Adjustment Procedures; High Bias - SPHINX Project Eighteen Service Manual

SPHINX Project Eighteen Service Manual

7. ADJUSTMENT PROCEDURES

The Project Eighteen has eight adjustable settings,
meaning four adjustment procedures per channel:
1. High Bias: to set the bias current of the power-
transistors for normal use
2. Low Bias: to set the bias current of the power-
transistors for standby mode
3. Offset: to set the minimal DC voltage for the
output
4. Common Mode: to maximise the common
mode rejection of the balanced input
Re-adjustment of one or more might be necessary
due to ageing or when parts have been replaced or
repaired.
Attention:
When re-adjusting any setting please ensure that
there is no loudspeaker connected to the output!
Otherwise the loudspeaker may be seriously
damaged.
Attention:
The amplifier is able to generate high output
voltages of over + or -60 V.
Please be very careful during the adjustments!
After removing the top cover plate you will clearly
see the two separate channel sections.

High Bias

With this procedure you set the proper bias level for
the power transistors. This ensures their Class A
operation at low power levels.
Connect the amplifier according to the drawing
"Connecting diagram for testing the project 18"
(page 13).
The input of the amplifier must be shorted (by way
of the MUTE function of the oscillator).
•
Switch the amplifier ON (not in Standby
Mode!) and wait until it has reached the proper
operating temperature (this takes an hour).
•
Set the millivolt-meter to the DC-range.
•
Place the two measuring clips of the meter
across one of the emitter resistors
Left Channel: R468, R469, R472, R473, R476,
R477, R480 and R481.
Right Channel: R268, R269, R272, R273, R276,
R277, R280 and R281 (see schematic at page
15).
•
The level should be 60 mV DC (±3 mV).
If not: adjust potmeter P402 (Left) or P202
(Right) until the level is 60 mV.
Attention:
All emitter resistors should show the same 60 mV
value. If not, the corresponding power transistor may
be defective.
•
Switch the oscillator on and set it to 1 kHz and a
level of 0 dBu.
•
Check the distortion with a THD analyser: it
should be at the specified value (0.01% IHF-A
@ 1 kHz).
•
If this is correct the procedure is finished.
•
You may now switch off the amplifier or
continue with another adjustment procedure.
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