I have a Pronomic PM-1002FX powered mixer used for band rehearsals (it is a cheap crappy mixer, I know, but is ideal for rehearsals).
It was used regularly until last March when Covid put a stop to rehearsals; it was working perfectly.
We managed to get a rehearsal in September, plugged the mixer in and got horrendous hum with all levels (including master) at min.
Took it home, set it up on the bench with nothing connected except speakers, and got full volume hum with masters at min.
Dismantled and checked a few things over and found 30v P-P 50Hz on the main outputs (both channels). Checked the PSU and it is smooth enough. I have disconnected the channel boards, just have the FX, EQ, sub masters and masters. Any ideas where to check?
This isn't a ground loop problem because there is no loop; single mains lead to mixer and no inputs connected.
It was used regularly until last March when Covid put a stop to rehearsals; it was working perfectly.
We managed to get a rehearsal in September, plugged the mixer in and got horrendous hum with all levels (including master) at min.
Took it home, set it up on the bench with nothing connected except speakers, and got full volume hum with masters at min.
Dismantled and checked a few things over and found 30v P-P 50Hz on the main outputs (both channels). Checked the PSU and it is smooth enough. I have disconnected the channel boards, just have the FX, EQ, sub masters and masters. Any ideas where to check?
This isn't a ground loop problem because there is no loop; single mains lead to mixer and no inputs connected.
sound like output stage failure i.e. bad output transistors, but could also be an open or loss of ground.
do you have a DIM bulb tester available?
any DC on the output?
do you have a DIM bulb tester available?
any DC on the output?
This with the output stages completely disconnected (and power unplugged from the output board).
The 30v 50Hz is on the line level output from the mixer board, detected with an oscilloscope.
The 30v 50Hz is on the line level output from the mixer board, detected with an oscilloscope.
well that's certainly odd makes me wonder if there's a fault with the low voltage supply? such as open filters....or a failed diode...what does the waveform look like?
after a quick net search it may be hard to find schematics to help with the troubleshooting...
after a quick net search it may be hard to find schematics to help with the troubleshooting...
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without a schematic i would think that zener's are used for low voltage supply regulation and there's open filter caps... being that you say the voltage is 30v that would lead me to think that there would be two 15v zener's and a bipolar configuration to supply the op amp rails.
without being there (or a schematic) all i can do is present my thinking/suppositions.
without being there (or a schematic) all i can do is present my thinking/suppositions.
Check the voltage at pins 4 & 8 of the op-amps with respect to ground.
There should be two simetrical voltages.
There should be two simetrical voltages.