644 group 1 always on

clixor

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Jun 20, 2020
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Gear owned
Tascam 644
Hi guys,

I finally got time to properly testdrive my 644 during the holidays. I had already replaced the belt and i must say this is one fine sounding machine!

I do got one (major) problem. When i route channels to group 1/2, group 1 seems to be always giving output, regardless if i have the sliders at zero. Group 3 and 4 don't show this behavior, it only outputs sounds when i turn up the (group) sliders.

In the matrix screen group 1 is also showing output when the slider is turned down.

What's worse is that if i turn UP the group 1/2 slider, the sound seems to add to the already present group 1 output giving some balancing and phasing issues.

Needless to say this is a bit of a bummer as i can't properly use group 1/2 and have to route everything through 3 and 4.

Hope someone can help me, as all inputs work fine otherwise i was thinking to investigate the slider part of the schematics.

cheers, Bas
 
I can not say what is causing this without seeing the unit. The group 1/2 fader may have a disconnect to ground so that signal is always there on the fader. People sometimes put heavy items on top of mixers as if they are made for this which they are not. I have seen faders have the phenolic element pushed out the back and also some that were cracked which required the fader be replaced. It could be as simple as a bad solder joint as well. These deck were not made with aerospace grade soldering and I have seen some consumer products that I repaired without any reading of the complaint as solder joints being broken was almost 100% the problem. These were Fisher receivers probably made by the Chinese. That is the kind of work they do. The final answer would be found by going in and checking the item with a scope and schematics on hand. The best service manuals come from Stereomanuals.com , that is where I get the ones I do not have already.
 

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