Behringer PMP1000 amp , audio output died in use

Not got inside yet and before doing so, less I disturb something. Inserting a mono jack in one of the input channel I/O Insert sockets, kills throughput to phones and also amp hiss at the phones output. The PMP1280 SM ,out there, does not seem to have a control line attached to the I/O insert socket switches at all , so different there , unless there is some sort of auto lack-of-signal mute circuit somewhere. At the moment I wanted to rule out something silly in the way of a wrong switch setting combination or something. The user manual shows nothing that could cause this.

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N_Cook
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PA and PREA work separately and as normal but low output at normal signal input and control levels. I don't think I've come across this situation before. The ground or common to all the "level set" pots on all the channels is missing presumably. You can only get normal throughput with the pots set in range 95% to 100% of each of their tracks, jumping from very low to max in a couple of mm of movement and all such pots would not have failed at the same time. So inside we go.

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N_Cook

Made 2007 Although the pmp1000 uses 4580 opamps like the 1280, not the same in the prea area. The wipers of the level pots there are at ground, "low" and "high" via Rs/Cs to opamp inputs. The 1000 has stereo pots, the pairs of low-side, wipers or high-side are not connected together, but similarly close connected to opamp inputs. Without a SM this will be a pain. Next thing is check +/-rails for the opamps.

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N_Cook
+/-15V on the "level" opamps, and 48V and 5V for digital. Looks as though I'll have to trace out the schematic around one of these pots and opamps to see what could be common to all the channels, requiring removing all the hardware to get to the other side of the pcb.
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N_Cook

I've just realised I've not seen a Level Set LED light on any channel with any input. No such LEDs on the 2800 schematic. Try and find the common voltage to the transistors or whatever buffer to those on each channel, before removing all the hardware.

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N_Cook

Getting somewhere now. To save desoldering a shielding plate I'd been probing the stereo channels. Problem is with 1 to 4 channels only, so removed shield. +/-15V ok on those opamps. LED supplies are off the -15V line, one zener per channel, giving nominal voltage of about -8.4V, varying a bit on each channel. Just pins 2 and 3 of each of the 4 off level double-opamp ICs show that same negative voltage on each channel, the B opamp of each is normal. Still a mystery what the problem in common is to cause that to each channel, but zeroing in.

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N_Cook

There are 2 simple -8.4V zener drop supplies per channel. One of those

-8.4V is connected to pin 2input and the other to pin 3 output of the opamp, what is that about? Still no wiser what the interchannel coupling problem is either

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N_Cook

PMH1000 schematic is very similar, 2 zener drop per ch, 2 SMPS ICs the same etc I didn't bother checking the pots function as 4 pots would not fail in the same manner - wrong. Logo B in a triangle, blue body, marked 05 C5 02 on the underside. Very iffy resistance at both ends of the track. Used here as simple variable resistors,wiper tied to min gain end , max gain at zero ohms position.

5K Log track, mid track 300R, but last mm or so jumps from roughly 100R to 0R, similar the other end. Last 3/4mm or so very irregular resistance Took apart and cleared out the grease, same patchy response on reassembly. I don't think its wear as all pots the same at both ends (min gain end does not matter here in use as imperceptible). Perhaps the grease reacts with the track surface , scraping back the track surface , seems to make the situation worse. Same patchiness on the part of the track not swept by the wiper. Main problem was perhaps SMPS going into protect, or "digital amps" goung into protect ,as unusually amp was used outdoors in sunshine or failure of a PA-Prea insert switch if one channel failed
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N_Cook

Got those 4 replaced, but that would not make the amp cut out. I'd decided the amp cutting out was due to the dI/dt protection part of the IXDD404 of the driver IC of the "digital" amps protecting, due to bad contacts of the crimps of the internal speaker wires. Then on rassembly noticed the undersides of the 3 main DC supply wires from ps to pa, the unobserved top sides, against flat steel of the mixer were not compressed. Laying over the thin edge of the steel plate vertically dividing ps and pa . Squashed against the steel screen plate , like a guillotine, under the mixer until the red one had its insulation so sqashed into a grove, as to touch ground . Cable now tied down to the vertical side of that divider , not the top.

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N_Cook

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