Behringer Tuner Repair Observation

[Incubus]

I'm in the middle of trying to repair my Behringer TU-100 tuner, and I've "sort of" been able to make it work.

I saw a video on youtube which has the Behringer and a Boss TU-2 side by side, and the behringer works perfectly.

The link is here:

http://youtube.com/watch?v=BxUgZKv6i38

Any signal that I input reads lower than it actually is. When I inject a 110 Hz sine wave, it reads about 40 cents too low....it does manage to read it as an "A" however.

The thing I've found is that if I reduce the supply voltage from 9 volts to 6 volts (variable power supply), it reads 110 Hz perfectly in tune. As I reduce the voltage from 9 to 6, the LED's creep up gradually until when I get to 6 volts, it seems to work. When I try to inject frequencies for the other strings with supply voltage at 6 volts, they are also in tune.

When I check the VCC's of the chips on the board (an LM358 and an ATMEGA8 chip), with 9v supply into the pedal, VCC of both is 5 volts and with a 6 volt supply going into the pedal, the VCC of both is around 4.5 volts.

My question is, can anyone think of anything that may be happening which would account for this ?

If I had an identical pedal that actually worked, and if I reduced the voltage on that pedal, would the reading also creep upwards as I reduced the supply voltage ?

I've checked the signal going into the ATMEGA8 and as I reduce the voltage, the signal stays at the same frequency, so it's not that.

I'll keep digging anyway...but if you have any ideas, feel free to post.

[Roobin]

I've got a Behringer TU600. Not sure what the differences are (new chip?), but mine works perfectly well. Dumb question, and not trying to sound patronising, but are you sure its calibrated to 440Hz? When you plug in, does it display a '0'?

[Incubus]

Yeah...it's set to 440 and displays "0". I just can't think what is being affected by reducing the supply that would make the reading change.

I didn't know that behringer came out with another pedal tuner.

The one I looked at just now was the TU 300...how long ago did you get yours ?

I guess they addressed the problem, but I'd still like to fix mine.

[Roobin]

Sorry, I meant the TU300  Mine has the date code of 0703 on the bottom.

[stumper1]

How old is your pedal?  Behringer has been very good about replacing defective product - sometimes even after the warranty expires. 

[m-theory]

Sounds like a perfect excuse to upgrade to a Peterson!