Allen & Heath Mixwizard Channel 5 very low gain

I have an A & H Mixwizard (not a 3), it worked fine the last time I used it, but last night channel 5 started acting up.
Everything seems to work ok except:

The gain is really low.  It requires full gain, no pad.
The gain control does control the gain, it just is very attenuated.
The pad still seems to work normally, making the gain even less.
It does it on both the XLR input and the 1/4 input.
There is a faint very high frequency noise.
Exercising the gain control, pad button, channel in/out button, 100hz roll-off all have no effect on it.

Thoughts?

Wild guess: gain pot? (Since you can't control the level/gain anymore. If the connection to the gain pot gets 'open', the gain of the amplifier stage will be dramatically lower.)

The gain pot still works, the gain is just much lower than it should be, but turning it down lowers it even more, could that still be a bad pot?

No, not likely. I overlooked the fact that you could still control the gain, my mistake.
Usually the input circuit of an active balanced microphone input looks more or less like this.
(Most of the time they also use this circuit for the line input, after a lot of attenuation of the line signal.
Not ideal, but that's how it is often done...)

Here is a schematic from an A&H mixer, you can see a lot of similarities.
I suppose your mixer will use the same input circuit.

By the way: what happens if you inject a signal into the insert jack? If the level for channel 5 is the same as for the other channels, there is certainly something wrong with the input circuit. But if the signal is still lower, the problem is after the input circuit.

Well, go figure, I pulled it out of the trailer and powered it up, channel 5 is perfectly normal, I tried all 16, no change from one to the next. I tried the xlr with a mic, and an odd thing happened, 5 was fine, 4 was fine, 6 was really low, a turned up the gain and it got better, but not bad pot better, it ramped up slowly, or at least slower than a bad pot making connection suddenly. This was like I turned the knob all of the way up, and when it was nearly all of the way, the gain came up over the course of a second or so, and I wasn't turning the knob any more, I'm going to keep messing with it.

wilsonsk said:

..I'm going to keep messing with it.

Click to expand...

described symtoms more likely are, you would be messing with a broken mic cable instead ...

No, yesterday we tried literally 5 or 6 mic cables on channel 5,  today I grabbed from a completely different case of cables, highly unlikely that many cables are bad. Also, it did it yesterday with 1/4" in the high impedance input too, I tried running the break music through it, same symptom, super low gain, but variable with the gain control. Now today I briefly seemed to have the same symptom on channel 6.

Loose ribbon cables?  But I don't know if that mixer uses ribbons...just a guess.

Bri

I went back down to try it again, same thing with channel 6, set the gains on 4,5 and 6 a 6 o'clock position(no pad), channel 6  noticeably lower than 4 and 5, turning the knob almost wide open and it came up, more abruptly this time and a kind of  rumbly pop sound. After it came up it works fine, just like it should, no scratchy sound, absolutely normal.  Again, the issue last night was channel 5, and no amount of gain knob turning would correct it, and I am sure it was 5, channels 1-4 and 6-16 were in use.  This is really baffling.

Problem is back on 5, exactly like it was last night, attenuated gain, but teh gain control works, high frequency staticky type noise when the gain control is maxed out to try to get some gain. The other channels are very clean.

This may drive me insane.

wilsonsk said:

Problem is back on 5, exactly like it was last night, attenuated gain, but teh gain control works, high frequency staticky type noise when the gain control is maxed out to try to get some gain. The other channels are very clean.

This may drive me insane.

Click to expand...

What Brian says above -- loose ribbon cable inside the mixer. Open it up and start poking around.

That seems to make sense, This morning channel 5 is still bad, 6 is fine. I will open it up and take a look.  Does anyone have images of what it looks like in there? Anything in particular I should look for?

Thanks,

Steve

wilsonsk said:

Anything in particular I should look for?

Click to expand...

Look for ribbon cables and any other kind of board-to-board interconnect.

The connections are all in a separate enclosure, it can be moved so that the connections are on the back or the bottom of the board, and inter connected completely with ribbon cables

Here is a pic of teh board opened up, you can see the connection portion at the top.

I got a service manual from A&H, 4 channels  share a ribbon cable to the connector board.  so 5,6,7 & 8 are on one ribbon. I will get some Deoxit and have at those, is makes sense I think, the way 6 was acting. I'm hoping anyway.

Check the insert socket.  Look at the schematic, as to whether the insert has normally closed switched contacts when no plug is inserted, (this is the usual configuration). Either clean the switch terminals, or make a plug with the in/out linked to check that this isn't breaking the signal. If the insert is a single TRS in/out make the link by bridging the tip and ring on a jack.

Plus one the insert jsck failure make a cable to bypass the switches in the jack

Never thought of that, I have had that problem on a couple of guitar amps. I will give it a try.