Joined: Mar 15, 2010
|Posted: Thu May 06, 2010 3:44 pm Post subject:
Problem with FS1a
|I have recently built an FS1a but it rapidly developed a strange fault that I cant seem to get to the bottom of.
After the build and setup i noticed that the UP output out was very loud and distorted. I used a sine as an input and on investigation I found the SumOut was just a squarewave output switching from +/- 13.5V all the time instead following the diffOut which goes from a few hundred Mv up +/-12V when it starts to clip depending upon the input. I traced it back to the pin 12 output of U17B LM13600. Only when the input is at a very low level a few hundred mV does the SumOut have a positive sine top and a flat -2V bottom as is was almost half wave rectified, any more and snaps back to a full output squarewave.
I checked the inputs on pin 14 and 15 and these are about +15mV to -20mV (there a little bit of negative DC offset) and clearly shows the two outputs from the 1496's fine and out of phase. There is -12V in to pin 16 and 1 from the compander section and that is working all OK.
The other half of the LM13600 for DiffOut is working fine, so I have no problems with the DOWN output. The whole thing had been working OK for about 30 minutes or more after which the problem occured, which pretty much rules out and wrong value resistors as it would not have worked at all with that issue.
To see if was the output of the LM13600 or the op-amp that was causing the problem I cut the track that joins them and still had +/-13.5V on the output of the LM13600. So I replaced the LM13600, everything worked OK and the output was normal level again. I ran thru the calibration again then the problem reoccured after about 30 minutes, exactly the same problem of the LM13600 acting like a switching comparitor rather than an amplifier. I have checked all the components, joints and signals in and out, I even replaced the LM13600 with a third one but this time it did not fix the problem. I have adjusted R127 and R133 but to no avail. The oscillator output is balanced and all the other calibrations are as they should be.
I have come to a dead end as to why this half LM13600 would be do this hard switching instead of amplifying. THe only thing I can think of is the phase of the inputs but that would be determined by the design which obviously works. Do you have any idea as to why this maybe occuring??
UPDATE : Problem found, dry joint on a resistor shunt in the LM13600, Thank god for freezer spray !!