light it may be a
problem in the Modulator circuit. Carefully check the modulator parts (Q9-Q15, IC7, panel
controls R110-R113 and associated components). Check the LED, it may be in backwards or
may be bad. It's human to suspect the ICs but most problems relate to the mechanics of
assembly - the wrong part in the wrong place or facing the wrong way or not soldered well.
3) Connect a signal source such as
a 1V to 5V p-p 300 Hz. triangle or sine
to the "R" input and an amplifier to the
"L+R" output. Rotate the Pan control
back and forth and observe that at "L"
the output is exclusively the white noise
source and at "R" you just hear the pitched
source. At intermediate settings you should
hear a mix of the two.
Problems here probably involve IC3, IC5, IC6,
Q1, Q2 or the other components associated with
the L and R VCAs. Patterns of failure will help
narrow the possibilities; e.g., if when panning from
L to R you hear the pitched source increase in
volume but hear no noise it means that at least
the R VCA is working OK. But you can infer more
than that - since sections of all the ICs mentioned are part of the working VCA and it's
unusual to have only a single section of an IC go bad (though it does happen) probably all of
the ICs mentioned above are functional. You can check the VCAs individually by listening to
the L and R outputs.
If you hear no noise it may be the noise source itself is not working. You can check this by
removing the amp input from the L+R output jack and inserting it VERY GENTLY into the "L"
input jack (just enough that the tip of the plug touches the Tip connection of the jack without
going in far enough to activate the switch that is part of the Jack). If the noise source is
working, you should be able the hear it here and if not focus your attention on the components
around Q5-Q7. If noise is OK, check on the components associated with the R VCA (see
schematic and Design Analysis sections starting on page 22 for more details) and particularly
the parts that are unique to that block, Q1 and D1 for example.
If one of the VCAs is on and the other off no matter the setting of the Pan control, focus
your attention on the connections to the Pan control R109, the common element.
4) Disconnect the amp from the L+R output and listen to the A output. You should hear the
noise/tone mix being modulated in volume following the changes in the ADSR output. An
equal mix of noise and pitched source modulated by a fast attack and somewhat slower
Release should sound like a snare drum.
