8
ADJUSTMENT/ALIGNMENT
Normally no adjustment will be necessary after the ex-works alignment procedure. However, with
time some drift of POT settings and component ageing may require some re-alignment to be made.
After component replacement, re-alignment and testing to the original specifications should be
carried out. THIS MUST BE DONE BY COMPETENT PERSONNEL WITH CERTAIN
MINIMUM TEST EQUIPMENT. (See Appendix 1.)
A copy of the test specifications PTS
E45
is available on request.
ADJUSTMENT OF THE VARIOUS PRE-SETS ARE AS FOLLOWS: (References for Ch. B in
brackets.)
a. VR101 (VR201) CMR
* Inject 1kHz @ +10dBu unbalanced with signal to relevant input XLR, pins 2 +3 (hot and
cold) and ground to pin 1.
* Monitor J103 (J203) pin 1 on the oscilloscope.
* Adjust VR101 (VR201) for minimum output level.
* Repeat for Ch. B.
b. VR103 (VR203) GAIN
* Inject a balanced 0dBu @ 1kHz signal into the relevant input XLR connector.
* Monitor the unloaded speaker output with a suitable audio analyser or audio level
meter.
* Adjust VR103 (VR203) for an output level of 32dBu. (Gain link set for 32dB, Pins 1-2)
* Repeat for Ch. B.
c. VR104 (VR204) BIAS CURRENT
* Connect suitable dummy 4 ohm load to relevant speaker output.
* Inject 4dBu @ 1kHz balanced into relevant input XLR.
* Monitor the output across the 4 ohm load on a suitable audio analyser capable of
measuring THD + N down to 0.003%.
* If possible monitor the residual signal from the analyser monitor output on an
oscilloscope.
* Set VR104 (VR204) fully anti-clockwise.
* Slowly adjust VR104 (VR204) clockwise. The THD + N should reduce gradually to a
minimum level of around 0.009% and then start to increase with continued rotation of
the POT. The correct setting is as far anti-clockwise as possible consistent with minimum
THD + N. NOTE: We recommend using an IEC ‘A-WTD’ filter for this measurement.
* Repeat for Ch. B.
Alignment of the limiters should only be carried out if clipping is evident when the amplifier
display indicators show a limiting condition and is carried out as follows:
THIS MUST BE CARRIED OUT TO ONLY 1 CHANNEL AT A TIME.
Ensure that all PRC switches are out (off) and gain controls set for maximum.
Set a level of +6dBu from a tone generator at 1kHz sine wave applied to Ch. B input of the
amplifier. Increase the level until the Ch. Limit LED illuminates.
Monitor the amplifier Ch. B output, loaded with 4-ohms, on an oscilloscope.
Turn VR1 on PCB700 slowly clockwise until clipping of Ch. B is evident on the oscilloscope.
