Wednesday, August 1, 2012

Sansui AU-6500 (***Updated***)

The baby of the infamous "black face" series, the AU-6500 is nevertheless a superb sounding unit if properly maintained.
Front view
I happen to chance upon a unit which look like it was just taken out from the box!!!

First obvious issue is the faulty power-on light.

Rear view

View from top

Next discovery was the unit only sounds great for the first 5min or so, before the high's are reproduced in a off-tune manner with the bass reproduce improperly aka half-baked!

Suspect the issue lies in the pre-amp stage as it sounds just as terrible when I connected the AU-6500 pre-out to the B-2101.

Observed the above issue seem to be prevalent when the volume is above 9 o'clock. Likewise when I listen via the headphones. Hence there could be an issue with the power availability from the power supply board or the audio reservoir capacitor.

I will recap the unit and change any component(s) which are not working properly once I have time to trouble-shoot the unit. 


Finally had the time to look into the above issues.

1st step was to recap the EC(s) only - using proper quality caps in the signal path. Caps used were Panasonic ECA/FC, Rubycon, United Chemicon and a few Jamicon(s) (in the PRE areas). Did not change the EC(s) on the power on relay board as those look to be in reasonable condition.

Previous issues seem to be 50% gone but reproduction seems too grainy when the volume was turn above 9 o'clock with vocals breaking up as well as drop-outs during playback.

Investigating further, notice the transistor on the PSU board was not the original 2SB507 but a Hitachi A771. On paper the A771 seem to be a compatible replacement. And there was a dark brown patch at the feet of one of the diodes on the PSU board. Hence I purchased the 2SB507 (... prefer to the original transistor as much as possible) from eBay. Swapped the original heat sink for the 2SB507 with a much larger one - the original heatsink was even smaller than those for the 7805 in my Philips CD players!!! Also installed small heatsinks to 2xTO-220 transistors on the Power Amp board. Replace all the PSU diodes with the leftovers from the 303 refurbishment eg 1N4004. Bought a new larger value Zener, as the original was no longer available. A slightly larger value Zener should be ok as it is only required during the initial power up. Took the opportunity to replaced the 2 transistors on the PSU board with modern equivalents.

Does the above help to resolve the issue(s)?

Well during the first hour of usage, the breaky vocals were still evident - was wondering if I had been barking up the wrong tree!?!

In the 2nd hour the breaky vocals seem to only occur after a duration, which tend to be further&further apart with more usage.

After about 3-4hrs, everything seem to be back to normal aka sounds decent again!

Will now slowly increase the volume in the followup usage periods to run the old gal in.

Once bedded down, will initiate recap of the non-EC(s) in the pre-power signal path(s)s.

Update the blog again in the near future ... this is gonna be fun!