Yes I truly agree, with you.
The AU-9900A was butchered to death when I got to work on the amplifier, and it was bought for a reasonable price from the seller, as he was not able to fix any of the below mentioned issues.
It had a shorted fuse using wire strands, biasing was set too high that the amplifier was putting out enormous amount of heat .
Apart this there were burnt resistors which i had replaced, and also a damaged midrange potentiometer's wafer plates.
1. Damaged Fuses
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2. Burnt Resistor Identified During Restore Work
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3. Damaged Wafers
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With respect to your other two points, i have removed the ill fated Glue using locally available thinner.
Use a small Q-Tip bud and remove the capacitors completely before attempting to removing the glue.
There are cases where the glue even damages resistors in that particular section.
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After cleaning the Glue:
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The PCB's also reek of slimily residue i belive due to the Glue or Oxidation:
It is better to clean them properly and recoat the PCB's.
Pics from my X11 restoration.
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And yes most second hand amplifiers are already tampered on too much to even work or fix it is a big hassle.
From obsolete transistors to obsolete potentiometers it is a big pain in restoring these amplifiers.
I would though recommend to check output transistors for authenticity before proceeding with purchase of a vintage amplifier.
The performance of the amplifier will never be the same with replacement spares !!.
I have confirmed this by using low FT transistors for my AU-X11 during the restore work, when measured with a scope the output of the amplifier starts clipping way before rated power specification . The highs were also a bit harsh to listen to .
After the restoration work i replaced the output transistors with the original sanken's and it made a big difference.
The amplifier could easily put out 160watts at 8 ohms with no clipping, and the harshness in the treble was gone.
I would though recommend bourns multi turn trim potentiometers for the biasing and offset calibration.
After the restore work it is best to load test the amplifier atleast for 10 minutes to watch for clipping distortion etc .
EDIT: The black flags can be replaced with silver micca from Cornell Dublier, I have replaced all the black flags with Silver Micca, in the unobtainium list are certain pre driver transistors and the SV02/SV03 diodes, certain FET's .
To add on these amplifiers are really worth restoring if you can get proper authentic spares, i was faked by eBay sellers many times and had to apply for a refund. So restoring this amp is a pain especially when you need to hunt down obsolete spares.
Cost involved with customs charges will burn a hole in your pocket, it would easily range between 30k till 1lakh depending on the complexity of work and the spares that one needs to change. Calculating a net effective customs charge of 40% on Product cost + Shipping cost.
I have seen these amplifiers sell for exorbitant prices in-spite of being in bad shape to buy them or not is once choice .
My 2 Cents !!.