Behringer DJX900USB power supply repair / maximum current - Page 1
Forum section: electronics / repair

author: frankvnk
author_rank: Newbie
timestamp: April 26, 2025, 12:29:06 pm
content: I have a Behringer DJX900USB DJ mixer in which the power supply failed.It turned out the transformer primary winding failed. I disassembled the transformer and noticed the thermal fuse embedded in the windings was blown (2A - 250V - 115°C). Since it is a linear power supply, i replaced the transformer with two toroidal transformers (2x15V and 6V) and powered the device up. For about 30 minutes, everything seemed to work fine but then the +15V rail failed.I noticed both the +15V and -15V regulators have a 1 ohm 1/4W resistor in series with their input. The one in series with the +15V regulator failed. I replaced both resistors, powered the device again and measured the current for each supply rail (all switches on the device set to on):+15V @ 600mA-15V @ 600mA+5V @ 550mAIt is no surprise the 1 ohm 1/4W resistors fail as they have to dissipate 360mW.I know this is a simple way to prevent damage to the electronics in case of overcurrent.As i don't know the average current consumption for each supply rail, i wonder whether there still are issues with the device.So, i would greatly appreciate it if anyone owning a DJX900USB is willing to open his device and measure the current for each supply rail.EditMy DVM played tricks on me (battery depleted). I measured the supply rail currents again, this time with a fresh battery in my DVM.+15V @ 510mA-15V @ 510mA+5V @ 550mAThis means the 1 ohm 1/4W resistors are pushed to theirr limits as they have to dissipate 260mW.Should i keep this as is?I found the schematics for the DJX700. This one is similar to the DJX900. The 900 has an USB interface and slight modifications in the power supply: addition of capacitors parallel to the bridge rectifier diodes and the 1 ohm resistors before the regulators. An original replacement transformer fits both.

author: MathWizard
author_rank: Super Contributor
timestamp: April 29, 2025, 05:35:35 am
content: I don't like that either, that's too much power. Depending on how much voltage overhead there is for the regulators at full load, I might just add another 1 ohm in series. I have a bag of 1W resistors, so that would even better. I think 1/8W in a 1/4W resistor, gets hot to the touch. As a rule, most people just use 2x the power usage, but they can still get pretty hot.

author: frankvnk
author_rank: Newbie
timestamp: April 29, 2025, 08:35:48 am
content: I think derating does not apply here as the resistors are probably used as a safety measure since they are only 1 ohm and probably need to fail when the current gets too high?If these resistors were chosen to drop the voltage, their value should be more than 1 ohmAs the transformer had a center tapped primary winding, i was able to measure the transformer output voltage by applying 110V to the primary winding part that still worked.Measured output voltages: 2 x 20VAC for the 7815/7915 and 12VAC for the 7805However, i am not certain if this is correct as the two primary windings are not equal (see attached drawing).

author: MathWizard
author_rank: Super Contributor
timestamp: April 30, 2025, 01:03:34 am
content: Yeah they might be doing that, but also maybe some inrush current limiting, and power sharing w/ the regulators.So is there actually a problem down stream from the +15V regulator? How much stuff can be disconnected from the PSU, to make sure it's ok ? Did you check around for shorts, like on the power pins of op-amps?IDK about DJ or mixer boards, I can't access the schematics of the 700series, but it's only rated 32W. So I guess it's all low power signals, in/out. So besides some op-amps, or and IC's burning out, or caps shorting, there's probably not much that could go wrong, and overload the transformer winding, if that's what actually happened.You could always buy some fuses for the secondary side of the transformer. But what size, or how quick they would act, IDK. And then put in higher power resistors, or more of them.Can you upload to link to the DJX700 schematics ?

author: frankvnk
author_rank: Newbie
timestamp: April 30, 2025, 09:35:50 am
content: Schematic of the 700 -https://www.synthxl.com/behringer-djx-700/This can only be used as a basic reference as the power supply on page 9 does not correspond to the one in the 900. According to the schematic, there is no separate AC input for the 5V whereas videos on youtube do indicate a separate AC input for the 5V. These schematics (dated 2004) are probably for first releases and since then redesigned.Inrush current limiting - also seems rignt to me, perhaps together with the fail safe due to the selected 1/4W (or cost saving, push parts to their limits....)Before powering up, i checked for shorts, burnt parts, damage - everything was fine. As the input/power supply board can be disconnected from the main board, i first tested this separately - worked fine using replacement transformers, 2 toroidals: 2x15VAC and 1x6VAC + 1A fuses on the secondary side.Next, i connected the main board and measured the power supply currents - results as indicated in my previous message.But, when the 1 ohm resistor broke down, i wanted to know the current consumption of an original setup.However, i should have made some calculations before posting because your remark about the 32W power rating is the perfect clue:2 x 15V @ 510mA + 5V @ 550mA = 18W. This is way below the 32W rating.I ran a full test on the mixer, everything is working fine.As a result, i'll keep using the toroidal transformers and replace the 1/4W resistors with 1W ones.Thanks for pointing me into the right direction.