Behringer Xenyx 802 replacement PSU

[richardash1981]

As related in the Microphone Preamp thread, the external power supply for my Behringer Xenyx 802 mixer has gone pop, more or less literally. It was sat on the bench, acting as microphone-to-headphones preamp for a Trantec S3500 radio mic receiver which was picking up a beltpack I was working on - so not actually being interfered with at all. After a a while, whilst I had the headphones off (because I was soldering LEMO connectors round backwards) there was a pop from the mixer. I at first assumed it was something picked up on the radio mic receiver, but then realised the (bright blue) power LtED was out on on the mixer. Investigation shows that the primary of the Behringer PSU is open circuit (but the 3A plug fuse is not blown).

The power supply I have got says "Behringer Eurorack Power Supply Model MXUK3" on it, and is rated at "2 * 18 V~ / 2 * 150 mA" with a 3-pin miniature DIN plug on the end. I take this to be a 37V transformer winding with centre tap (although I seem to remember measuring rather more than that off load). The low voltage side measures 9.5 ohms end to end and 5 ohms either end to centre, which seems to match up.

The manual available from the Behringer website has the same power supply specification, but quotes part as PSU MX3EU.

So I tried searching and got a page on Behringer's website which states that the spare for this mixer is PSU3-UK. So that's three part numbers. The photograph used is (when you zoom in) of a 120V input MXUL3 which has a second 30V output on it ...

When I looked on Studiospares (I need to order something else so would avoid postage), they don't have a PSU3-UK, but do have PSU4-UK and PSU6-UK. The latter is the listed PSU for the larger 1002FX 1202FX in the same range, but I can't find an actual output specification anywhere (it's not in the manual for those mixers), so I don't know whether it would actually work or not.Rapid seem to have about the only stock of the PSU3-UK in the usual UK suspects apart from one B-stock at Gear4Music.

Should I be reasonably confident that (despite not matching the old PSU or the manual, not withstandanding the product photos) PSU3-UK is the right replacement part to order?

[sandall]

A bit surprising - Xenyx mixers usually fail because the internal PSU regulators overheat. If it's any help the PSUs for earlier Xenyx 1202s are model MXUK5, rated at 20w, input 105mA, outputs 2x 17.5V @ 650mA. These are bigger & heavier than the MX3, but would work, but your lower current draw might mean a higher voltage, so more work for the regulators. The MX3UK (or MXUK3) is presumably just a MX3EU with a 13A plug instead of a European one.

 

There is a thermal fuse in the transformer, so it may be that a regulator has gone, causing this fuse to go open-circuit. It may reset when cold, so it might be worth checking whether you get an output when the transformer is off-load.

[sunray]

A bit surprising - Xenyx mixers usually fail because the internal PSU regulators overheat. If it's any help the PSUs for earlier Xenyx 1202s are model MXUK5, rated at 20w, input 105mA, outputs 2x 17.5V @ 650mA. These are bigger & heavier than the MX3, but would work, but your lower current draw might mean a higher voltage, so more work for the regulators. The MX3UK (or MXUK3) is presumably just a MX3EU with a 13A plug instead of a European one.

 

There is a thermal fuse in the transformer, so it may be that a regulator has gone, causing this fuse to go open-circuit. It may reset when cold, so it might be worth checking whether you get an output when the transformer is off-load.

There didn't appear to be thermal fuses in the 2 that I've looked at, unless it's completely hidden within the winding [without an extra tag]

 

These are simply a transformer and you may make your own replacement, which reminds me I need to do the same for a soundcraft where it stayed on site when the mixer went home...

 

[sandall]

There didn't appear to be thermal fuses in the 2 that I've looked at, unless it's completely hidden within the winding [without an extra tag]

The 2 MXUK5s I've got in front of me both have (different) icons for a 115 deg C fuse, but I haven't attempted to cut one apart to see what they look like.

[timsabre]

It could also just be the primary winding in the transformer has failed. Seems to happen more than you'd expect.

[sandall]

Before spending too much on a replacement PSU it's worth bearing in mind that at around £50 retail the mixer itself is practically a consumable

[richardash1981]

Unfortunately the power supply still measures open circuit on the primary after several days cold, so I think it is permanently open circuit. Mine also has the 115 degree thermal fuse symbol, so presumably there is one in there somewhere. It certainly wasn't red-hot when it failed, but it could have gone on a mains spike or a primary winding fault - it has been dragged around quite a lot, although not maltreated or used that many hours (it's basically been my bench / lend-to-people-who-only-need-a-few-channels mixer, never regularly gigged). I remember changing a lot of pinspot transformers, and it was the cheaper, newer sort with unvarnished windings which generally had the primary fail, usually with a nice spray of metal vapour over the surroundings! In earthed kit, it usually took the RCD out via the metal coating ... My guess was always that the wire moved and rubbed the enamel off, then you got a shorted turn followed by no turn at all but a pretty cloud!

 

Maybe I ought to open the mixer up and power it with some benchtop DC supplies on the main reservoir caps in case the whole thing is self-toasted before I spend money (or time) on a new PSU.

It's interesting that the MX5 is a lower voltage (by 1V) rating, although much higher current. I would assume there is plenty of headroom to the regulators, as 7815 needs 15+2.5 = 17.5V DC, and 17.5V AC through a rectifier gives 23V peak! I wonder why the larger PSU has a lower voltage rating, when I would expect the same PSU circuit to be used (ideally with bigger heatsinks!). In practice between higher-than-240V-mains and the transformer lack of regulation, I don't think there is any risk of the regulators coming out of regulation. Unless the unregulated ~24V is doubled to make the 48V phantom power rail ...

[DrV]

Unless the unregulated ~24V is doubled to make the 48V phantom power rail ...

Highly likely, but you can check out the rest of the mixer with this method.

[sandall]

It's interesting that the MX5 is a lower voltage (by 1V) rating, although much higher current. I would assume there is plenty of headroom to the regulators, as 7815 needs 15+2.5 = 17.5V DC, and 17.5V AC through a rectifier gives 23V peak! I wonder why the larger PSU has a lower voltage rating, when I would expect the same PSU circuit to be used (ideally with bigger heatsinks!). In practice between higher-than-240V-mains and the transformer lack of regulation, I don't think there is any risk of the regulators coming out of regulation.

I'm not a great fan of over-working regulator chips (13V AC with decent-sized caps should be adequate for a 15V regulator) because of the unnecessary heat-sinking required. Unfortunately on the little Behringer mixers I've met the caps are barely adequate for the job & the heat-sinking & ventilation are almost non-existent, so that corner of the PCB gets very hot.

 

Funnily enough the only time I've ever had a mains transformer spontaneously fail was in my Seaward PAT tester.

[richardash1981]

I know this is an old thread, but both I and the mixer are still here. Some things have moved on:

This site has the schematic diagram for the mixer, including the internal PSU - https://www.g7syw.co...xenyx-802-mods/ - which shows that the 48V phantom power is indeed derived from a voltage doubler circuit on the incoming AC. So DC power schemes will loose the phantom power rail (which is regulated with a discrete regulator at 48V). The 37V AC input is voltage doubled to ~50V DC after diode drops, just enough to regulate to 48V.

As a result of this you can't reduce the AC input voltage hardly at all without the phantom power coming out of regulation (in theory). Which is no doubt why the odd PSU voltage was chosen.

 

 

Having discovered this, I finally did the power-up-from-DC test as suggested here, and I'm glad I did (with a bench PSU). The +15V rail was drawing a heavy current as soon as it got past 5V or so - half an amp if you let it.Removing the +ve regulator from the PCB cured this and the rest of the mixer (minus phantom power) works fine. I think what must have happened is that the +15V regulator failed more or less short circuit, and the current from that put paid to the mains transformer in the external PSU. Interestingly, although the +15V regulator is marked on the schematic as "7815", it's not a TO220 part. It and the -ve regulator are in some sort of compatible all-plastic package. This is important because they are sharing the same metal (earthed ish) heatsink, with no separate insulator. If you replace (especially the -ve) with standard TO220 parts you will need to add a heatsink insulator kit (which won't do the thermal performance any good).

Edited July 17, 2021 by richardash1981

[Dave m]

Probably no point now but between Xmas and June we had a series of thefts that have hopefully stopped now.We also lost 2 802s but I am pretty sure we have a working PSU still as the thief wasn't too bright

I can have alook? One was covered in tape but the other was in better condition