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Old 1st December 2012, 04:16 AM   #1
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Default Trouble with Denon PMA-520 bias adjustment

I'm trying to adjust the bias on an older Denon PMA-520,following this translation of the German directions from the service manual (corrected from here):
BIAS CURRENT

-Setup
1. Install device in a normal position and protect from direct draft of A/C or fans. Carry out adjustment at a room temperature between 15C and 30C.

2. Set control elements to the following:
POWER BUTTON -> OFF or STANDBY
VOLUME CONTROL -> CCW [counter clockwise] until stop
SPEAKER TERMINALS -> open (connect neither speakers nor load resistors or similar)

-Adjustment:
1. Open top cover, then [put a] DC voltmeter on the measuring points of PCB number 1U-1608-1 (EQ.- and amplifier module).
2. Plug in power plug and switch power button to "ON". Within 10 seconds turn up VR403 (left channel) and VR404 (right channel) so much, that the voltmeter shows 0,5 mV +-0,2 mV.
3. Warm up for 5 minutes, then adjust VR403 and VR404 so that the voltmeter shows 3 mV +- 0,5 mV.
4. After 15 minutes of warming up, adjust VR403 and VR404 so that the voltmeter shows 5 mV +- 0,5 mV.
5. Apply a signal of 1 kHz and 100mV RMS and turn up volume to maximum.
6. Now the voltmeter should show a slightly higher value (max. about 10 mV), now adjust the trimpots VR405 (left channel) and VR406 (right channel) so that the voltmeter shows 20 mV +-3 mV.
7. After another 2 minutes, adjust VR405 and VR406 to 35 mV +-3 mV (PMA-720) respectively 20 mV +-3 mV (PMA-520).
8. After 10 minutes of warm-up, adjust VR405 and VR406 to 40 mV +-3 mV (PMA-720) respectively 20 mV +-3 mV (PMA-520).

I got to step 5 without incident, but when I hooked up the signal generator and turned the volume all the way up, the voltage at the test points went to around 1.4mV rather than 10mV and was unresponsive to adjustments from either set of trimpots. As I slowly turned the volume up the voltage increased from 5mV to a maximum of over 20mV part way up then decreased. Does anyone have any idea what might be causing this?
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Old 1st December 2012, 04:56 PM   #2
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Here's the service manual - in German, but it's the only one I could find. Denon PMA-520 Service Manual
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Old 10th December 2012, 10:57 PM   #3
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Any help here?
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