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identified by unplugging each input source from the
mains. One remedy for this is to use an interconnect
which only has the screen connected at one end. Other
causes of low-level hum can be from adjacent
equipment, so experiment with moving equipment
around to see if this makes the hum better or worse.
Interference Problems
The amplifier design incorporates features and devices
which make it resistant to mains-born interference. But
some CH boilers/fridges/cookers etc can generate RF
(radio frequency) interference which travels through the
air (and walls). Although rare this can be very irritating.
In this case a simple capacitor is often all that is needed
to effect a permanent cure (ask us).
One channel missing.
Usually ‘bad’ connection on either the input or the
speakers. Try swapping the connection over to establish
if the cause is:
(a) Input to the amp.
Sound will move to the other
channel.
(b) Amplifier or speakers.
Sound will not move.
Strange noises coming from speakers:
Turn volume to minimum on unused input, if problem
corrected either fault with source unit or with connection.
If noise persists, problem with amplifier.
If a whole output valve glows red (other than the heater),
often accompanied by a hum through the speakers,
switch off immediately, and refer to Icon Audio or a
service engineer, as this could be valve failure.
A valve that is lit up is not a guarantee that it is working
properly; conversely a valve that is not lit up will not be
working (usually cold to the touch).
Valve Replacement
(see also section 7)
Valve life will depend upon such things as hours of use
and number of on/off cycles, Do not switch on and off
unnecessarily (see
Standby).
Also it is not good practice
to remove the valves unnecessarily as this can strain the
pins and cause tiny air leaks.
Service:
Should you suspect a problem, you could
return the unit to Icon Audio for a periodic service or
return the valves for testing free of charge. You should
carefully remove the valves (the KT88/EL34s should be
held by the base when removing, to prevent damage)
numbering them with a marker from left to right as you
do so in order that that may be replaced in the same
position. They should be well packed in cardboard &
foam or similar, and returned to Icon Audio for testing.
(Valves are very rugged if packed properly).
Mains Supply
This amplifier is hard wired to work on 230/240v ac. The
transformer may easily be re-configured for 110/120v ac.
Contact for more information.
6. Bias Checking & Adjustment
If you are unsure about any aspect of bias
contact your retailer, Icon Audio or a competent
service engineer.
The Stereo 40 MKIII uses the ‘Fixed bias’ method
of valve operation. This has the advantage of
higher power, lower feedback and cooler running.
However we recommend that once or twice a year
you check the bias reading using the supplied
meter to ensure best performance from the
amplifier.
This is a safe procedure which involves
measuring less than 0.5 volt in the four sockets next
to each KT88/EL34 and adjusting if necessary.
1, Tools you will need: The supplied meter or one set
to measure approx more than 2 or 3 volts DC, and a
small flat blade screwdriver. Adjustments are done at
zero volume with speakers connected. Run the amp
for at least 15mins (if it is working correctly).
Otherwise do a rough adjustment and check later
when the ST40 is hot.
The Icon meter reads in mv (millivolt=1v/1000). So
will read “.400” EL34 or “.450” KT88.
2, Connect: the black probe to the chassis ‘earth’ by
unscrewing the ‘0’ speaker terminal and tightening
the probe in the exposed hole. And the other in the
test socket adjacent to the valve on test. Set the
‘Icon’ meter to 2000mv (2v) or the ‘black mark’. See
pics.
Making the ‘earth’ connection.
3,
Checking: You should get a reading of 400mv
EL34, 450mv KT88 if valve is conducting correctly.
But bear in mind that your mains voltage fluctuations
can affect your readings up to 10% Check all valves,
they should be within about 20% of the ideal voltage.
4, Adjusting: If the reading is incorrect, set this by
using the bias adjuster in the centre,
NOTE: There is
only one adjuster for all four valves
. They are very
sensitive so adjust very carefully; use tiny turns of the
screw. If the reading appears a little unstable this is
normally due to mains fluctuations.
5, If one or more valves are showing erratic readings
or you cannot set the correct voltage, then that valve
is probably faulty or out of specification. If you are
unable to set the reading high enough this means the
emission of the valve is low.
