that the voltage on these test points (TP14 and TP15) must not go below 13.3V otherwise Q28 and
Q29 will be damaged. VR32 and VR33 are interactive. Connect a DVM to TP16 and check that the
voltage on this point is <+/-1V. If not, slightly adjust VR32 or VR33 ensuring that the voltages on
TP14 and TP15 remain within 13.4V to 13.5V. Note that it may be necessary to add a low value
resistor in the meter lead close to TP14 and TP15 to prevent instability in the amplifier. Connect a
voltmeter to TP8 and adjust VR28 for <1mV. Connect a DVM to TP17 and adjust VR31 for <3mV.
Frequency multiplier to 100 range, squarewave, adjust VR30 for a flat top squarewave. Note that
the oscilloscope must be dc coupled for this adjustment.
Symmetry, select 1k range and adjust VR9 for equal mark: space ratio using the oscilloscope x10 X-
multiplier to increase resolution and the trigger slope switch.
Before commencing the vernier calibration procedure, turn the dial fully clockwise and check that
the 005 mark aligns with the mark on the front panel. If not, remove the cap in the centre of the
knob, using a knife blade and loosen the nut one to two turns. The knob can now be slid around on
its shaft to align the marks. Tighten the nut and check that the dial rotates freely against the front
panel. Refit the knob cap. On later instruments a PTFE thrust washer is fitted between the dial and
the front panel.
Dial Multiplier
Adjust
Reading
5.0
10k
VR3
49.5 to 50.5 kHz
5.0
1M
VC1
4.9 to 5.1 MHz
5.0
100k
VC2
490 to 510 kHz
5.0
100
VR15
495 to 505 Hz
0.005
100k
VR2
465 to 475 Hz
Sinewave distortion, select sinewave, 1k range, dial at 5.0. Display the distortion meter output on
CH2 of the oscilloscope. Position VR18 and VR22 fully clockwise. Adjust VR19 to give a flat
response between peaks and VR23 to give a symmetrical display. Note, this is not minimum on the
distortion meter, do not adjust these two presets to give minimum distortion at this stage.
15
