Arfa Daily wrote in message
...
"N Cook" wrote in message
...
Arfa Daily wrote in message
...
"N Cook" wrote in message
...
Ten years ago I would have considered it impractical or even immoral.
But having to replace yet another effectively all plastic 12 mm pot
because
the wiper is almost invisible in thickness, perhaps time for a
rethink.
This type of pot has a metal covering but its only function is as a
shroud
for fixing to the board, not even earthed, it does not strengthen the
pot
body to restrain abuse at the other, user end. No longer are such
pots
fixed
to the front pannel.
I'm thinking that with the next one to replace it will be with a
standard
small 12 mm or so enclosed preset that takes a standard plastic shaft
in
the
adjustment slot, fudging the other end of the shaft to take the knob
and
through pannel bearer. The internal mechanism is just a pro-rata
scaled
down
conventional paxolin backed track with double wiper nibs.
If this replacement physically breaks in use then that seems more
justifiable than the modern flimsy wipers losing spring action or
just
aging
of the lubricant inside to wedge under the wiper and loose contact or
whatever their problem is, the tracks are always perfect (so little
wiper
pressure presumably).
Are the resistive tracks of presets more frail/easier scoored than
pot
tracks, so for that reason cannot be used say for volume control ?
Anyone any opinions?
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
I would have said that the description " preset " tells you all that
you
need to know on that score. These components are designed to be set and
left, pretty much for the lifetime of the equipment, so I can't see
them
standing up to the rigours of regular use for long. Add to this that as
they
are not designed to be rotated repeatedly as a user control, the wiper
/
track interface has not been designed to be specifically quiet. I also
think
that the control will feel bad to a user, as it will not have that "
stirring treacle " feel that a proper user control has, courtesy of the
drag
grease that the manufacturers put in the spindle bush. Why not replace
controls with manufacturer's originals ? Most group amp builders supply
spare parts. Failing that, there are plenty of reasonable quality pots
available - even the range that Maplin keep are pretty fair quality -
and
if
you really want to fit top quality pots, these are also available from
audiophile outlets.
Arfa
I had not considered the feel, which agreed is very 'scratchy'.
Squirting silicone oil in one and turning back and forth a hundred times
is
very much improved, and resistive track still ok.
What is the point of the feather-light touch of the flimsy pots if the
downside is they just don't last.
I don't see the point of replacing with an exact match when the original
is
say one of 8 and has only lasted for use once a week for 2 years before
internally failing , ie not abuse. In fact they seem to be the pots that
are
not used much if at all, that's why I suspect clumping or something in
the
lubricant.
I'll try a 1000 times and re-measure to see if the track fails.
--
I'm actually really surprised that you seem to come across so many bad
pots
in group amps. I have been repairing these for many many years, and do
lots
of all makes and models, for a local shop, and I really don't have to
replace many pots that aren't physically broken through taking knocks. I
have found in general, that most manufacturers appreciate that this gear
is
going to last its owners for a lot of years, and tailor their design
criteria, and component spec-ing to suit. I would agree that the pots
fitted
these days tend to be Japanese ( probably Chinese actually ) small-bodied
types, which are not as good quality as say the large body Wellyns or the
typical RS components pots that you used to find in them a few years back,
but still, I don't seem to have that much trouble with them. If you really
think that the problem is insufficient spring tension on the wiper
fingers,
just pile a bit more on. It's easily done with a pair of long-nosed pointy
surgical tweezers.
Arfa
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
The present problematic is a Carvin Pro Bass 100, PB100 which will come
bouncing back next weekend.
The new fault , could well be due to another pot failed, symptoms over the
phone so far, the previous one I replaced is fine.
I still have the previous original one lying around .
No makers name , a logo like cross-section of a flying saucer, a thin elipse
laid over a circle, just the outermost lineforms.
The wiper is only 0.15 mm thick , 0.6mm wide and using a force gauge, it
starts deflecting at only 12 grams pressure.
Followup on my endurance test on a preset, I survived 1000 turns back and
forth , whole track, as did the pot. No wavering on mid-track resistance
readings. The rough feel has returned but I doubt that is due to the wiper
action. Cracking it open, one obvious shiny, but not broken through to
paxolin, track from one nib and vague partial thin trace from the other nib.
I would estimate it could have done more than 10,000 rotations before both
tracks were worn through.
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/