OT: Help with Headphone circuit!!

[wokeupfalling]

I have an issue with my mixing board headphone circuit.  The left side sounds great, while the right side sounds distorted and compressed and awful.  I've narrowed it down to the headphone circuit itself, and even swapped out the OpAmp, thinking it was the culprit - but still the same problem.

I am attaching a drawing of the right channel, which is a mirror of the left (but on the drawing I labeled it as the left channel, oops).  I excluded the power area to the OpAmp.

Having a compressed, almost LP filter + distorted signal - Can anyone help me narrow down a possible culprit here??

The Left channel sounds prefect, but the Right...



http://a677.ac-images.myspacecdn.com/images01/43/l_0715691b72db65b52b49af6c063044f4.jpg

This drawn from the original schematic - a scanner would be nice ;-)

[GibsonGM]

No schematic came thru, falling.  I'd start testing components and use the good channel as a benchmark.  Could even be a bad output jack, so use an audio probe in there to listen to the signal...

[wokeupfalling]

Okay, I updated the post with a link to the schematic as well.

I will get in there and start poking around this afternoon, but I was checking to see if the symptoms rang a bell for anyone.

In the manual it states: "CAUTION: Mono Headphones will cause Circuit Failure" "2 wire (mono) Headphones will short out one of the amplifiers driving the headphones, which will cause it to burn out."

I assumed that was what happened, so I replaced the 4560 - but the problem is the same.

This is on an old Tascam M308 Mixing Board, FYI.

[GibsonGM]

Check for voltage on the IC and at that power rail, then audio probe thru the circuit, man...put on some tunes, start at the 10uF input cap, check the output of the opamp.  See if you get sound.  I'm assuming you know how to make an audio probe? If not, search the forum for 'how to', it is easy, just a probe with a cap in the signal path to block DC.  If no audio at the output of the IC, check the resistors and caps associated with it - a problem around the opamp would produce distorted crappy output.   If you get any audio (clean w/no distortion) after the opamp at the 470R resistor, on to the next phase:

Next phase, it's likely either a transistor blew, and/or those diodes, so get checking with the multimeter.  Post voltages on the bases, emitters & collectors here; and compare them with the readings on the working board.

   It's a pretty simple circuit, so it should be easy to fix.  The transistors take the relatively weak audio signal produced by the amp IC, and boost its power to drive headphones.  So, using an audio probe with an amp will let you hear signal.  Look out for any high voltages in there! 
Post back, this should be a pretty easy repair.

~Mike 

[wokeupfalling]

Awsome, thanks for the help!  It was the 1 ohm resistors - they were WAY off!

All fixed!

[GibsonGM]

Glad to hear it was actually that simple!!  Luckily they failed before the transistors.
Cool!