A friend of mine brought a Denon PMA-980R in for repair. He wanted to upgrade the sound and swapped opamps. Unfortunately he used a single opamp instead of a double and now the amp is not working anymore (turns on etc but no sound).
The opamp he replaced is IC402, a 2082 (right top of schematic). The single opamp blew up. Any ideas what might be wrong with it? I'm trying to understand the function of IC401/402 but is it some kind of DC-servo?
The opamp he replaced is IC402, a 2082 (right top of schematic). The single opamp blew up. Any ideas what might be wrong with it? I'm trying to understand the function of IC401/402 but is it some kind of DC-servo?
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More likely a mute, power on delay or protection circuit. The optocoupler looks like it will pull TR 521/522 base to the negative rail.
It turns out two resistors are blown, R841 and R842. It looks like those are the inputs for the +/-16V power supply. Will change them tomorrow.
Replaced the resistors and the amp worked but started smelling funny after a while. Checked the power supply for the opamps and this was +/- 50V!
The transistors in the low voltage power supply are also blown. New ones should arrive tomorrow.
The transistors in the low voltage power supply are also blown. New ones should arrive tomorrow.
Make sure the zeners in the psu are Ok otherwise you could get +/- 50 v again. If you can connect the psu without any load and test it, that might be worthwhile as it would indicate whether the fault that is causing these issues is downstream in the low voltage area. A short in the low voltage circuits would drive the pass transistors to ground and they would basically fry themselves.
Yeah, I managed to remove all connections between the powerrails and the opamps (either by removing the opamps or the cables). Will test first before connecting them again.
I'm a bit surprised the opamps didn't blow and were actually working. +/-50V is way out of spec...
I'm a bit surprised the opamps didn't blow and were actually working. +/-50V is way out of spec...