Use of the fuse on the XLR output of a dj mixer - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: July 22, 2021, 06:34:46 pm
content: Hi guys,Got a Pioneer dj mixer suffering from buzzing on the XLR output. Turns out its a common problem and the culprit is the fuse on the 'hot' signal out. The issue and the fix is shown in the following video.Same as the owner of the video I also thought what would be the purpose of this fuse. At first I also thought it could a guard against accidental plugin into phantom powered input - but since phantom power is limited to mA range and though its not ideal it could not cause much harm in this situation and its not common to see a fuse on this end in other similar mixers.Also the exact fuse is hard to source and its sold as 'pioneer fuse'. Which leads me to think its kind of a manufacturing decision to make the life of the repair tech harder.What are your thoughts on the possible use for this fuse?Thanks in advance.P.S. any suggestion on an equivalent replacement. Its rated as a 1A fuse.

author: SeanB
author_rank: Super Contributor
timestamp: July 22, 2021, 07:35:41 pm
content: I would say it is to protect if somebody plugs an amplifier output into the mixer. Probably there is a SMD fuse that fits, but simpler is a polyfuse that will fit in place of the fuse, and which will self reset on overload.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 22, 2021, 08:30:47 pm
content: This has been a puzzle for a long time, first came accross this maybe 6 or 7 years ago.I doubt its to protect against plugging into a power amp output, power amps haven't used XLR outs since the 70's/80's.The dual zeners will protect from 20v or so, phantom power doesn't count because as stated it just doesn't have the capability of blowing a 1 amp fuse.Gooseneck lamp outputs might possibly be an issue if they are 24v.I have a feeling it might be to protect against something in the lighting world that also uses XLR's.DMX uses XLR's but that's only signal. Maybe there is something in the lighting Universe that could possibly send DC or AC down an XLR?Dunno, not my Universe.Fact is, these fuses DO blow. so Why, and How?I have never seen such a fuse on anything other than a Pioneer mixer.Perhaps they are best shorted, this seems to be a ridiculous design decision that nobody else deemed necessary to implement. Ever.

author: techricky
author_rank: Contributor
timestamp: July 23, 2021, 05:29:10 am
content: Would be a pretty specific case, but the Clear-Com Partyline comms system uses pin 2 for the +30V power supply. Capable of at least 2 amps..A lot outputs just put a couple of 50V+ caps in series with the signal lines don't they? To avoid issues with phantom power etc

author: Yamin
author_rank: Frequent Contributor
timestamp: July 23, 2021, 07:08:56 pm
content: Thanks everyone for the input, it is a mysterious fuse hehehe..I think I'll jumper the pads, but then again I think to myself what could cause the fuse to blow? Nothing else in the path is faulty and the mixer has always been used the same way with the same setup. Could it be just a dud brand of fuses perhaps?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 23, 2021, 07:39:07 pm
content: I suspect some designer/approval system made a poor decision.Pioneer would never admit that to be the case, of course, they would just hope it goes away quietly.And of course they could have been supplied with duff fuses, but again would probably not admit that either.

author: lazyman
author_rank: Contributor
timestamp: July 23, 2021, 07:54:43 pm
content: Hi. I have often see different type of output fuses, or reistors, blown from mixers and controller used in live situations, mostly from small bars, with improper instalation. Problem is that ground can be on some potential and this will blow fuse or output ic, but better is fuse. Also, laptop charger colud sen shock, or any other connected equipment. Seen blown output more often on dj controllers, but never see blown output on pioneer cd players, which has no separate ground.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 31, 2021, 10:43:05 pm
content: Guys another (maybe) interesting curve ball - basically when my customer brought in the faulty mixer in to my shop his complain was that there was a buzzing (the sort of buzzing you get when you have the jack connected in half way into an amplifier) sound from the PA when he had the mixer connected to it. Nothing connected to the inputs just XLR outputs to the speakers. This has been his setup for years and no issues. So he changed the mixer with an exact working mixer - same cables,PA,power cord/outlets etc. and no buzzing.When I got the faulty mixer before opening up as usual I checked it and I didn't get the same issue as him. I didn't hear any buzzing and I got good signal out of my speaker. So after trying to recreate the problem that the customer had. I only got the buzzing when I had one end of the RCA connected to the Line input and the other end of the end of the cable not connected to anything. The buzzing goes if I connect the loose end to my signal source. I noticed that when I had the buzzing, when I touched the outer jacket of the RCA which was connected the buzzing noise does change.When I opened the unit the fuses were open. Bypassed them put the unit back and it works. I don't get the buzzing when I have one end of the RCA connected and the other end loose as before.I was hoping someone could explain why this happened, and why it was different scenario for the customer. Also I wonder why I got any sound in my speaker with the open fuses, because it's my understanding that with a differential input (which I believe my speaker would have) needs the 'hot' and 'cold' signal to amplify the signal or one of them needs to be grounded which wouldn't have been the case with an open fuse at the 'hot' output of the mixer.Thanks in advance

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 31, 2021, 11:59:35 pm
content: Here's a curve ball.Has someone tried to PAT test this unit?Don't know if you do PAT tests in your world, but it basically involves a machine that might put several 10's of amps between the mains earth and the chassis of the unit under test.This is to test that the mains earth has a good connection with the chassis, so should a fault occur that put mains on the chassis, you won't die.Now, it can be the case that someone PAT tests your gear who doesn't have much of a clue of PAT testing, beyond the fact that they can make money on everything they test.Which means that if they cannot find a convenient chassis ground to connect their tester to, they may think that any kind of RCA input or output ground would do instead.So their PAT test gear sends 10's of amps between mains ground and RCA ground. The RCA ground will be connected to mains ground via thin traces on a circuit board, so tens of amps will be sent down these traces during the test, which will destroy them.Nobody will know this has happened, until finally, they take this in to you for repair.

author: Yamin
author_rank: Frequent Contributor
timestamp: August 04, 2021, 09:08:37 am
content: Quote from: Audiorepair on July 31, 2021, 11:59:35 pmI don't think so.Quote from: Audiorepair on July 31, 2021, 11:59:35 pmWouldn't it take out all the things which are common to mains earth, like the micro controller etc...?Now that you mention its a very likely scenario that there could've been a fault at the mains ground connection. That's pretty common, but also in those cases I see more damage done on different parts of the circuits.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 04, 2021, 09:27:24 am
content: The fact that there is only a fuse on one side suggests to me it is to protect against something very specific - either on just that pin, or across pins 2/3. Maybe another product of theirs that used XLRs for power

author: Audiorepair
author_rank: Frequent Contributor
timestamp: August 04, 2021, 05:55:50 pm
content: The PAT Test Test done properly requires you to plug the mains cable of the item under test into the PAT test machine, and then you manually take a ground test cable attached to the machine and attach this via its crocodile clip or whatever to the metal chasssis of the item under test.The machine then sends often several 10's of amps from the ground pin of the items mains cable, through the chassis, and back to the PAT test machine, whilst measuring the resistance of this loop.For safety reasons, you would want this loop to be a very low resistance one, meaning the earth from the items mains plug is securely bonded to the chassis, and can take a large current and still maintain the bond, and people won't die.This current can't really flow through anything else other than this loop.This is the procedure for a Class 1 device, i.e.one that has a mains earth on the mains plug, and a metal chassis that could, under fault conditions, become live and dangerous.A Class 2 device that doesn't have a mains earth connection or a metal chassis that could become live and kill you, is not tested with this method, it is tested with a CLass 2 method that doesn't involve sending 10's of amps through the unit under test.If you are a Numpty, you might attempt a Class 1 test on a Class 2 device and send 10's of amps down the earth connection of an RCA socket or XLR, and blow stuff up.Quote from: Audiorepair on July 31, 2021, 11:59:35 pmWouldn't it take out all the things which are common to mains earth, like the micro controller etc...?Now that you mention its a very likely scenario that there could've been a fault at the mains ground connection. That's pretty common, but also in those cases I see more damage done on different parts of the circuits.

author: Yamin
author_rank: Frequent Contributor
timestamp: August 04, 2021, 10:35:32 pm
content: Quote from: Audiorepair on August 04, 2021, 05:55:50 pmAh I see, thanks.