On Mon, 24 May 2010 07:33:52 -0700 (PDT), "William R. Walsh"
wrote:

Hi!

Nice work :-)

Good job. :-) If I had a prize, you'd win it.

I would not dare to call myself an expert, but I think it is important
to do what I can to point out that things can usually be fixed instead
of just thrown away. I'm definitely learning...

William

Probably a guess, but I suspect the mystery part is a high value
resistor as Phil stated. Its purpose is for "leaking" any DC static
voltage that could build up in the circuit. That would probably happen
if you were connecting a speaker up, and you were charged up (could be
anywhere from a few volts to tens of thousands of volts). Walking on a
nylon carpet in a dry house (less than 20% humidity) can generate
enough static electicity to jump a gap of more than a centimeter. In
northern Canada during winter, I've generated 2-3cm sparks this way.
The DC voltage or the transient current may stress or damage
components.
Touching the case or metal knobs of your amp when you are charged,
may also stress components in your amp, especially if the case isn't
grounded (has a 2-wire cord).
The resistor allows for a discharge path to the AC line, so that an
excessive high voltage is not impressed across the insulation between
primary and secondary of the power transformer/switcher. The
interwinding capacitance also allows transient current flow, that
reduces the instantaneous voltage difference between primary and
secondary. Without the wierd little component, your power
transformer/switcher would have to withstand up to 10-50kv. That's
$$$$$! Typical transformers are rated 2-4kv.
The DC currents that are generated while you build up a charge
(scuffing your feet across a rug) are quite small, in the nano- or
micro-amp range. However, once you are charged (humans have about
150-300pf self capacitance) to many kilovolts, you can make a fairly
nasty zap. If I remember correctly, the human body model for
eltrostatic discharge has about 1500 ohms effectively in series with a
few hundred picofarads. For testing equipment, that capacitance is
charged to 3-5 kv. The transient current is quite significant! The
risetime is quite small, and transient currents can induce voltages in
places you wouldn't expect.
Sometimes you also find a ceramic capacitor in addition to (or often
instead) of the resistor. I've seen values of around .001uf, rated at
several kv. Be very careful replacing these components.... should that
capacitor or resistor fail, you could die. You should be very
concerned about the reliability of these parts.
When you discharge a current into the electronic circuit (it
ultimately heads for ground) you create a circuit between your body
capacitance (200pf), body series resistance (1500 ohms), and the power
transformer insulation in parallel with the protective
resistor/capacitor. You can calculate the voltages applied to the
power transformer or switcher if you know winding and stray
capacitances.
Colour TV's and monitors are a problem, since you can get
discharges off the front of the screen. That introduces a similiar
current to ground that must be dealt with. Switched power supplies
will have similiar issues plus dealing with protecting their control
circuits.
Of course the "protective" resistor or capacitor is chosen to limit
the "leakage current", should you touch the unit and be at ground
potential. That current depends on your electrical code.... values are
from a few microamps to a few hundred microamps. For me, I can sense
tens of microamps, and it starts to tingle around half of a milliamp
(AC 60Hz). I can't feel DC until quite a few ma., then I sense it as a
muscle pulling.
The protective resistor/capacitor is to both save your amp from you,
and you from your amp. It protects you in case you over-stressed some
part because of your charge, and that overstressed part breaks down
and applies AC to the circuit and you. It protects your wallet from
other non-lethal circuit failures. It appears to give you some risk in
the form of a small leakage current.
If you doubt the component, measure the AC current (not voltage)
between amp and ground, for both polarities of the plug (if you have a
choice of polarity). Put a resistor (say about 10kohm) in series with
your meter so you don't blow your meter up should the case or amp
circuit be live to ground. Any current greater than a few hundred
microamps, and I'd retire your amp.

Paul G.