Denon DN-X100 Phono Inputs stop working after some time

[KRiSh05]

Hi !

2-3 years ago i fixed a Denon DN-X100 by replacing the 2 channel sliders and also the crossfader.
The service went smooth and testing the mixer using Line Inputs for some time seemed perfect.

Now my friends took it back to me again for repair, because they tested it with Phono Inputs and they said something about them was KO.
It took me some time to borrow a Technics 1210 to test the Phono Inputs and when i did initially it seemed everything was fine. But then I was very surprised.

It took around 1, maybe 2 hours of continuous use for the issue to show up.. :

at some point, the audio simply stops. The mixer don't show any issue, all commands respond, but simply the audio coming from the Phono Inputs stops.
During the issue showing up, i swapped the Technics on Channel 2 and the issue was still present, so when it happens it influences both Phono Input channels.
Powering Off the mixer and then Powering On (Hard Reset) even without waiting seconds before powering on, magically fixes the issue !

1) What kind of issue can this mixer have where suddenly BOTH Phono Inputs stop working at the same time and powering off/on fixes the issue ?
Of course the issue showing up is totally random. It may happen after 1 hour or 2 or 3...

I attach the full Service Manual which is done good and has schematics and parts list.

2) Looking at the schematics where would you start by debugging this kind of issue ? Which test points you would check ?

3) Considering the issue is random, what is a good methodology to catch this kind of issue when they show up ?
(a turntable cannot play vinyls for hours, and anyway the mixer must be open without the fader board installed to be able to access electronics of Phono Inputs..)

I still need to take some time to analyze schematics in deep. I have a Tektronix dual channel scope (2215 60mhz).
Judging by how the issue is random and what it is i tend to suspect something about overheating ?
some kind of overloading that happens in some digital lines and somehow affects both Phono Inputs ?

mmmmh.....

Thanks !

 

[ccaudle]

When phono 1 and phono 2 inputs stop working, is it on both L and R channels, or just one or the other?
Did you check the line inputs after the phono inputs had stopped working? Do you know for sure it is only the phono inputs, or could it be all inputs?
Do the cue paths to the headphone output work? If so that indicates that the input phono amps are working, and the problem is downstream from the cue split. If not the problem is upstream (or at the point where the cue split is taken).
When the problem occurs does the talk over switch still work to let a microphone signal go through? That would tell you whether the problem was in the output circuitry or input circuitry.

The mixer is controlled by a microcontroller, that would get reset when switching off and back on quickly. The microcontroller controls VCA devices that actually handle the level control and cross fade. If the problem is in the microcontroller fixing will be tricky, but you should be able to see that by checking the control signals to the VCA devices.

 

[KRiSh05]

@ccaudle wow ! thanks for the excellent debugging proposals. I will try to setup the mixer with the turntable + line inputs + microphone connected and do all the tests you proposed. I thought too about testing the Line Inputs as well. I did some of these tests couple of months ago but my memory about them vanished so as soon as i do them again i will note down everything and come back here with a solid report. Thanks again!