Mains hum on a guitar effect unit! Help (circuit enclosed)

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I've been at this all night & frankly I'm at a loss.

This is a guitar effect unit - it takes a guitar signal in, adds some effects to the guitar signal & outputs the treated signal onwards for amplification ....there is a line level headphone socket on the unit

The fault symptoms are that the there is mains hum mixed in with the guitar signal (about equal in volume to the actual guitar signal - therefore the guitar signal is fighting to be heard over the mains hum!). The guitar unit functions in every other aspect.

The hum intensifies as I touch the metal in an around the input/output jack sockets...on the surface it would seem to be some kind of grounding problem. (though I tried 'star' feeding the signal ground (the negative leg of C216) to all of these input/output ground pins - not a lot of difference observed.

The input to this amp is 9VAC

Here's the PSU circuit diagram...

An externally hosted image should be here but it was not working when we last tested it.


I'd I'm reading it correctly, the ground from the power cable is not used (the ground looks to be on the shell of a DIN plug that feeds the unit - but in fact there didn't appear to be any mains ground on the shell)...but instead the unit has it's own ground used throughout the unit?

All of those regulators outputs are correct but on top of the DC is something that looks a bit odd - like a sawtooth with a rounded point at 50mV peak to peak. It's not constant...but more like a line of DC with the this aforementioned pattern 'waving' along the DC line.

The line feeding into the regulators has about 1.5V of ripple (essentially measured across the biggest cap in the PSU - C216.). Now I'm no expert on PSUs, but I'd have thought there should be less ripple at that point? I doubled up the capacitance...it only reduced the ripple a little.

I'm open to suggestions...have scope & multimeter & can take any readings required!!

Many thanks in anticipation.

PS (all those diodes at the top is some kind of pump circuit to get the 9V AC input rectified & ultimately cranked up to a couple of hundred volts DC for a valavle that is in this unit.

PPS Complete schemtaic here, in case you need more info..... http://freepdfhosting.com/18d232f8e3.pdf
 
Sorry.. I should have said in my original post....this is a Digitech commercial effects unit that was working fine - now it's got the aforementioned mains hum. The extract from the schematic (the PSU), is their design....it hasn't been modified in anyway.

The unit is about 8 years old & way out of warranty...Digitech have been very good in that they have supplied me with a schematic (attached to the original post)

I've just noticed on the schematic 3 different symbols relating to the ground ...

1. J5 left hand side.
2. U40 pin 2.
3. u45 pin 3.

Can anyone explain what's going on there? (ie what are they getting at by having three separate symbols)

Also what's the purpose of L12 ...the signal coming in on J5 is 9V AC ...& I always thought that inductors were used for smoothing DC (in PSUs at least)...but L12 is before the rectifier diode D42?
 
HankMcSpank said:
I've just noticed on the schematic 3 different symbols relating to the ground ...

1. J5 left hand side.
2. U40 pin 2.
3. u45 pin 3.

Can anyone explain what's going on there? (ie what are they getting at by having three separate symbols)

Also what's the purpose of L12 ...the signal coming in on J5 is 9V AC ...& I always thought that inductors were used for smoothing DC (in PSUs at least)...but L12 is before the rectifier diode D42?

#1 is the symbol for a chassis ground, #2 is for analog ground, #3 is for digital ground. A little search will give you plenty of reading about they are.

L12 and L13 are ferrite beads, once again, a little search will get you more than you need to know.

Since you say this is a commercial unit that was working properly at one time, did you thoroughly go through it for mechanical problems first? Always do that with stompboxes first: make sure the jacks are making good contact, reflow solder joints, shielding is intact, etc.

EDIT: I meant do a mechanical check in both the unit and the PS, since it sounds like you have no evidence of where the problem is.
 
Second to everything Lead said.

Mech. stress on pins soldered to PCB without strain relief eventually can get an open circuit due to cracks around the solder joint. These are very hard to see without a magnifying glass. Happens alot with electronic esp. w something that gets hot when on and cools when off. (ie temperature cycling)

1) Just reflow all PCB solder joints with off mounting to jacks, plugs , switches, sockets, esp at PCB to chassis interfaces. Would pay close attention to AC power J5 and the mating transformer/cable as well.

2) If the guitar level has decreased requiring more gain then could be a component failure.

3) doublecheck all DC supplies with a DMM esp. HV section. This is a diode multiplier and leaky caps can cause lower voltages.
 
Well, you were right - I was getting in too deep & trying to get all analytical. I reflowed the solder joints on all the inputs/outputs & the 'power in' socket...hum now totally gone!

Sometimes it's good to step back, take 5 & throw the problem open to the internet community...9 times out of 10 they're right!

Many thanks for your tips!

Hank.
 
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