Need help with Entech 205.2 / OPA2134

[shon]

I have an Entech 205.2 Number Cruncher DAC that has developed a problem with one channel - at first I thought I had a problem with the gainclone that I am building, but now I know it is the DAC.

Researching a little inside the board, I found a Burr-Brown OPA2134 dual op-amp. The voltage from V+ to V- is an unwavering 30.55 VDC, well within the tolerances and believalbe (since I do not have a schematic, and the board is multi-layer, e.g. very difficult for me to read). I hooked up a little sound "probe" so that I could tap into the IN and OUT sound of the BB chip and ran it through my headphone amplifier. On the A side, both +IN and OUT sound great. On the B side, +IN sounds great, but I get no output on the OUT.

So I suspect there is either a problem with the B channel itself on the BB chip, or somehow a supporting device is bad. The 4 resistors closest to the chip (which I suspect form a divider) all measure equal at 2.05k ohms. Just a bit further out things get a bit quirky (and I know I am on a slippery slope trying to measure things while mounted on the board, but really it's all I have right now...) R9/R11 are marked 20K ohms, but on the good side, the reading is some 16K ohms (so there is *something* else affecting the reading), and on the bad side the reading is 20K ohms (so there is *nothing* else affecting the reading). And then finally I measure the resistance from OUT to -IN (to try to gauge the feedback resistance) and on the good side it is 26M ohms but on the bad side about 77k ohms.

I am going to try to find a source here locally for the chip, and replace it and see what happens, but hopefully someone more knowledgeable than me can provide some information or insight on where/how I should look to better pinpoint things.

Thanks in advance.

[David Nessim]

Shon,
Can you make a small drawing of the circuit? Also, is there a capacitor in the feedback loop or on the output? You could have a shorted muting circuit on the output. The chances are that if you measure correct resistances on the four resistors, they are probably very ok. Try measuring the output terminal on the 2134 (pins 1,7) and see if both are half way between the rails. To verufy chip death, you can time being use NE 5532, 4558 and varients, LM 833, even LF 353 or TL 072. All will work and should be available everywhere. If they work, there is the answer. If not, your 2134 is probably ok. But I suspect it is not.
Best of luck!
David