Mains hum on a guitar effect unit! Help (circuit enclosed) - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th March 2009, 01:05 AM   #1
diyAudio Member
 
Join Date: May 2008
Default Mains hum on a guitar effect unit! Help (circuit enclosed)

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...

Click the image to open in full size.

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
  Reply With Quote
Old 17th March 2009, 02:52 AM   #2
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Back up a bit: what are you troubleshooting hum on? Is the unit itself commercial but you built the power supply, or is both the unit and PS commercial? Was everything ever working properly at some time in the past? Is all of it commercial design?
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 17th March 2009, 07:42 AM   #3
diyAudio Member
 
Join Date: May 2008
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?
  Reply With Quote
Old 17th March 2009, 03:56 PM   #4
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Quote:
Originally posted by HankMcSpank
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.
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 17th March 2009, 06:05 PM   #5
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
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.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 17th March 2009, 11:11 PM   #6
diyAudio Member
 
Join Date: May 2008
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.
  Reply With Quote
Old 17th March 2009, 11:34 PM   #7
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
good to hear it's as good as a brandspankin new un.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 18th March 2009, 01:42 AM   #8
diyAudio Member
 
unclejed613's Avatar
 
Join Date: Dec 2006
just curious, but what is the voltage multiplier for (the big diode/cap ladder at the top) i didn't see a tube anywhere in the schematic.
__________________
Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net
spammer trap: spammers must die
  Reply With Quote
Old 18th March 2009, 01:58 AM   #9
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
Hi
I wouldn't mind seeing the rest of the effects schematic.
Gotta love tube opamp hybrids.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 18th March 2009, 08:33 AM   #10
diyAudio Member
 
Join Date: May 2008
The rest of the schematic was in my post above (the one with the pdf hosting URL). Here it is again http://freepdfhosting.com/18d232f8e3.pdf
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
is thier away to turn alphadrum schematic into a guitar effect prorms Digital Source 0 22nd February 2008 03:38 PM
DOD JuiceBox guitar effect schematic wanted. darkfenriz Instruments and Amps 0 20th June 2007 04:25 PM
Tube Preamp as a Guitar Effect fresno Tubes / Valves 4 28th March 2007 02:20 AM
Need DIY Bass Guitar slap effect??Help!! thomgun_lc Instruments and Amps 24 25th July 2006 07:59 AM


New To Site? Need Help?

All times are GMT. The time now is 01:05 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2