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Content Management System

Fostex Line Mixer Model 2016

Front Panel

On the bench is a line mixer with a intermittent right side of the the 2nd output.

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Interesting design inside, very late 80's...

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Comments

Boswell Thu, 07/01/2021 - 08:58

By "intermittent R side" are you saying that the audio is sometimes present and sometimes not? I assume the second photo is showing the L channel passing signal but nothing on the R channel.

Does flexing the boards, cables and connectors bring the R channel back to life, or does it suddenly return on its own?

Do you have schematics? Can you tell from scope probing whether there's a point in the R channel signal flow where, for example, the signal is stuck at a power rail value (saturated)?

Link555 Fri, 07/02/2021 - 06:49

In reply to by Boswell

Boswell wrote: By "intermittent R side" are you saying that the audio is sometimes present and sometimes not? I assume the second photo is showing the L channel passing signal but nothing on the R channel. Does flexing the boards, cables and connectors bring the R channel back to life, or does it suddenly return on its own? Do you have schematics? Can you tell from scope probing whether there's a point in

From What i can tell the problem is at the final output amplifier for the right side of the 2nd bus output.

Spot on Boswell, yes the channel returns if i wiggle or touch the main channel 2 output pcb the signal returns.  I thought at first it was cable problem, but I can not pin point which cable is causing the drop out. Also when I increase the input signal to the board the R channel returns to the output.  Could it be a return path issue....?  Flexing the board seems to have the same response as touching the pcb, the signal returns. 

 

I do not the schematics, nor can I find them.

 

Boswell Fri, 07/02/2021 - 07:53

From how you describe the problem, I would be inclined to think it's a dry joint or a faulty component rather than a cable.

If you can get it to stay in the failed state, it should not be too difficult to chase the signal through the circuit using a scope. Send the same input signal to the two output channels, then you would have something to compare against as you walk through.

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