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One of my clients stopped by with the power supply from his Neve 8248 4-channel Side Car (channel strips), with the problem that after being on 20 minutes or more, hum would come up, then it would shut down. I had worked on some of his Neve modules back in 2016...stand-alone mic preamp modules, though not from this series. I figured I'd have a look at the power supply first, as that sounded like where the trouble is.
Output connector is a 5-pin Switchcraft A3F, which I didn't have, so had to drive up the street to Pacific Radio to fetch one, then come back and wire up a power supply breakout fixture to connect my loads and meters. I didn't have the documentation on these 81380 modules from the 8248 series console, but, from what I recalled, it was loaded with 8-pin IC's, though don't recall how many. So, I figured each module would draw anywhere from 50mA to 125mA each. I don't recall there being anything exotic in them like their older vintage cousins. I had a connector strip from one of my proto boards back home, and brought that in, it having five Binding Posts that I could wire up to a cable from the Neutrik 5-pin male XLR I bought. Then fetched the box of Dale 50W power resistors I had down the street at my shop, hoping I had what I needed to properly load the supplies. I found a pair of 100 ohm/50W, 10 ohm/50W and three 40 ohm 50W load resistors. I also brought along a large aluminum plate that I often use to set atop open Fender amp chassis' when on the service cradle. Figured I'd wire up the loads, solder on some small banana jacks on the ends, then coat the bottom of the resistors with thermal grease and stick them onto the plate, wiring them up with Pomona Banana Plugs.
Using 110 ohms (100 ohm in series with 10 ohms, both 50W) to load the bipolar 22.2VDC supplies to 200mA each, and a 2.2k/5W to load the 48V phantom supply to 22mA, I powered up the supply and let it run to see if it shut down. After an hour, with no change, and the scope showing just low white noise and no ripple, supply pulling 352mA/30W from the mains, I then increased the load current, swapping out the 110 ohm loads for 40 ohm loads (both 50W), and started over again. 22V into 40 ohms is 550mA each. Started it up again to see if that made any difference, now pulling 620mA/56W from the mains.
I let this run for an hour, with no shut-down, nor any change on the scope's residual noise, full sensitivity 5mV/Div. Each supply channel was dissipating 4W into the 40 ohm resistors. Plate was now quite warm, resistors too hot to leave my fingers on them, and the pass xstr heat sink on the back of the supply was now warm, but nothing had shut down.
So, I suppose the next move will be to have him pick up the supply, and I'll follow him back to his studio...which fortunately is in the same building where my regular shop is, and I can just bring some tools and a power analyzer to see just how much power is being drawn. Then, remove one module at a time, so see if it's a channel strip causing this. Figured the supply would be the place to begin...just didn't yet reveal the problem.
The two power transformers...each being 12VAC C/T, have their primaries in parallel, secondaries in series to provide 24VAC C/T to the power supply PCB. I didn't pull it up to see how the 48V supply was wired up. Onward
Output connector is a 5-pin Switchcraft A3F, which I didn't have, so had to drive up the street to Pacific Radio to fetch one, then come back and wire up a power supply breakout fixture to connect my loads and meters. I didn't have the documentation on these 81380 modules from the 8248 series console, but, from what I recalled, it was loaded with 8-pin IC's, though don't recall how many. So, I figured each module would draw anywhere from 50mA to 125mA each. I don't recall there being anything exotic in them like their older vintage cousins. I had a connector strip from one of my proto boards back home, and brought that in, it having five Binding Posts that I could wire up to a cable from the Neutrik 5-pin male XLR I bought. Then fetched the box of Dale 50W power resistors I had down the street at my shop, hoping I had what I needed to properly load the supplies. I found a pair of 100 ohm/50W, 10 ohm/50W and three 40 ohm 50W load resistors. I also brought along a large aluminum plate that I often use to set atop open Fender amp chassis' when on the service cradle. Figured I'd wire up the loads, solder on some small banana jacks on the ends, then coat the bottom of the resistors with thermal grease and stick them onto the plate, wiring them up with Pomona Banana Plugs.
Using 110 ohms (100 ohm in series with 10 ohms, both 50W) to load the bipolar 22.2VDC supplies to 200mA each, and a 2.2k/5W to load the 48V phantom supply to 22mA, I powered up the supply and let it run to see if it shut down. After an hour, with no change, and the scope showing just low white noise and no ripple, supply pulling 352mA/30W from the mains, I then increased the load current, swapping out the 110 ohm loads for 40 ohm loads (both 50W), and started over again. 22V into 40 ohms is 550mA each. Started it up again to see if that made any difference, now pulling 620mA/56W from the mains.
I let this run for an hour, with no shut-down, nor any change on the scope's residual noise, full sensitivity 5mV/Div. Each supply channel was dissipating 4W into the 40 ohm resistors. Plate was now quite warm, resistors too hot to leave my fingers on them, and the pass xstr heat sink on the back of the supply was now warm, but nothing had shut down.
So, I suppose the next move will be to have him pick up the supply, and I'll follow him back to his studio...which fortunately is in the same building where my regular shop is, and I can just bring some tools and a power analyzer to see just how much power is being drawn. Then, remove one module at a time, so see if it's a channel strip causing this. Figured the supply would be the place to begin...just didn't yet reveal the problem.
The two power transformers...each being 12VAC C/T, have their primaries in parallel, secondaries in series to provide 24VAC C/T to the power supply PCB. I didn't pull it up to see how the 48V supply was wired up. Onward
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