Nice mod!
I bought one of these recently and it arrived with a very healthy channel imbalance across all outputs!
Tried to get a schematic but no joy at all. I don't suppose you have one?
Anyway, working blind my tech friend changed the LF353 opamp but no joy.
All the resistors around the chip seemed fine But he was getting weird readings around the pins either to Ground or to themselves.
Anyway, he bit the bullet and did a complete blanket change of all the electrolytic capacitors, some of very dubious make! He also removed the 8 pin socket which looked as though it could be contaminated, possibly chemicals from an electrolytic and thoroughly washed the board. Didn't have a spare 8 pin socket so made one from a 20 pin one.
Caps replaced:-
33 & C34 3.3uF 50V
C37 & C38 47uF 25V
C39 & C40 100uF 50V
C43 & C44 3.3uF 50V
C47 & C48 10uF 25V
C81 & C82 10uF 25V
Second board wash after fitting above and it now works, err well almost. The NORMAL outputs are spot on but the tube outputs have a small imbalance, the Left out is -1.2dB down. After 15 mins the imbalance drops to -0.8dB so maybe it is just the 6N3 valve? I going to try a 5670 GE..
Anyway, the joys of buying from China!
VPI Scout 1.1, Hana ML, Vanderveen MC10 SUT, WAD Phono3, Rpi3B+/Digione Signature, Xiang Sheng DAC-01, WAD KiT6550 and Reiver Edingdale SR