On 12/29/07, prophei put forth:
>--- In cgs_synth@yahoogroups.com, "prophei" <prophei@...> wrote:
> >
> > i still can't figure this out. ugh. i am certifiably baffled by
>this one. :)
> >
>
>
>anybody have any other idea's? still no cv activity at all. sound
>just goes straight through... no
>vca action, no changes with cv... etc
I haven't got around to building mine yet. I got all the parts
including a nice MOTM-format panel from Bridechamber, but I've been
way too busy.
Look on the bright side, if the signal goes straight through, at
least you know the audio path works :)
Although maybe it's working too well. Make sure that you didn't
bypass the tube circuit in some way by accidently connecting
something or using too much solder. With power off, check the
resistance between pins 5 of the TL072 with and the audio input
circuit, and check the resistance between pin 6 of the TL072 and the
audio input circuit, making sure you didn't accidently connect the
output amp to the input amp or rest of the input circuit.
If the audio path seems fine, then check the CV circuit. With the
power on, the pots turned all the way down, and no signal at any of
the inputs, check the voltage at pin 5 of the tube. If it shows a
significant voltage I would measure the voltages around the CV amp.
Check the power pins of the LM358.
If there is a significant voltage at pin 7, then check the output of
the the trimpot and gain pots -- make sure they are at zero. If
there is still a voltage at pin 7, turn the power off, and check the
resistance between pins 7 and 6. It should be the value of the
feedback resistor. If the resistance is near zero, check for a short
and measure the cap if you have a capacitance meter. If it is near
infinity, check for a bad solder connection. Check that there is no
significant resistance between pin 5 and the ground on the power
connector for the module.
If there is no voltage at pin 7, but a voltage at pin 1, turn the
power off, and check the resistance between pins 1 and 2. It should
be the value of the feedback resistor. If it is near zero, check for
a short and measure the cap if you have a capacitance meter. If it
is near infinity, check for a bad solder connection. Check that
there is no significant resistance between pin 3 and the ground on
the power connector for the module.
Check the CV filter. Check all the connections. Check the cap.
Measure the resistance from pin 7 to pin 2. If the inverting input
(pin 2) of second amp is disconnected or shorted to ground, you won't
be able to adjust its output with the trimpot.
Check to see that the trimpot is working and that you can adjust a
negative voltage at its 100K resistor.
When the CV circuit is working properly, changing the value of the
trimpot should directly change the voltage output of each amp (pin 1
will go down as pin 7 goes up), and therefore the voltage at pin 5 of
the tube. If this does not work, check the CV circuit again. (Be
careful with the amount of trim, as each inverting amp has a gain of
around 2, too much negative voltage from the trimpot will clip the
amp. I'm not sure why its connected directly to the negative rail.)
If you are getting a negative voltage at pin 1 of the LM358, but not
pin 5 of the tube, check the connections. With the power off, does
the resistance between pin 1 of the LM358 and pin 5 of the tube
measure 100K?? Is pin 5 shorted to ground??
If that works, and you can adjust the voltage at pin 5 of the tube,
and adjusting the trimpot (with no external CV input and the gain pot
all the way down) resulting in a negative voltage at pin 5 of the
tube that doesn't mute the audio, then turn the the power off and
check all the connections around the tube with an ohmmeter, going
from the tube itself to legs of all the components connected to it.
Measure the resistance between the plate and the positive power
connector for the module, and between the cathode (pin 2) and the
negative power connector. Since in this particular design, the
cathode is connected to a negative voltage, the tube may conduct if
either of the grids are floating (as they would be positive in
relation to the cathode). Is there any resistance between the
cathode (pin 2) and the suppression grid (pin 4)?? With the power
on, but no signal, what is the DC bias at pin 6?? What is the
voltage at the suppression grid??
I hope this helps :)