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 15°C and 30°C.
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?
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 15°C and 30°C.
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?
Here's the service manual - in German, but it's the only one I could find. Denon PMA-520 Service Manual
That whole circuit including the opto is common to both channels so if both channels are acting up it's pretty good odds it's that circuit. The voltage on pin 4 of the opto will tell you if the rest of the circuit is working.
Craig
Craig
With no signal I'm getting 14.9V instead of 15.5V at pin 4 of IC503. Later gonna try with signal (but getting 10 mV RMS may be tricky with my multimeter).
Set the voltage between TP1 - 3 and TP2 - 4 to 5mV with no signal and no load with the bias pots and that's it, done.
Hello all !
Recently i had also the same problem: a friend of mine gave me his PMA520a to repair because a channel was weak vs the other. It was logic brecause i measured the Bias current of that weak channel near 0mA… So I wanted to re-trim correctly this Bias current:
From schematics, the overall Bias system of that amp, for each channel is set by 2 Trimmers: 1 for Static (no signal) DC bias & 1 for Dynamic Bias (Opto isolated).
I found on your website the setup in german, translated by Schubert (many thanks !):
I followed that procedure points from 1 to 5 that was OK (100mV RMS on input AUX). But the same problem arrived: Test Point TP1/3 DC Static bias set to 5mV by VR403, but now increasing the VR405 position led me to 5mV to approx. 10mV, reaching 20mV but decreased itself quickly (#0.5s) returning to 5mV Static bias… no way to avoid that !
I did some experiments:
With no input, I checked the 2x VCE of Opto iC503: the same # 1Vdc on each, so the opto was good because only 1 channel was weak.
But with a input, the signal Peak Rectified AC to DCbias dynamic feedback chain iC502, was also rectified “sidechained” via TR501 via the D505 and 10k//3.9k x 100µF. So, if there is a smaller but continuous signal, that loop saturates TR501, cutoff both TR502 + Opto iC503: so the amp returns in DCbias Static mode: that’s is the problem caused by VOLUME Pot set at Maximum in the Documentation procedure !
SOLUTION: as indicated my input was around 100mV, but i set the Volume to half from the Max (position at noon, that is approx. 1/20th of the Power): as this Side Loop was not activated all was OK, and I was able to trim VR405 correctly at 20mV (which is the Dynamic Bias for large Transient signals)
NOTEs:
_ The sensibility of the DC Static Bias trim is high (L=VR403 & R=VR404), so it is impossible to maintain the small +/- 0.2mV tolerance, but it is not important vs the 5mV (23mA DCbias on 0.22 Ohm equivalent).
_ The dynamic Signal to DC Opto Isolated (L=VR405 & R=VR406) are inactive when no signal input, so they will be trimmed after the DC static bias.
_ I rised slightly all the Bias current values (respectively 6mA & 25mA) because the heatsinks of this amp are very big, and easily support these increased bias currents.
Recently i had also the same problem: a friend of mine gave me his PMA520a to repair because a channel was weak vs the other. It was logic brecause i measured the Bias current of that weak channel near 0mA… So I wanted to re-trim correctly this Bias current:
From schematics, the overall Bias system of that amp, for each channel is set by 2 Trimmers: 1 for Static (no signal) DC bias & 1 for Dynamic Bias (Opto isolated).
I found on your website the setup in german, translated by Schubert (many thanks !):
I followed that procedure points from 1 to 5 that was OK (100mV RMS on input AUX). But the same problem arrived: Test Point TP1/3 DC Static bias set to 5mV by VR403, but now increasing the VR405 position led me to 5mV to approx. 10mV, reaching 20mV but decreased itself quickly (#0.5s) returning to 5mV Static bias… no way to avoid that !
I did some experiments:
With no input, I checked the 2x VCE of Opto iC503: the same # 1Vdc on each, so the opto was good because only 1 channel was weak.
But with a input, the signal Peak Rectified AC to DCbias dynamic feedback chain iC502, was also rectified “sidechained” via TR501 via the D505 and 10k//3.9k x 100µF. So, if there is a smaller but continuous signal, that loop saturates TR501, cutoff both TR502 + Opto iC503: so the amp returns in DCbias Static mode: that’s is the problem caused by VOLUME Pot set at Maximum in the Documentation procedure !
SOLUTION: as indicated my input was around 100mV, but i set the Volume to half from the Max (position at noon, that is approx. 1/20th of the Power): as this Side Loop was not activated all was OK, and I was able to trim VR405 correctly at 20mV (which is the Dynamic Bias for large Transient signals)
NOTEs:
_ The sensibility of the DC Static Bias trim is high (L=VR403 & R=VR404), so it is impossible to maintain the small +/- 0.2mV tolerance, but it is not important vs the 5mV (23mA DCbias on 0.22 Ohm equivalent).
_ The dynamic Signal to DC Opto Isolated (L=VR405 & R=VR406) are inactive when no signal input, so they will be trimmed after the DC static bias.
_ I rised slightly all the Bias current values (respectively 6mA & 25mA) because the heatsinks of this amp are very big, and easily support these increased bias currents.