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PSU Modification A30.2

Written by Bjorn Holmqvist for the manufacturing industry.
[email protected]

Confidential !
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This modification solves a stability problem with the power regulation for the
amplifiers driverstage.
The first symptom for instability are Speaker relay flickering during startup, other
symptoms might be distortion in the sound when amplifier is warm.
If there is oscillation in (REG1, REG101, REG2, REG102) it can easily be spotted by
monitoring the output voltage at test points (+-48V) with the oscilloscope.

When servicing this circuit I also recommend that the resistance over R63, R163,
R64, R164 are checked, if the resistance are different that the stated 4.7R please
replace all four of them.

1. Remove mains power and the topcover.

2. Remove the heatsinks on REG1, REG101, REG2, REG102 in order to free up
space for the soldering iron.

3. Solder 22uF/63V 105 centigrades capacitor in parallel with C18, C118, C19,
C119 (or replace the original 3.3uF capacitor). Use a small soldering tip when
carrying out this work and watch the relays !

4. Put back the heatsinks on REG1, REG101, REG2, REG102.

5. Test the amplifier and measure the voltage at +-48V test points on both
Note! I have used 10uF capacitors in these pictures, the correct value is 22uF.

Left CH

22uF /63V

Right CH

22uF /63V

//Bjorn Holmqvist