Weird measurement of Vbe in Q2 of Vox wah

Started by PietS, October 17, 2023, 11:39:28 AM

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PietS

I recently bought a used Vox wah with the intent of doing some modding (it's the US made V847 from the 2007 or so). It works fine, but thought I check out the bias of the 2 NPN transistors (MPSA18).
Q1 read fine, with collector at 4.1V, the base at 0.7V, emitter at 0.1V.
But Q2 has me stumped. When using circuit ground as zero point, my cheapo multimeter reads: collector 8.8V, base 2.7V, emitter 3.3V. Which equates to a Vbe of -0.6V. But when I measure Vbe directly from the legs of the transistor, I do get the expected Vbe of +0.6V.
I thought that there must be something wrong with the transistor so I swapped it for a 2n2222, and I get the same thing. Lower voltage at the base then emitter when using circuit ground as zero, but normal Vbe of 0.6V when measured directly.
I'm a bit of a newbie and this has got me stumped. Any idea what's going on?

FWIW the circuit is stock and the PCB, components and wiring all seem fine.

Eb7+9

your DMM is actually drawing minute current from the circuit voltage it's trying to measure - a perfectly zero-current voltage measurement does not exist in this corner of the galaxy - very low yes, common/cheap DMM not so much ... measuring base voltage to ground on a biased bipolar transistor tends to be "all over the map" so to speak because this minute (DMM drawn) current is potentially/likely in the same ballpark as the base current that is responsible for turning on the transistor in the first place ... in this way your meter is pretty likely, almost guaranteed, to alter the reading from the (un-loaded , ie., true) base voltage by robing base bias current in the process ...

antonis

"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

PietS

Eb7+9 and Antonis,
Big aha moment here. It suddenly makes sense. Thanks for the quick and clear answers.