822 and 831 Builders & Users FAQ – Updated 2/14/02

Send me a question or comment on the 822 or 831 and it will end up here for
all to read. Of course, all names will be removed to protect the innocent

02/14/02 wire-to-rotary switch connection:

On both my 822s & 831s, I found that for me the far easiest way was to solder the short wires to the PCB first, then fix the switches to the panel, then solder the wires to the switches, using a meter along the way to check finally the PCB is coaxed into place and fixed on the bracket.


This is another method that P.B. finds works for him. My instructions show having the
PCB attached to the bracket first. But, I can see how this might be easier to get to all
the connectors on the rotary switch. Also, A.W. has a suggested method to connect the rotary switches before mounting anything. These tips and photos appear on the 822 and 831 construction photo page and are linked from the
news page.

02/07/02 calibration issue:

I did have one minor issue with the calibration. I was not able to get steps 10 and
11 up to the correct voltages with R12 at 2.8k. Even with the trimmers cranked all
the way up to the clicking point, the output voltage was always flat. Perhaps my 50
ohm trimmers were all a little under their optimum values (e.g. 45 ohms). I didn't
have a 2.7k 1% resistor so I cheated by putting 100k in parallel with 2.8k. That was
enough to shift the voltage ladder so that the trimmers were all near their middle
positions after completing calibration.
You are correct in your assumption that trimmer value tolerance is the culprit behind this
problem. I noticed this on the last two 822 modules that I built. And, your solution is
the correct one. I have now even documented this in the parts list. Anyone having
similar problems should change R12 to the next resistor size down in the 1% range.
That is 2.74K ohms. In fact, I recommend getting both 2.8K and 2.74 K resistors
when ordering parts in case your trimmer tolerance causes this problem.

01/29/01 Question

Some of the resistors have an "A" or "B" designation.
I guess I'm not real clear on the "correct" placement.
If you are building the 822 or 831 with matched resistors from me (I think
you all are), then all resistors go in the "B" positions. The completed 822
will have NOTHING installed at R21A and R27A. The completed 831 will
have NOTHING installed at R14A, R25A, and R36A.

When you install these resistors in the "B" positions, you will notice you
are going over the top of where the trimmers would have been installed.
These can be built without hand matching resistors by installing trimmers
and putting the resistors in the "A" positions. If you follow the traces
you can see that the "A" position would have been in series with the
trimmer. Since I thought the PCB supply might outlive my desire to hand
match resistors, it was made with both construction options. So, your "B"
resistors all end up with two holes under them with nothing in them.

Now, look onto these empty resistor positions with JOY. Because..... later
we will have some DIY development schematics for cool add on stuff that
will use these resistor positions. Stay tuned, but not real soon.

02/05/01 Question

If I want to use my 822 for a quick key change, do I need one channel
in my 822 for each oscillator I am using?
No. Each channel of the 822 is capable of driving 4 oscillators at once
without difficulty. You could use one channel only for key change for all
oscillators controlled by the same keyboard CV. The other channel of
your 822 can still be used for interval / octave shifting.

Here is a typical patch to do that.
Keyboard CV out to 822 A IN.
822 A OUT to osc1 CV and to 822 B IN (using a mult)
822 B OUT to osc2 CV

In this example, the A channel of your 822 provides key shift for both
oscillators while the B channel provides interval and octave switching
to oscillator 2 (relative to osc # 1) and within the key selected by the
822 channel A.

02/05/01 Question:

If I am using my 822 or 831 with other CV processors like the MOTM-
820 VC Lag, does it matter which comes first / last in the CV source?
Yes. For reasons outlined by Paul, the 820 VC Lag should always
be the first module connected from your MIDI-to-CV or other CV
source. I recommend that the 822 or 831 be the last module prior
to connecting to your VCO VC input. And remember, anything you
put in your keyboard CV line must have the high accuracy circuitry
found in the 822, 831 and modules like the MOTM-820.

02/09/01 Question:

I came upon soldering "via holes" associated with the 831
switches. Would you elaborate on what you are referring to?

Soldering the via holes is NOT required. But, is generally accepted
as "good practice."

Via holes are places in the PCB where a connection goes from top to bottom
through a hole and there is no component located there. They are easier to
see from the bottom, but might be easier to solder from the top because of
some close ground plane tolerances on the bottom of the 831 PCB..

You will find them located between the locations where the traces run along
the board so that the A, B and C switches all get their voltages from the
same resistor / trimmer divider. You will see these holes. Notice they are
smaller than other component holes and have no markings for anything to
attach. If you look at both sides of the board, you can see how the traces
attach traces on the bottom to traces on the top through what are called
"plated thru" holes. If you fill these holes with solder, just as if you
were soldering a component there, you will strengthen this connection.
there are a couple of ground plane via holes too. I would not worry about
them. You can solder them, but they require a lot of heat because of the
larger surface are of the ground plane trace.

03/10/01 Question:

I just realized my 822 & 831 don't fit in my cabinet.
[Is there] a part number for a slimline replacement switch?

There is a lengthy discussion of this subject here
The slimline part numbers for the switches that you need are there. The
only switches I found to work are the ones made my Electroswitch. For the
831 you want the "adjustable stop" (-A) version. For the 822 the full rotation
(-C) version is best. However, it is hard to find. Dave Hylander wrote some
very fine instructions on how to modify both the standard and slimline switches
for full rotation. You will find those modification instructions also at the above

back to main projects page