One Tascam 244 preamp WAYYY too much gain, troubleshooting SOLVED

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AnalogApples

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Has anyone encountered this problem? I’m setting the VU level calibration and putting a 1,000hz tone into each channel.
Channel 2 is slammed in the red no matter what I do with the preamp trim pot. (Clipping LED is lit too) I can’t even get it to go down with the VU calibration trimmer. It happens when I put the tone into the send/return input too. And it records slammed into the red too.

If I pull the fader down and have the preamp all the way down, I can get the test tone to a reasonable level.
Any suggestions would be much appreciated!
 
What happens if you inject the tone at the input jack, but have the SEND/RCV jumper removed? Tell me the overload LED is dark. And how does the tone sound when you monitor at the SEND jack? Is the level normal?
 
When I remove the jumper, VU goes to no signal, no overload LED. Then if I take the Send signal and inject into channel 1, I get a normal VU level in channel 1, and the tone sounds good-no distortion on headphones.
It’s also interesting that the overload LED for the trim pot comes on at the same point for each channel, despite the VU meter for #2 being slammed. I don’t hear any distortion to the signal in #2 either, until I turn up the trim pot.

This is starting to look like my VU meter for this channel is malfunctioning.
 
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Could it be that the circuit that adjusts VU meter sensitivity is messed up? The 1khz tone is not distorting despite the meter being way in the red. The master overload LED is lit, but it’s lit for each channel with that test tone. The trimmer overload LED is not lit. Seems like the internal VU calibration trimmer is broken.
 
Update: if I lower the fader until the VU meter says 0, I still can’t get the VU meter to move with the VU calibration trim pot. My trim pot is not working. Interesting.
 
If you have distorted audio that is hotter than it should be, it’s not caused by a faulted meter amp circuit, if even your meter amp circuit is faulty. There is a buffer amp in the meter amp circuit that effectively separates the meter amp circuit and the audio path.

So the shotgun blast of information has me confused now. Is the signal through channel 2 distorted at the various outputs as well as on tape, or no?
 
The signal through channel 2 is not distorted in any outputs or on tape.
Sorry for all the sudden information, you started me thinking about this in a different way and I thought I should post these observations.

That VU calibration trim pot is not affecting the VU sensitivity at all. This looks like where my problem is. I might try replacing that little internal trim pot. Worth a try?
Thanks for your help with this, Sweetbeats. You’re a kind neighbor.
 
Final update: problem solved. The contact for the calibration trim pot was bent up and totally away from the surface it glides against. I removed that circuit board, used a tweezers to bend the metal back into place, using a multimeter to test that it was working, and reinstalled. I can now adjust a 1khz 0db tone to say zero on the #2 VU. Hooray! Thanks to Sweetbeats for making me think like a technician.
 
Interesting. Okay. Yeah you’re welcome.

So are we talking about VR32? That might be the issue. The trimmer resistors sit in the feedback loop of the meter driver opamp and trim the gain of the feedback loop. They’re a 5K trimmer. Hypothetically if the trimmer was zorched in a shorted failure state the opamp would be running at open loop gain which would much too much. You could always test the theory if you had any 5K resistor handy and temporarily install it between pins 1 & 2 of the driver opamp which is U831.

BUT…none of that would explain why the overload LED is lit. Hm…
 
Oh…missed your last post. I think you posted while I was composing mine.

Good job figuring it out, and cool that’s all it took!

But I’m still perplexed as to how the overload LED was lighting up…
 
There’s the overload LEDs at the master fader, and the ones at each channels trim pot. Only the master fader LEDs are lighting up. I’m honestly probably not supposed to be using a 0dB signal to calibrate the VU meters, that’s probably why I’m lighting those. But there’s no distortion.

I wish I knew where to get an appropriate test tape cassette I can use to calibrate playback levels. Can you suggest where to buy one that’s 9.5cm/second?
Also, should I be using a less hot signal than 1khz 0dB to set the VUs to 0dB?
 
That’s why my master channel overload LEDs are lighting, because I’m using way too loud of a test tone. Thanks for all your help, Sweetbeats.
 
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