On Mar 19, 4:20*pm, Ian Jackson
wrote:
In message
,
Tabby writes

On Mar 18, 4:59 pm, Terry Casey wrote:
In message

on Fri, 18 Mar 2011 08:52:07 -0700 (PDT)
Tabby wrote:

I have occasionally seen a volume pot carrying dc, but not wired up
correctly, ie arranged so that any slight poor connection produces a
lot of noise.

What do you mean by 'not wired up correctly'?

Carbon pots carrying DC are much more prone to be noisy.

The change from valves to transistors, with their lower impedance
circuitry and
leaky electrolytic coupling capacitors, considerably increased the
incidence of
noisy volume controls.

Some can be cleaned easily and give long service before more attention is
required, whereas, if the track is badly worn, relief, if any, is only
temporary, no matter what you use.

In such cases, the only remedy is replacement.

These can typically be silenced just by connceting the
unconnected pot end to the wiper ...

There shouldn't be an unconnected end! The pot will typically be connected
between signal and ground with the output being between the wiper and ground.

Carbon pots carrying DC are much more prone to be noisy.

The change from valves to transistors, with their lower impedance
circuitry and
'leaky' electrolytic coupling capacitors, considerably increased the
incidence
of noisy volume controls.

Some can be cleaned easily and give long service before more attention is
required, whereas, if the track is badly worn, relief, if any, is only
temporary, no matter what you use.

In such cases, the only remedy is replacement.

... or if that cant be done fitting a fixed R to the wiper and
ground or one
of the other terminals.

That will modify the 'law' of the pot and change the wiper position for most
settings. If the track is worn in the area that is most usually used (not
surprising!) moving the operating point to a relatively unused section can
work.

Fine as a diy work around but not worthwhile if a replacement is to hand as
that will give a better guarantee of a long term solution.

I wasnt quite clear was I. Sometimes the pot is wired such that if the
wiper goes oc, the output, which connects to the wiper, is o/c. Adding
a fixed R across the pot's output prevents this, ie from wiper to
ground end. It can be high enough value to have only a trivial effect
on the 'law' of the pot, which is a fairly trivial matter anyhow.

I wouldn't have thought that made the slightest bit of difference. If
the wiper's o/c, a resistor between the wiper and the ground end of the
pot still won't provide any output.

However, if the earthy end of the track has become disconnected from the
earthy connection pin, but is otherwise intact, the top of the pot and
the wiper is still a variable resistor. The volume will probably be
maximum, and more-or-less uncontrollable. Adding a resistor between the
slider and the earthy end will give you of degree of control.

The same is true if the top end of the track becomes detached from the
connection, except that the volume will be zero. Adding a resistor will
restore some output (albeit less than maximum), and a degree of control.

When the wiper connection goes o/c, you often get a dc offset showing
up on the output wire, creating major noise. Adding a resistor
prevents this, and can turn a very noisy pot into a silent well
behaved one, some of the time.


NT