AX-900 Preamp Section Problems

[absolve2525]

When I connect the preamp out-main in jumpers, the DC offset jumps from below 10mv on both channels, to 50mv on one side and the other side bounces all over the place, going up and down to 250mv erratically. Weirder still, when I press the CD Direct button, the main relay clicks and cuts power to the speakers. When I remove the jumpers, the amp section is fine. Any suggestions would be helpful. Thanks.

 

[avionic]

Pre-amp issues. If you have a seperate pre amp you can verify that the 900's amplifiers are squared away.

 

[avionic]

As for the 900's pre-amp. You can monitor the pre-amps DC offset at the pre-out jacks.. Look around for the obvious things first . Like cracked solder connections. If this model utilized thin copper buss strips mounted above the circuit board. Check there solder connections. TO-126 and TO-220 transistors run warm/hot. Check there solder connections as well.

 

[clinic-audio]

Hello

If I good understand you change the 2 x 2SA970GR for nothing , some days ago ?
Check what voltage you have on each channel at the preamplifier output sockets ? See if any DC offset change when CD direct knob is activated

 

[avionic]

While monitoring the DC voltage out of the pre-out jack. Tap around the circuitry with a non-metallic probe. This can assist you in locating bad intermittent solder connections.

 

[absolve2525]

Thanks for the replies. Yes, I did swap out the the amp section differential pairs for KSA992, and it lowered the DC offset of the amp section, when separated from the preamp. The preamp still has issues though. It's behind the front faceplate, so I will remove that and inspect further. Where do I hook up the leads exactly to test preamp only DC offset?

 

[avionic]

Where do I hook up the leads exactly to test preamp only DC offset?

The pre-out jack on the rear panel. Red test probe to the RCA center pin. Black test probe to the metal outer shield of the jack. Start with the channel with the erratic DC offset.

 

[absolve2525]

Great! I will check it out and look for cracked solder joints.

 

[absolve2525]

I took off the front panel, and tapped around the board for loose solder joints. I didn't see any there, or on the main amp board. I'm getting 7v(!) on the left channel, preamp output with jumpers pulled. The right channel measure near 0mv. So something is up with the left preamp channel. Any suggestions on what might be causing it? Thanks

 

[zaibatsu]

Unsolder and swap Q105 and Q106 (both 2SC2878), then see if your 7v offset moves to the right channel.



If the offset moves, it means Q105 was bad, replacement here.
If the offset stays on the left channel then don't mind me.

 

[absolve2525]

I will give that a shot! Thanks for the reply.

 

[absolve2525]

I received the new transistors today from B+D. I swapped out the old Q105 and Q106 for new 2SC2878 replacements. Success! DC offset on the bad channel has gone down from 7v to around 10mv. Thanks so much guys!! I'm excited to finally do some listening with this amp! I just have to put it back together. Can't wait.

 

[zaibatsu]

 

[absolve2525]

Put back together! I've seen some photos of this model with nice lacquered wood side panels. They really make it pop. I'd love to find or have some made some day.

 

[JAK]

Old thread but dude that 900 looks sweet