Au-3900 restore

shamrock.

Active Member
I picked up an Au-3900 as part of a cheap deal with a bunch of stuff

the lid has a bit of surface rust which looked a bit ominous but I plugged it in anyway and gave it a burst .

Everything worked and sounded pretty good. I did an ab with a fully recapped and restored (by someone else) Au-505 that I own .
The weatherbeaten original old au3900 had a lot more mojo than the restored au505
it sounded way better in the low end and lower mid range.
They have similar power output specs, but the 3900 also seemed to be more powerful.
The only negative with the 3900 was that anytime the vocalist sang s or t it sounded really distorted and bad.

I was just going to flick the amp as is but rethought this as it would be a pretty good amp to do my first recap with. I have started dismantling the unit and cleaning it up.
To remove the indicator plug from the front panel can I just pull it until it comes out (Ive put up a photo), dont want to break it.

Ill respray the top, and look to replace the caps,clean everything up inside, deoxit all the pots and switches, remove the transistors and replace the paste on the heatsink .
Anything else I should pay particular attention to?
 

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Yes, the plug just pulls off the l.e.d.

I would relace the 10D1 and 10D2 diodes, specifically in the rectifier bridge. When I first powered mine on, one of the filter caps exploded and I took out the outputs on one channel.

My AU-7900 stopped coming out of protection due to one of these too.

They don't seem to be known as problematic but due to my personal experiences, I replace them as a matter of course.

I used 1N5393 for both.
 
Also replace the 4 fuse resistors on the main amp board and change the bias pots to multiturn types.


Cheers

John I
 
Cool thanks guys, speaking of bias pots I think I should check and set bias before I get into changing out components. I have downloaded a schematic but still havent found a service manual with instructions for setting bias.
If anyone knows where a service manual online is, or can tell me how to set the bias on this Id appreciate?
 
Hifiengine.com should have it.

Free easy registration and your set. The AU-3900 & 4900 share the same service manual.

Sometimes an old bias trimmer can go wide open unexpectedly and blow the amp up. If you're going to rebuild it anyways, it may be safer to replace them before making adjustments. I've adjusted a bunch of amps and receivers using the old pots without a problem, but you never know when it could happen.



Sent from my iPhone using Tapatalk
 
cheers guys I have the service manual now.
I checked out the old trim pots and they seemed ok so I took my chances and checked the bias settings
One channel was close and I adjusted it to 1.3 mv as the manual specified.
the other channel is reading zero and adjusting the pot makes no difference.
Which I found interesting because the amp still plays music equally in both speakers.
I metered the adjusting pot and its fine. Possibly a bad transistor?
 
More likely that the fuse resistors have gone high in value, this will result in low or no bias. Measure the value of R29, R30, R31, and R32. They should each measure close to 220 ohms. You can measure them in circuit, no need to remove them (with there're off of course).

In any case, these should be replaced with 1/4w metal film resistors as the originals will fail.

Cheers

John,

PS - the bias current measurement/adjustment you made was 13mv, and not 1.3 mv?
 
Right on the money Skippy three of those resistors have drifted right out, and yeah bias measurement was 13mv, I dunno where I got that decimal point from
 
If you replace the resistors, don't forget to wind the bias adjustment pots back to minimum before powering on with the new resistors fitted, then adjust the bias to 13mv.

Cheers

John
 
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