channel problem with Tascam M208

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ozpata

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hey guys , I just got my mixer back from a friend who borrowed sometime ago, the machine is clean and everything works fine but the channel 8, if I crank the trim knob to max with the fader in the normal position (between 7 and 8) I can see the needle moving but it is very weak in comparison with the other 7 channels, I took the fader down and it was clean as the rest of them but I can see the needle moving if I jiggle it a bit, if I turn the trim down just a bit I don't see movement on the needles, the other 7 channels work perfectly fine .

any ideas ?
 
With headphones on, are you hearing any abnormal sounds beyond the normal electronics hiss for that elevated level? Stuff like humming or excessive intermittent static?

If so, there might be a bad cap in there somewhere or a bad ground.

But, if a signal passes through cleanly at more normal trim settings, then I probably wouldn't be too worried about it.

Cheers! :)
 
if I put it at let's say 75% of the trim in all first seven channels I can hear it perfectly and needles move but in channel 8 nothing, no sound at all
 
Ah OK. Now I get the picture. You're saying channel 8 is essentially not passing any sound and just a tiny bit of meter movement if you absolutely crank the trim control. Is that a correct description?

If so, you might try a couple of simple things like pushing the Mic/line input switch in and out several times. An oxidized contract on that switch might be the problem. You can also try inserting a TRS plug into the insert jack of that channel a few times to exercise/clean the jumper contact, which also if oxidized, will prevent the signal from passing through the channel.

Try those two tests and let us know if that did anything, and if not, we can look deeper.

Cheers! :)
 
ok, I have tried that and no improvement if anything the signal appears to got weaker as now I can't see movement on the needle at all, still can barely hear the sound coming through on the headphones, as I said I tried on the other seven channels at the same settings and everything is fine, but channel 8 just that barely audible sound, of course that if I cranck the monitor volume up I can hear it perfectly, if I rock the fader slider for channel 8 up and down just a little bit, the needle moves as a reaction of the movement, not the sound, I have tried with a guitar, mic and keyboard.
 
OK. So we have ruled out the possibility of dirty input switching, dirty internal jumpers on the insert jack and now know that the problem is there whether its the Mic or line which is being tried.

So, that leaves us looking further down the signal chain which leaves us with the EQ section, the buss assignment buttons and the final channel fader stage. Looking at any of those is going to require opening up the mixer and having the technical know-how to trouble shoot defective components and replace them. If you don't posses those skills, then its time to seek out a local repair shop and have them find the defective/blown part(s) and replace them.

You said that you had loaned the mixer out to a friend. How was the mixer being used by him during that time? Was he using it for playing live in a bar/club? Overloads during live use are pretty common from things like microphone feedback that go unchecked for too long before some can get to the fader to turn the volume back down. I can't say if this what happened in this particular case. Just saying its a likely possibility as these mixers are pretty reliable and tend not to die unless abused.

Cheers! :)
 
thanks for the help,
he used the mixer to do some audio at his house, some mix down and have to say it was given to me back in a very clean state, he used mostly the first four channels but did mention to me one day that the channel 8 sounded kind of low which I thought it was just dirt or something silly.

what will be required to trouble shoot the areas that you mention?, I have done some repair long time ago on some cassette and vcrs decks, but I don't have specialized tools
 
What will be required to trouble shoot the areas that I mentioned would be first of all the service manual for the mixer which contains the schematic diagrams and parts lists and at minimum a meter to test for signal continuity, specified voltages, open and shorted components And the technical experience to know what you're doing. The fact that you're even asking what's involved tells me that you don't posses this knowledge...no offense intended.

You have mentioned that the channel fader if jiggled can cause some meter movement/noise. It's possible that the insides of it are gummed up with dirt or rust or even possibly damage from fluid entry such as a drink that accidentally got spilled into it at some point. You could try getting some fader cleaner product and attempt to clean the insides of that control. And if that doesn't fix the problem, then its time to take it to a qualified repair shop and have them look at it.

Cheers! :)
 
the fader is clean as the rest, I took it apart because that was where I thought the problem was, and did the same with a couple more and they looked the same, the last thing that I wanna do is fool around with the mixer, my question was just intended to find out if there was any other test that I could do to try to narrow it down, I know my limitations and always if in doubt , don't touch it, chances are that I can make it worse instead of better.

I will see if I can find somebody around the poconos area that can do it.

thanks a lot
 
my question was just intended to find out if there was any other test that I could do to try to narrow it down
Click to expand...
Beyond what I've offered up, nothing else comes to mind. Sorry.

Maybe someone else here can think of some other things to check for but if that doesn't happen, off to the repairman it must go.

Good luck!

Cheers! :)
 
Have you tried swapping faders from a channel that works? I'd do that first because even if you opened up the fader and it *looked* clean doesn't mean that the wipers might not be making poor contact. If you take, say, the fader from channel 7 and plug it into the channel 8 card and it works, then you can assume its a problem with the fader and then I can tell you what to try with the defective fader. If the faders are soldered in to the channel cards then you'll obviously have to heat up the iron for the test, but I'd still check that first to rule it out because I've had those low profile ALPS faders get punky after 25-30 years...the wipers have to be *gently* tweaked to restore proper contact with the resistive element.

I have to say though that my FIRST instinct was blown cap or blown opamp...maybe the input coupling cap or the first line level gain stage. I'd have to look at the schematics, but the behavior sounds very much like what I dealt with on one channel of my Tascam MX-80 8-channel rack-mount mic pre and it turned out to be a bad opamp.
 
I don't think that I want to fool around with swapping faders while they are solder, but what I have tried was connecting both faders through some wires with alligator clips( don't know if is the right name), don't know if that would do anything but it was just a thought on something simple to try.
 
Upon further pondering, if the fader IS bad wiring in another good fader in parallel won't work because if the bad fader is bad because the wiper isn't contacting then the audio is being shunted to ground and that's the path of least resistance. I think I've got that right. So the audio signal will still go through the bad fader straight to ground which equals "hey...i don't hear anything."
 
ok, here is what happened, after further cleaning ,I was plugin a keyboard to the insert channel and I can hear pretty good , certainly better than yesterday, now if I plug the mic I can hear ok, and in the line in, I can hear it a bit better than yesterday, the needle moves just a little but the clip light is just at the point of blinking and in the headphones it sounds a bit distorted, only in the insert the sounds gets close.
 
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