strange hum

[steve-bbb]

ok not another earth hum thread i hear you say....

slightly different one here that i (and others ive asked) have not encountered before

the rogue amp in question is a [b]trace elliot boxer65[/b] combo

when plugged in with input gain <1 and also master volume <1 ..... plug instrument lead in .... hold lead without touching the shield...then touch only the signal tip of the jack .... results in monster loud earth hum at the sort of volume you would expect if both input and master where up at 9-10

when the bass is plugged in and playing clean without any other equipment connected there are no issues whatsoever - only when touching the signal tip alone

however if i go through the pedal board with the pedal power supply plugged into the same 4gang multiblock then i get an extremley loud constant earth hum

ive tested my sockets in the wall with a 3pin plug with the led's to check for any earthing issues and the sockets are fine - the kettle leads i use for the combo are ok ive checked them and use them with my main rig with no problems likewise the 4 gang multiblock/rcd which i normally use

any ideas please? thanks

Edited November 17, 2012 by steve-bbb

[RAY AGAINST THE MACHINE]

A few years back, a friend of mine sold me his Peavey combo(TNT series /scorpion?)
Immaculate condition, but when turned on in the bedroom there was a hum.
I had 2 tech people look at it in different repair shops.

Think there was an issue with transformer.

In the end , I just got rid and part exchanged for a combo and a set of strings,)

[JapanAxe]

No advice to offer about the direct-to-amp hum without being able to check out the amp in person.

For the pedalboard, it sounds like the earth on the power supply transformer is connected to the signal earth, giving you the earth loop we all know and [i]don't[/i] love! The solution is to keep the safety earth connected to the transformer primary, but break the connection to the secondary side of the circuit. I had this issue with a Harley Benton Power Plant - to fix it I just needed to break a track on the circuit board. This left the safety earth connection intact, so the metal enclosure is still earthed via the mains cable, but left the 9-volt outputs 'floating'.

WARNING: Don't attempt any mods like this unless you know [b]exactly[/b] what you're doing!

Edited November 18, 2012 by JapanAxe

[steve-bbb]

[quote name='JapanAxe' timestamp='1353239862' post='1872828']
No advice to offer about the direct-to-amp hum without being able to check out the amp in person.

For the pedalboard, it sounds like the earth on the power supply transformer is connected to the signal earth, giving you the earth loop we all know and [i]don't[/i] love! The solution is to keep the safety earth connected to the transformer primary, but break the connection to the secondary side of the circuit. I had this issue with a Harley Benton Power Plant - to fix it I just needed to break a track on the circuit board. This left the safety earth connection intact, so the metal enclosure is still earthed via the mains cable, but left the 9-volt outputs 'floating'.

WARNING: Don't attempt any mods like this unless you know [b]exactly[/b] what you're doing!
[/quote]
thanks - i only ever take it to a local amp specialist for stuff like that

the 9v power supply for the pedals is just the standard one from maplins

i took the head out of the box to see if there were any 'obvious' loose or stray wires but nothing visibly apparent - the only thing that was really obvious was that the four power transistors had been fixed on to edge of the pcb and heatsink panel with some sort of white putty type substance - very messy job indeed and looked like a possible repair as i would assume that TE would probably not allow something so obviously shoddy to pass any sort of QA

one other thing i have noticed is also that the power supply is creating quite a loud pop on switching off - resigning it to the cupboard now until it goes over for a service/repair

the 9v power for the effects chain is fine with my other rig no hum at all even when the head and the effects are both plugged into the same 4gang multiblock

schematic [url="http://www.britishaudioservice.com/schematics_2009/B&C/Boxers.pdf"][b]here[/b][/url] (pdf file)

Edited November 18, 2012 by steve-bbb

[geoham]

Hi,

I know this thread is a few weeks old now.

Just want to add, that I also have a Maplin power supply powering my pedal board ([url="http://www.maplin.co.uk/9v-power-bank-for-guitar-effect-pedals-31292"]This one[/url]), and it is often unusably noisy It behaves very strangely - the noise changes depending on what pedals are being used and what order they are in - even when they are all off.
I had resort to essential pedals only powered by battery at my last gig. Same problems on my own amp and at rehearsal room.

[steve-bbb]

well in other places my maplin supply is absolutlely fine

what has just occured to me though is that my pc is plugged into the same ring and whirring away - i will test it soon but i have a sneaky suspicion the pc may be the cause of the problem

its been checked over by a local amp specialist and is fine - he did advise check the cheap grey block RCD device i was using as they can sometimes cause similar problems (apparently they are suitable for lawnmowers but can introduce unwanted noise into audio equipment)