[sdiy] Analog drum machine start/stop

[Ben Bradley]

After a few minutes looking at that schematic, the pop on start/stop
is definitely a design issue. Under the voltage measurement of 11VDC
is the label "to Q14" that connects on the next page to "FROM
FLIP-FLOP, IC1" at the top of a 22k resistor. This provides the source
voltage for the output amplifier stage, so it's sure going to pop when
the run/stop flipflop is toggled. This appears to make sure the output
is dead quiet in stop mode, but it hardly seems worth the pops.

Disconnect that connection, and connect the top of that 22k to the
"+12VDC" further down that page at the junction of the transistor
emitter, 100uF and 10k. That should do it.

On Fri, Jun 30, 2017 at 7:46 PM, Nicholas Keller <nirokeforums at gmail.com> wrote:
> I've managed to fix my friend's Electro-Harmonix Rhythm-12 preset drum
> machine, and had planned to pack it up to ship back to him, but while
> testing I notice a loud "pop" in the audio out when I start and stop the
> unit.
>
> The original problem with it when it arrived was that the Zener diode and
> transistor in the power circuit kept exploding, and the tropical fish caps
> were crumbling.  Turned out that whoever worked on it for him on the past
> (which he didn't mention until well into my troubleshooting and confusion)
> had installed an EBC transistor rather than BCE.  It didn't help that I
> didn't have a schematic, but I should have put 2 and 2 together from the PCB
> traces.  Eventually, I found this schematic (see below) of a later revision
> of the machine where the transistors were all changed to EBC and the PCB was
> modified accordingly, I presume.
>
> http://www-personal.umich.edu/~damont/ElectroHarmonixRhythm12.pdf
>
> I replaced all tropical fish caps and electrolytics.
>
> While testing after these repairs, I noticed that the tempo would
> occasionally slow.  I cleaned the tempo pot and this seems to have stopped
> that issue.
>
> The last "problem" I'd like to sort out is this pop in the audio out.  I
> don't know if it is a by-product of the circuit design or from a fried part
> from the power issues it had. Maybe someone can look at that schematic and
> make some suggestions. The start/stop function as designed is performed by
> touching the grounded case and the socket's ring/sleeve with your finger in
> order to bridge the connection.  You can also use the footswitch control
> input in the same S-Trig manner with the same result (except you ground the
> tip instead).  Starting and stopping the machine causes this loud pop.  I
> can't imagine anyone would enjoy hearing this, especially over a PA system.
>
> Can anyone see a part that should be preventing this pop, or can anyone
> suggest a modification that would remove the pop?
>
> Thank you so much!
>
> Nick
>
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