Page 6
Hum Problems
If you experience hum, try disconnecting all inputs, if
hum persists this is probably an amplifier fault.
If not, identify which input is causing hum. Connect one
input at a time. A
common cause is a ‘hum loop’ caused
by having too many earths, and may be 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.
Strange noises coming from speakers
Turn volume to minimum on unused input, if the noise
disappears, the fault is with the source or the connection.
If noise persists, the problem is 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.
Valve Replacement
(see also section 7)
Valve life will depend upon such things as hours of use
and number of on/off cycles, the Standby/HT Delay
Circuit if fitted will extend the life of the valve by not
stressing the cathode when it is cold. 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 your dealer or Icon Audio for a periodic
service or return the valves for testing free of charge.
You should carefully remove the valves (the KT90s
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 be re-configured for 110/120v ac by a
qualified engineer. Contact for more information.
6 Bias Adjustment
Read these notes all the way through first.
If you are unsure about any aspect contact your
retailer, Icon Audio or a competent service
engineer.
The MB 90s use
the “fixed bias’ mode of valve
operation. This has the advantage of higher power,
and cooler running. However occasionally (once a
year) it is advisable to check the bias reading using
the supplied meter to ensure best performance from
the amplifier. This is a safe procedure which involves
measuring 0.5v (500mv) DC in the 2 sockets next to
each KT90 and adjusting if necessary.
1, Tools you will need: The supplied meter or one set
to measure approx approx 2v (2000mv), and a small
flat blade screwdriver. Adjustment is done at zero
volume with speakers connected. Run the amplifier
for about 10 mins (if possible) first.
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 or the ‘black mark’. See pics.
Making the ‘earth’ connection.
3, Checking Bias: You should get a reading of ideally
500mV if each valve is conducting correctly. But this
will vary from about 450mV to 525mV. Bear in mind
that your mains voltage fluctuations can affect your
readings by up to 10%.
Showing a probe in output valve test point
