PelliX
Well-Known Member
I snagged a broken DBX 166 compressor for a couple of tenners the other day. Broken in this case was a big question mark, there were two knobs missing off the front and "something about the meter on channel #2".
The mains flex was a mess of tape insulating exposed wires where the sleeve had been sheered off, a terminal block in the middle, etc. They creatively used yellow tape - nice touch, a bright colour to warn you. Unfortunately, I doubt that was the rationale behind it. Anyway, off that came, together with the wirenuts connecting the mains lead to the transformer primaries. Like most DBX compressors of this type it's always on, no power switch.
The transformer and so on seem OK, though I gave the electrolytics a hard stare. They don't appear to have failed yet (entirely) and feed a pair of 7815/7915 voltage regulators. The positive side reads about 14.8V, the negative side read about 8V, so something it/was off there. I disconnected the output of the negative supply from the rest of the circuit, but that wasn't dragging it down. Ripple on the supply lines looks pretty OK without any signal loading things down. I intend to replace the caps later on anyway, for now they'll suffice. After probing about a bit I found that the -15V rail had climbed/jumped to -14.8V, matching the positive side. Hmmm... wiggled and poke everything... couldn't reproduce the fault. Touched up the solder joints on the regulator, checked the little film caps across the rectifier.. all fine. My guess is that a cap in the negative section of the power supply was struggling and a little juice and warmth brought it back to life. I don't trust the LM7915, though but I also don't have a spare strangely enough (!).
The pots for the 2nd channel output gain and peak threshold are destroyed anyway, they got forced into the chassis, snapping the wafer of the pot. They're also unmarked. Found a schematic, but the photocopier must have been on its last legs and while I've found that one is a 20K linear pot, the other is "6K7 log". What, a 6K7? Yup. I removed the broken pots and cracked them open to measure the carbon track - 20 and 6.7K. Well, that's going to be fun to track down.
Apart from that the whole unit is basically just a lot of LM339 DIL's, TL072's and a couple of 4558 opamps. The switches were dirty but after a little cleaning all working again.
I'll drop some pics here when I've received the replacement pots and parts. Nothing else looks fried and channel #1 is "OK", channel two is a bit hard to evaluate with controls missing.
The mains flex was a mess of tape insulating exposed wires where the sleeve had been sheered off, a terminal block in the middle, etc. They creatively used yellow tape - nice touch, a bright colour to warn you. Unfortunately, I doubt that was the rationale behind it. Anyway, off that came, together with the wirenuts connecting the mains lead to the transformer primaries. Like most DBX compressors of this type it's always on, no power switch.
The transformer and so on seem OK, though I gave the electrolytics a hard stare. They don't appear to have failed yet (entirely) and feed a pair of 7815/7915 voltage regulators. The positive side reads about 14.8V, the negative side read about 8V, so something it/was off there. I disconnected the output of the negative supply from the rest of the circuit, but that wasn't dragging it down. Ripple on the supply lines looks pretty OK without any signal loading things down. I intend to replace the caps later on anyway, for now they'll suffice. After probing about a bit I found that the -15V rail had climbed/jumped to -14.8V, matching the positive side. Hmmm... wiggled and poke everything... couldn't reproduce the fault. Touched up the solder joints on the regulator, checked the little film caps across the rectifier.. all fine. My guess is that a cap in the negative section of the power supply was struggling and a little juice and warmth brought it back to life. I don't trust the LM7915, though but I also don't have a spare strangely enough (!).
The pots for the 2nd channel output gain and peak threshold are destroyed anyway, they got forced into the chassis, snapping the wafer of the pot. They're also unmarked. Found a schematic, but the photocopier must have been on its last legs and while I've found that one is a 20K linear pot, the other is "6K7 log". What, a 6K7? Yup. I removed the broken pots and cracked them open to measure the carbon track - 20 and 6.7K. Well, that's going to be fun to track down.
Apart from that the whole unit is basically just a lot of LM339 DIL's, TL072's and a couple of 4558 opamps. The switches were dirty but after a little cleaning all working again.
I'll drop some pics here when I've received the replacement pots and parts. Nothing else looks fried and channel #1 is "OK", channel two is a bit hard to evaluate with controls missing.
