Sansui AU-9900 Repair And Restoration
This Sansui AU-9900 integrated amplifier is ready for some repairs and restoration. Like many vintage units the amp was put out to pasture years ago.
The faceplate of the Sansui AU-9900 is now removed. Warm water and soap to be followed by some WD-40 for a shine.
With the faceplate removed the chassis behind the faceplate can be cleaned and deoxit used on the pushbuttons.
Have the top covers now removed on this Sansui integrated amplifier so more dirt and dust can be cleaned out.
As with all vintage stereo restorations you have to do some disassembly to get to the controls that need cleaning.
Every unit is different and as shown below just removing a couple of nuts and pushing can give you access to the control you need to clean.
Removing Four screws on the Sansui AU-9900 gets you access to the RCA jacks and the phone switches.
Four more screws on the other side of this Sansui integrated amplifier gives you access to the speaker connectors and pre/main jacks.
Now the bottom panel is removed. You guessed it.....Getting some more dirt and better access to some controls.
You can now get the deoxit to controls that you could never clean properly without the bottom cover removed.
During testing it was found that the right channel had no output through the phono section (equalizer board F-2570)
After troubleshooting by injecting a 10mv 1000HZ signal through the phono stage a bad transistor was found.
One half of the defective transistor pair tested good. This gave the gain (Hfe) of the transistor so that we can gain match the replacements.
The defective transistor on the left is not available so I'll make my own matched pair out of the two transistors on the right.
These are matched pairs so temperature makes a difference in their performance. Putting the cases together helps this issue.
Using heat shrink tubing will keep the transistor casings in contact with each other.
The image below shows the defective dual transistor and the gain matched transistor that was constructed from two separate transistors. .
This Sansui AU-9900 now has both phono channels working properly on the bench. Sounds good with music also!
Before this Sansui AU-9900 restoration is complete the amplifier bias/offset should be checked and adjusted by the service manual.
Now it's time for this Sansui AU-9900 integrated amplifier to be bench tested for power output and distortion.
The top covers are reinstalled. This Sansui AU-9900 repair and restoration is complete. Time for some music.