MOTM

--- In motm@y..., "Paul Schreiber" <synth1@a...> wrote:
> I offer replacement pots (both Spectrol and Bourns) for $6.50ea.
> And, if you need a manual for a purchased module, send me an
> email.  [...]

Some of you are probably confused by this response and are thinking, 
buy replacement pots? Huh? And as you might expect, there is a little 
more to this story!

Right after posting the "why not just use one brand" question, I 
called Paul expecting to ask a completely different question.  
However, "Spectrol vs Bourns" still seemed to be on his mind - so here 
is my summary of potentiometers 101:

There are essentially two base technologies: cermet and conductive 
plastic.  (This refers to the type of electrically resistive element 
used inside the potentiometer.)  Conductive plastic has the longest 
life so, all else being equal, it is the technology of choice!  
However, all else is almost never equal.  Cermet offers significantly 
lower electrical noise (Paul claims 10x) and, as such, it is preferred 
in a few critical applications such as tuning the frequency of an 
oscillator or filter.  (But you already knew this based on other posts 
to this thread.)


So, why choose one *brand* over the other?  Here are the details:

Spectrol offers conductive plastic elements (148 series) and cermet 
elements (149 series) in an otherwise equal form factor.  These 
Spectrol pots are only available with PC board "pins" - that is, 
intended to mount directly to a printed circuit board.  The standard 
Spectrol 148 and 149 series pots are linear taper.  Log taper is only 
available by special order and you must commit to quantity.

Bourns only offers conductive plastic elements.  Bourns pots are 
available with PC board "pins" AND with solder tabs for termination to 
wire.  Bourns pots are always "special order" (no distributor stock) 
but are available in both linear and log taper configurations.

The form factor (specifically, the distance from the centerline of the 
shaft to the base of the pot) is different between the Spectrol and 
Bourns pots and, as such, it is not possible to mix and match the two 
brands of pots on the same printed circuit.


So as it turns out, there does seem to be valid reasons why you might 
choose one brand over the other based on the requirements of a 
particular module.  (A specific example from Paul - EGs need large 
value log taper pots and it is expected that these pots will get a lot 
of use.  In this case, Bourns is the preferred solution as these are 
the long life conductive plastic and log taper is available as a 
standard configuration.  Sure enough, there are board mounted Bourns 
pots in the 800 EG!  In other modules where there is a mix of cermet 
and conductive plastic types mounted to the circuit board, the 
Spectrol types are used.)


Fine - but you are probably still wondering about the replacement pot 
comment?  Notice in my original question that I mention the Spectrol 
248 series - specifically, that I found the 248 series to be 
particularly stiff.  Wait a minute, what is the 248 series???  Well, 
it turns out that very early on (before Paul managed to convince 
Spectrol to build 148 series 100k log pots) some modules were built 
with this 248 series.  This Spectrol 248 series has a plastic busing 
around the shaft where all the other types have a metal busing - 
logically, this explains the difference in feel.  The 148 and 248 seem 
to be interchangeable otherwise.

Because the 148 series log pots are special order, I asked Paul if I 
could purchase a few of these from him should I decide that I really 
can't stand the feel of the 248 series.  He agreed, which I think is 
pretty damn nice of him!


Paul also agreed to provide me with a copy of the MOTM 300 assembly / 
calibration / operation manual - which was the real reason that I 
called him.  My MOTM setup doesn't have any VCOs yet and, as Cuari has 
put his MOTM system up for sale, I offered to buy a bunch of VCOs from 
him.  (He didn't have the module documentation - and I wanted to be 
sure I could get a copy just in case.)  So, now that I have some 
VCOs on the way I can really make some noise!  Oh wait, I still need 
to work on that patch cable deficiency... ;)


Seth