Circuit ground is not at 0V...

[gutsofgold]

I am rehousing my Arion analog delay into a nicer metal enclosure. I noticed that if I follow the ground from my DC power jack (negative center) into the circuit, it eventually turns into about 3V DC. These readings are referenced directly to the center pin of the power jack. Is this normal?

For example... the MN3205 should get 5V DC on pins 5 and 8. Putting the positive (red) probe of my multimeter on either pin and the negative (black) probe on the DC power jack center pin... I get ~9V DC. However, if I put the negative probe to ground later on in the circuit (on PIN 1 of the MIX pot for example), I get ~6V DC.

The circuit does not output right now which is why I ask.

http://www.diyguitarist.com/Images/ArionSAD-1.gif

http://www.diyguitarist.com/Images/ArionSAD-1layout.gif

[aziltz]

i had this problem this week.  Make sure the PS ground and the circuit board ground actually meet some where.  That was my issue.

[JKowalski]

Yeah, you probably have a bad wire/solder joint somewhere thats messing with the resistance between your ground connections.

[Dan N]

I hope you'll post pics when you're done!

[gutsofgold]

Oh but they do! You can't see it in the schematic I posted but here is the layout.



The Red dot indicates where my GND connection is entering the circuit board (from the power jack center pin).

It then travels across that 330ohm resistor and then hits the CATHODE of a diode. The ground signal when referenced to the center pin of the power jack measures 0V on the cathode side of the diode and 3V on the anode. I have never seen a diode in series with the ground like that either. Also it should be noted that the DIYGuitarist site where I found the schematic states that... " the power supply required for the MN3205 is 5V, which probably explains why the SAD-1 will not work with other pedals sharing the same power supply as the SAD-1, because the virtual ground created in the SAD-1 is not the same as the ground in most guitar pedals using a 9V battery. "

Could he mean that this virtual ground is essentially 3-4V DC thus giving you 5V DC when applying the 9V to the 3205 and grounding the chip to this virtual ground ... 9-4=5.

[Paul Marossy]

Oh but they do! You can't see it in the schematic I posted but here is the layout.



The Red dot indicates where my GND connection is entering the circuit board (from the power jack center pin).

It then travels across that 330ohm resistor and then hits the CATHODE of a diode. The ground signal when referenced to the center pin of the power jack measures 0V on the cathode side of the diode and 3V on the anode. I have never seen a diode in series with the ground like that either. Also it should be noted that the DIYGuitarist site where I found the schematic states that... " the power supply required for the MN3205 is 5V, which probably explains why the SAD-1 will not work with other pedals sharing the same power supply as the SAD-1, because the virtual ground created in the SAD-1 is not the same as the ground in most guitar pedals using a 9V battery. "

Could he mean that this virtual ground is essentially 3-4V DC thus giving you 5V DC when applying the 9V to the 3205 and grounding the chip to this virtual ground ... 9-4=5.

I didn't exactly have anything specific in mind when I wrote that, it was more of a hunch. But your findings seem to support that ground in these pedals is not 0V. But then my SAD-1 did work on one of my common power supplies once, which was totally unexpected because it never worked for me otherwise - it had to have its own power supply to function. I'm still not eactly sure what the deal is with that.

[gutsofgold]

As of right now I'm not getting any output on my rehouse so I'm looking at all options here. Currently I'm using the traditional "ground the enclosure" method but if the enclosure is grounded to the power jack (ACTUAL ZERO VOLTS) then the circuit is grounded to it's 3V virtual ground. This leaves my input and output jacks grounded differently than the circuit.

The original pedal is obviously plastic along with the jacks so it should be assumed you need to isolate the jacks when using a metal enclosure and have their grounds go to the circuit ground. Now if I do this and the pedal still does not work I will have assume it's a chip or transistor at fault and then it's audio probe time.

[Paul Marossy]

I guess you could try isolating all of the jacks from the enclosure. That might work. Or at least the DC jack anyway...

[Dan N]

Um, did you replace the pots? Don't forget, their metal bodies served as jumpers on the pcb.

[MikeH]

i had this problem this week.  Make sure the PS ground and the circuit board ground actually meet some where.  That was my issue.

Oh but they do! You can't see it in the schematic I posted but here is the layout.

Did you check this connection with a continuity meter?

[gutsofgold]

Dont have a continuity meter but like I said... the voltage of the ground plane on the circuit board changes from before the diode (which is where the actual power supply voltage comes in so its 0V) to after the diode (3V)

EDIT: Measuring resistance from the ground connection of the power supply to anywhere on the ground plane AFTER that little diode results in overload/open circuit.

[gutsofgold]

Um, did you replace the pots? Don't forget, their metal bodies served as jumpers on the pcb.

Forgot to mention this! I indeed replaced the pots and made sure to jumper each pair of pads that were once connected via the original pot bodies.

[Dan N]

On a similar vein, the pcb mounted input jack was a ground connection too. You know, when you plugged your cable into the input the power turns on... So if you replaced the jacks too, you'll have to jump the input jack pads on the pcb. Sleeve to ring, that is.

[gutsofgold]

got it running. I had to isolate the input and output jacks (duh!) and the ground indeed sits at 3.6-4V when referenced to power ground. I guess it just makes for easy power to all the IC's in there. the green knob is a variable feedback resistor that gets added in while the repeats voltage divider gets taken out when you step on the momentary switch. the little flip switch adds in germanium diodes to ground in the ff path to keep oscillation under control. I love the way this sounds!

[Paul Marossy]

got it running. I had to isolate the input and output jacks (duh!) and the ground indeed sits at 3.6-4V when referenced to power ground. I guess it just makes for easy power to all the IC's in there. the green knob is a variable feedback resistor that gets added in while the repeats voltage divider gets taken out when you step on the momentary switch. the little flip switch adds in germanium diodes to ground in the ff path to keep oscillation under control. I love the way this sounds!

Wow, that's pretty trick. Interesting about the ground, I thought something was different about that.

[Dan N]

Good one! I have a somewhat picked over SAD-1 pcb. This looks like a fitting project to save that thing from the junk box.

Did you keep the original switching?

[gutsofgold]

thanks, and yes original switching. just replaced the on board switch with an enclosure mounted "normally open" momentary switch.

[gutsofgold]

after playing with this for awhile I've noticed this thing possesses an almost atmospheric character that none of my other analog delays have. it's a noisy machine for sure but it just has some personality to it I guess, I don't know but it's a real fun delay especially for what they sell for. I noticed that when I engage the the oscillation/crazy/freak out mode... it never gets as loud as the digital delay that I had setup in a similar way. I even brought the feedback resistor down the nothing so the entire signal was feeding back and it still wasn't that close to ripping up my speakers. my digi delay on the other hand was set out to rip my speakers and then my ear drums, and wanted to do it quick. is this just the inherent nature of analog bbd's? or is something else causing this? the compander?