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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.
When checking the bias ensure that any "Active"
pre-amp or source is "off" with zero volume to
prevent false readings. Passive pre-amps should
have "zero" volume. On a test bench "short circuit"
the input.
The MB 845s use the "fixed bias' mode of valve
operation. This has the advantage of higher power, and
cooler running. However occasionally (twice 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 0.4v
(400mv) DC in the 2 sockets next to each 845 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 '-ve' connection.
3, Checking Bias: You should get a reading of
ideally 400mV if each valve is conducting correctly.
But this will vary from about 350mV to 430mV. Bear
in mind that your mains voltage fluctuations can
affect your readings by up to 10%.
So if all the valves are reading slightly high or slightly
low, this usually means this is due to your mains. In
this case no adjustment is necessary.
TIP: If one or two valves are out of line with the
average, only adjust those valves to the settings of
the others.
Showing a probe in output valve test point
4, Adjusting: If all the valves are high or low, set the
400mv by inserting a small screwdriver in each
adjuster. If one valve is significantly higher or lower,
then it may be faulty or life expired. The adjustment
is very sensitive so adjust very carefully. 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 400mv, 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.
NOTE!
If there is no reading at all the internal fuse individual
to each 845 should be checked by a qualified
engineer. Spares should be located inside.
Page 7
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