User Manual

darTZeel NHB-108 model one Audiophile's technical manual Page 27 of 28
12. Special adjustments
12.1. Output DC voltage drift
Caution! The procedure described below
can present some risk, since the darTZeel
NHB-108 model one must remain powered
ON.
WE DECLINE ANY RESPONSIBILITY IN
THE EVENT OF ELECTRIC SHOCK DUE TO
INCORRECT MANIPULATION.
WARNING! ONLY A QUALIFIED PER-
SON OR A TECHNICIAN CAN SAFELY
PERFORM THIS ADJUSTMENT.
When darTZeel NHB-108 model one's eyes
are blinking, it means that an output DC
voltage greater than ±0.6 volt is present
at the terminal speaker outputs. An ad-
justment is then necessary.
Before doing any task, it is important to
ensure that:
- Ambient room temperature is be-
tween +15°C and +35°C.
- The darTZeel NHB-108 model one is
properly vented, as described in the
Owner's manual.
- The loudspeakers are connected to
the speaker terminals!
- The darTZeel NHB-108 model one has
been in idle state for a minimum of 2
hours, allowing an optimum distrib-
uted heat across the entire housing.
To make this adjustment, you need the
following tools:
- A No. 4 flat blade screwdriver for re-
moving the glass cover; this tool is
supplied as standard with the darT-
Zeel NHB-108 model one.
- The 2 suction cups supplied with the
darTZeel NHB-108 model one. Please do
not forget to slightly wet them, to get
firm vacuum on the glass.
- A No. 0 flat blade screwdriver for ad-
justing the 12-turn trimmer; this tool
is supplied as standard with the
darTZeel NHB-108 model one.
- A universal DMM (Digital Multi Me-
ter), not supplied.
Procedure:
1) Using the DMM, check that the DC
output voltage is more than ±0.6
volt. In practice th e value is gen erally
negative.
2) Before opening the glass cover, lo-
cate the trimmers, using a pocket
torch if need be; the trimmers are
close to the RCA input connectors.
On left channel, the trimmer is lo-
cated on PCB solder side, between
PCB and heat sink. A clockwise rota-
tion induces a negative voltage shift.
On right channel, the trimmer is lo-
cated on PCB component side, ori-
ented toward the inside of the ma-
chine. A clockwise rotation induces a
positive voltage shift.
When adjusting, it is important to
turn the trimmer's screw slowly.
Please allow 5 seconds at least after
each tuning step, to wait for the
value to stabilize.
3) Connect the DMM to the left output,
and open the glass cover.
DO NOT FORGET THAT THE AMPLI-
FIER IS POWERED ON!
NEVER TOUCH THE COPPER BUS
BARS, WHICH ARE AT A POTENTIAL
DIFFERENCE OF 115 VOLTS DC.
A SHORT CIRCUIT INDUCED BY THE
SCREWDRIVER WILL PARTIALLY
EVAPORATE THE BLADE BY IN-
STANTANEOUS MELTING OF THE
METAL OF THE LATTER!
4) Within the next 2 minutes, set the
DC voltage drift at -300mv (-0.3V).
5) Connect the DMM to the right output,
and perform, within the next two
minutes, the same tuning as for left
channel.
6) Put back the glass cover, without
screwing it yet. Wait for one hour.
7) Then measure again the output volt-
ages. They should be be-
tween -300mV and -400mV.
8) Pow er the amplifier OFF, and wa it for
one or two hours for total cooling
down.
9) Power the darTZeel NHB-108 model one
ON again, and recheck the outputs.
DC drift should not be greater than
+500mV when ambient room tem-
perature is +22°C.
10) If everything is okay, you can screw
on the cover glass.