What could cause a resistor's resistance to fall to 0.0?

[wellshuxley]

Hi folks, beginner here. I'm having a challenge with a build and I am hoping someone here can point me in the right direction towards a solution.

I have a 300 ohm resistor in my circuit that tests at right around 300 ohms when the circuit is unpowered, but when I power the circuit, the measured resistance across the resistor immediately drops down to 0.0 ohms, and remains there. Consequently, an audio probe reveals audio on the "upstream" leg of the resistor, but no signal on the "downstream" leg.

What issues in my circuit could potentially cause this symptom?

More detail:

• The circuit is a passive filter network bookended by clean opamp gain stages
• The 300 ohm resistor in question hangs off the output pin of the first opamp stage; it is in series with a 10u interstage coupling cap that sits in between the input gain stage and the passive filter stage
• The build is on veroboard, and I had the circuit working on a breadboard before starting on the current implementation
• When I power up the breadboard circuit, the equivalent 300 ohm resistor's resistance drops some, but stabilizes at around 280 ohms
• The circuit accepts 9V DC in, and increases that to just over 16V via an implementation of this NE555-based voltage doubler: http://tagboardeffects.blogspot.com/2013/02/ne555-voltage-doubler.html

I've checked my connections and replaced a few parts, but so far I have not been able to identify or fix the problem. Since my breadboard implementation is working, I assume the issue is either a faulty component or, more likely, a build implementation error.

I've attached the schematic in case I left out any pertinent details, sans the passive filter section. The filter section is based on the Pultec EQP-1A. I can share that if needed, but I thought it would just introduce noise.

Thank you in advance for any help you might be able to provide!

[GibsonGM]

Hi Wells, welcome!

You should always measure resistance with NO power applied to the circuit...and often you need to lift 1 leg of the R in order to get a true reading (the parts it's connected to can cause faulty readings).  No worries, we've all done it with power on   Lucky you didn't pop a fuse.

As for why it doesn't work, not sure.  If it was ok on BB, 99.999% chance you have a build error - the most common cause for failures! Check it again (and again)...if still no luck, let us know and we can help more.  Might need front and back legible pix of your build for that.

[ElectricDruid]

Welcome Wellshuxley!

+1 what Mike said above - you can't test resistance with the power on. The meter uses a small voltage to do the resistance test, so if there's any voltage in the circuit already, it completely confuses the reading and what you get is nonsense.

If your audio probe shows that you've got signal at one end of the resistor, but not at the other end, then the most likely cause is that the "no signal" end is grounded somehow. That could be a faulty component, or a short. The culprit could well be inside that blank passive filter area you haven't shown us.

[wellshuxley]

Thank you both very much! I'll focus my next debugging session on the path on the side of the resister that has no signal.

[antonis]

I'll focus my next debugging session on the path on the side of the resister that has no signal.

The resistor should have no CURRENT flowing through it..!!
(meaning, effectively out of powered circuit, as Sir Mike said above..)

P.S.1
As for what Sir Tom said: Meter applies a small current and measures voltage drop and not vice-versa..
(of course, all Ohm's Law involved parameters de facto pertain to philosophical dispute..)

P.S.2
Also Welcome..

[ElectricDruid]

P.S.1
As for what Sir Tom said: Meter applies a small current and measures voltage drop and not vice-versa..
(of course, all Ohm's Law involved parameters de facto pertain to philosophical dispute..)

Lol, ok!! You caught me, Antonis!! We'll just say that "R=V/I" and leave it at that, ok?!?

[wellshuxley]

Well, I'm embarrassed to admit that the issue was a trace cut on the trace connected to one of the 300 ohm resistor legs, which I had neglected to drill out. One of those "how did I spend hours overlooking that..." moments. But my prototype pedal lives!

Thanks you all again for your help.

[PRR]

> admit that the issue was a trace cut

Thanks. Such posts reassure everybody that 'headbangingly dumb' mistakes are common, routine, human.

I was betting on a solder-blob, my favorite zero-ohm resistor, but trace-cuts are good too.