Signal path shorted to ground

[nosamiam]

I'm trying to get a friend's storebought MXR Phase 90 working and not having any luck. I've discovered the signal path shows near zero resistance to ground. I've gone over it with magnification and can't find any solder bridges. The PCB is unmolested and looks to be in good shape. No components leaning over and crossing legs. I just can't figure it out. If I play through it I get effected signal but just very, very quiet as you'd expect. There was a tantalum cap that I went ahead and replaced with an electrolytic just because it was easy and I know these are failure prone. Didn't help.

I've had this happen before with my own DIY projects in the distant past and never successfully gotten one of those working. I've successfully brought ones to life with other issues and am not really that new to troubleshooting although I'm not trained at all.

So is there an approach one should take in trying to isolate and correct an issue like this? If the whole path is grounded there's no way to scope it or trace it that I can think of. Maybe cutting traces to figure out what part of the circuit the fault is in? Or lifting one end of a component that's in series?? I don't know... there are enough parallel paths that I don't think that would be too easy.

Any advice?

[GibsonGM]

Could it be a bad input jack, somehow shorting to gnd??  Something with the switching, on of the transistors there?   Seems like an odd malfunction, sounds mechanical more than 'a bad part', but I could be wrong....

[nosamiam]

Good thinking. I'll pull off the footswitch and jacks and try using test leads. That very well could be the problem.

[GibsonGM]

Good thinking. I'll pull off the footswitch and jacks and try using test leads. That very well could be the problem.

It's all that comes to mind - good luck!

[EBK]

Where in the signal path are you measuring this resistance?  Are you measuring with power applied to the circuit?

[electrosonic]

I just fixed a Boss PH-2 with the same issue. The input jack had rotated and was shorting to the case. Sounds simple but it took me a 1/2 hour to figure out.

Andrew.

[nosamiam]

Where in the signal path are you measuring this resistance?  Are you measuring with power applied to the circuit?

Measuring between the tip of the input jack and sleeve of output jack with power applied and effect engaged. Haven't had a chance to get back to it yet.

[EBK]

Measuring ... with power applied ....

That's a problem.  Not necessarily the problem, but a problem nonetheless.  You can't measure resistance in a powered-up circuit with a multimeter in resistance-measuring mode.  The presence of a voltage or current source in the circuit under test will often trick the meter into reporting zero ohms (and, in some cases, could damage the meter).

[GibsonGM]

Measuring ... with power applied ....

That's a problem.  Not necessarily the problem, but a problem nonetheless.  You can't measure resistance in a powered-up circuit with a multimeter in resistance-measuring mode.  The presence of a voltage or current source in the circuit under test will often trick the meter into reporting zero ohms (and, in some cases, could damage the meter).

Yep! If you have the function on your meter, "continuity test" is more indicated for this 'problem'.   You  need the "beeper" to see what is connected/low ohms, with no power applied.   

If you have an audio probe (or can make one now), you could try inputting a signal and audio probing the input, then the output from the 1st active device, an opamp (remembering to trip the footswitch to be sure it's "on", if you get no signal!).    Quick check to make sure that input jack IS working, and that the opamp input stage works.  Can't give you pin numbers, as I don't have a schematic to work from, but from what I know the first stage IS an opamp.    Take part number, get data sheet, then you know inputs/outputs....Most likely, it will have the pinout of a TL072, if you get stuck.

[nosamiam]

Measuring ... with power applied ....

That's a problem.  Not necessarily the problem, but a problem nonetheless.  You can't measure resistance in a powered-up circuit with a multimeter in resistance-measuring mode.  The presence of a voltage or current source in the circuit under test will often trick the meter into reporting zero ohms (and, in some cases, could damage the meter).

Yep! If you have the function on your meter, "continuity test" is more indicated for this 'problem'.   You  need the "beeper" to see what is connected/low ohms, with no power applied.   

If you have an audio probe (or can make one now), you could try inputting a signal and audio probing the input, then the output from the 1st active device, an opamp (remembering to trip the footswitch to be sure it's "on", if you get no signal!).    Quick check to make sure that input jack IS working, and that the opamp input stage works.  Can't give you pin numbers, as I don't have a schematic to work from, but from what I know the first stage IS an opamp.    Take part number, get data sheet, then you know inputs/outputs....Most likely, it will have the pinout of a TL072, if you get stuck.

Measuring ... with power applied ....

That's a problem.  Not necessarily the problem, but a problem nonetheless.  You can't measure resistance in a powered-up circuit with a multimeter in resistance-measuring mode.  The presence of a voltage or current source in the circuit under test will often trick the meter into reporting zero ohms (and, in some cases, could damage the meter).

I'm actually not 100% sure it was powered up when I measured resistance now that I think about it, although I didn't know that and thanks for pointing that out! I am 100% sure though that I used continuity mode and got the beeping when it was powered on and probing between input tip and output sleeve.

I don't have a permanent audio probe but always whip one up when I need one. I'll try the pointers re: the jack and the opamp. There are a TL074 and TL062 in the circuit. Not sure which one has the input stage (haven't found a decent PCB layout diagram) but I'm sure I can follow the traces visually on a single-sided PCB.

Thanks!

[EBK]

Continuity mode and resistance mode are the same thing, other than the beeping.  You can't measure continuity on a powered-up circuit for the same reason.

[amptramp]

Mallory used to make jacks for telephone patch panels that had a plain jack surface (not threaded) and a screw offset from the jack that was used to mount the jack to the panel.  This device could not rotate once it was mounted.  Maybe if we ask them nicely, they could bring back the Type X jack and end all this business of jacks that can rotate and short out the wiring.  The picture doesn't show it but there is a threaded hole above the jack input in the upper picture for mounting since the barrel of the jack is not threaded.



They also had offset jacks where the contacts were off to the side and it was easy to lock the jack in position because it had a broad, flat steel frame that sat behind the panel and could be prevented from rotating or drilled for a nut and bolt:

[GibsonGM]

Measuring ... with power applied ....

That's a problem.  Not necessarily the problem, but a problem nonetheless.  You can't measure resistance in a powered-up circuit with a multimeter in resistance-measuring mode.  The presence of a voltage or current source in the circuit under test will often trick the meter into reporting zero ohms (and, in some cases, could damage the meter).

Yep! If you have the function on your meter, "continuity test" is more indicated for this 'problem'.   You  need the "beeper" to see what is connected/low ohms, with no power applied.   

If you have an audio probe (or can make one now), you could try inputting a signal and audio probing the input, then the output from the 1st active device, an opamp (remembering to trip the footswitch to be sure it's "on", if you get no signal!).    Quick check to make sure that input jack IS working, and that the opamp input stage works.  Can't give you pin numbers, as I don't have a schematic to work from, but from what I know the first stage IS an opamp.    Take part number, get data sheet, then you know inputs/outputs....Most likely, it will have the pinout of a TL072, if you get stuck.

Measuring ... with power applied ....

That's a problem.  Not necessarily the problem, but a problem nonetheless.  You can't measure resistance in a powered-up circuit with a multimeter in resistance-measuring mode.  The presence of a voltage or current source in the circuit under test will often trick the meter into reporting zero ohms (and, in some cases, could damage the meter).

I'm actually not 100% sure it was powered up when I measured resistance now that I think about it, although I didn't know that and thanks for pointing that out! I am 100% sure though that I used continuity mode and got the beeping when it was powered on and probing between input tip and output sleeve.

I don't have a permanent audio probe but always whip one up when I need one. I'll try the pointers re: the jack and the opamp. There are a TL074 and TL062 in the circuit. Not sure which one has the input stage (haven't found a decent PCB layout diagram) but I'm sure I can follow the traces visually on a single-sided PCB.

Thanks!

Look up the data sheets, then you can get the pinouts and listen to inputs/outputs, to get an idea of where your trouble may lie    Don't short the pins together, tho!