NAD 3020B - Alignment Procedures; General Alignment Guidelines; Idling Current Adjustment; DC Offset Adjustment

ALIGNMENT
R653
RX1
VR6
RX2
ALIGNMENT INSTRUCTIONS
GENERAL:
When servicing, in addition to checking output power, THD etc., the power output stages IDLING
CURRENT and DC OFFSET should also be checked.
if ANY WORK has been performed on the output section, checking and adjusting of IDLING
CURRENT and DC OFFSET must be done.
A. IDLING CURRENT:
Checking:
1. Set Volumecontrol to minimum, disconnect any output loads.
2. Locate and remove soldershorts across sensing resistors-R 653 (left) and R 654 (right).
3. Connect DMM or VOM (with sensitivity better than 100 mV full scale) across R 653 and R
654.
4. Turn on and read voltage. Reading should be 5 — 20mV, typically it would be around 12 mV.
1F NOT WITHIN TOLERANCES GO TO ADJUSTING:
10.
11.
12.
If within turn-on tolerances, allow unit to warm up for 5 minutes.
The voltage should then be 30 mV, tolerances + 10mV.
IF NOT WITHIN TOLERANCES GO TO ADJUSTING:
If OK, reconnect the soldershorts across R 653 and R 654.
Adjusting:
Turn off, remove bias adjusting resistors RX 1 (left) and RX 2 (right) and temporary replace
with variable resistors 2 Kohms to 5 Kohms, set the variable resistors to maximum resistance.
Turn on and read voltage acress R 653 and R 654. It should be less than 10 mV. If too high,
turn off unit immediately and check the output amplifier section, especially Q609-0611-
0613-0615-0617 left channel or Q610-0612-0614-0616-0618 right channel.
If voltage is OK upon turn-on, adjust variable resistors to read approximately 12 mV on VOM.
Leave unit on for 5 minutes and reset voltage to 30 mV. Note that it takes a little time for
readings to stabilize.
Turn off and remove variable resistors and replace with fixed resistance with exatly the same
measured value. Use resistors in series or paralell if necessary.
Recheck idling current,
Reinstall soldershorts across R 653 and R 654.
ALIGNMENT INSTRUCTIONS (continued)
Notes:
1. Failure to re-install soldershort across R653 and R 654 will result in failure at high volume.
2. Decreasing value of RX 1 and RX 2 will cause idling current to increase.
3. Too little idling current will cause ‘“Crossouer” distortion. Too much idling current will result
in overheating.
B DC OFFSET: :
1. Set Volumecontrol to minimum, disconnect any outputloads.
2. Connect DMM or VOM across each channel’s speaker terminals. eres
3. Turn on and read VOM. Reading should be 0 V.DC. Tolerance + 50mV.
Adjust VR 5 (left) or VR 6 (right) as necessary.
Notes:
1. Before adjusting VR 5 or VR 6 soften the lock paint with laquer thinner to avoid destroying
them. Secure with lock paint (nail polish) when adjustment is completed.
2. Incorrect adjustment of DC offset can create pop when inserting/removing Headphone plug.
3. Offset more than 3 V is caused by defective parts, not by misadjustments.