> Why it was soldered,
Apparently they forgot it in the original circuit, thus it did not make
it onto the PCB...
> and do i need to unsolder?
No.
What does it do?
IC20 (an LM13600, again...) is the (final) VCA before the signal leaves
the KLM-368 module. It performes the overall VCA modulation (tremolo
effect) by the MG. C82 together with R186, R87 form a power-on mute
circuit: when the power is switched on and the +15V rail becomes active
C82 still is discharged and the right side of R186 is pulled to +15V.
R186/R187 form a voltage divider setting the base of dual transistor
Q24 to ca. +0.65V. At the same time similar things happen at the other
half of Q24: the base is held at 0V by discharged C81. The common
emitter is held at ca. +0.6-0.7 thus cutting off the collector current
of the right side half of Q24, which in turn sets the gain of IC20B to
a minimum, switching off IC20B output. C81 is charged to a voltage set
by the programmed volume ("ATTENUATOR") via R188 and R189 and R195. It
adds some lag to changes in the programmed volume, which is hardly
noticable. After C82 is fully charged the voltage at the right side
base of Q24 is held at 0V by R187. Thus the gain of the VCA built from
IC20B and IC21A is controlled by the programmed volume and the VCA Mod
signal (LFO wave).
So what the use of this unnamed diode?
When power is switched off C82 is still fully charged to 15V. The +15V
rail drops to 0V (the buffer caps in the P/S are drained by all
circuitry which are connected to it). So the right hand side of C82 is
at 0V but at the same time is still is highly charged since R186, R87,
and Q24 are a rather light load. This sets the left hand side of C82 to
about -15V. Which at least turns Q24 fully on but even worse may damage
Q24 and/or IC20. The diode clamps this negative voltage to a harmless
value of max. -0.7V.
So this diode is a protective measure which is necessary for the moments
after power is switched off.
Johannes
Sorry for the long post ..
On 22.11.2013 11:42, iorlovs@yahoo.com wrote:
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> At the top right corner of KLM368a board i have found soldered diod
> (/∗image∗/ a/∗ttached∗/). Why it was soldered, and do i need to unsolder?
>
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>
> http://groups.yahoo.com/neo/groups/PolySix/photos/albums/1404048030/lightbox/737952942
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