"Jim Lesurf" wrote in message
...
In article , Sala
Massimo
wrote:
Hi

I am the happy owner of one A1 amplifier, bought in 1991. It is 20 Watt
class A amplifier, build around MOSFET.

Nope, all bipolar, except for a couple of TL084 op-amps ;-)



In the last months the A1 started to malfunction, the volume knob works
very bad: when I turn the knob I hear strange noise ("shhh shhh" as
there is sand inside the knob, and also some "crack") and the volume
output isn't in accordance with the knob position.

I took the unit to repair, but they said me they cannot because they
haven't the potentiometer.

I ask you: 1) is really the pot the culprit ?

From what you describe I'd say that the pot has become damaged by d.c.
applied through its wiper (variable connection). The symptoms of
'whooshing' sounds when rotated and some cracks are classic signs of this
damage. The cause may be something like a capacitor in series with the pot
leaking d.c. and thus degrading the pot. Hence you'd need to replace the
relevant capacitor(s) as well as the pot to avoid any new volume control
rapidly getting the same problem.

Jim's right - it seems to be a stock fault with these. But there's no
capacitors to check in this instance - my feeling is that it's a design
problem.

Please feel free to have a look at my homepage - you'll find lots of info
there, including schematics. It's currently the first result on a Google
search for "Musical Fidelity A1" ;-)



In some cases, cleaning the pot and replacing the capacitors may cause the
problem to go away even if you go on using the old pot. However applying
cleaner to some of the better Alps pots can be problematic, so a new pot
makes sense if available.


2) where can I found one (or two) spare parts? It is a Alps 0071 50KAX2
(I cannot measure the value because it is soldered).

Farnell stock the Alps "Blue Velvet" pot - don't know the part number off
the top of my head, but it's easy enough to find. 50K Log is what you need.

Cheers,

Mark
--
http://www.mhennessy.f9.co.uk/