Need help troubleshooting a Pimped Alice circuit
- Thread starter skyy7
- Start date
Help Support GroupDIY Audio Forum:
Discrete semiconductors and passive components have well known behaviors in circuit. Voltage drops across semiconductor junctions can suggest whether devices are possibly faulty. Ohms law will suggest appropriate voltages across resistors in circuit.
This knowledge is not only useful for troubleshooting, but also for design.
JR
Tried resoldering and nothing. I am getting DC signal through the resistor and the DC values are correct but the AC kinda just turns to 0 after the resistor which I don't see how that makes any sense.
I also measured the resistor outside the circuit with my VOM and got the correct value and measured the voltage drop across the resistor using a cheapo power supply i have which was also correct.
Ok, so I did the calculation (with the help of simulations), and I'm getting everything where it should be right up until after the 10K resistors where the DC values are correct, but the AC value with a .1v ac signal @ 1k injected at the capsule just turns to 0.
Also, I believe I made a statement about not having the correct AC voltage at the legs of the 2sk209. Please disregard that the AC values I am measuring at the source and drain is correct.
Update: I think it's the output section as when the resistor is disconnected from the base of the 2N5087, I get a normal AC signal across the resistor.
Another update: I think I found the issue... I think I accidentally bypassed the 100K resistors.
Another update: I think I found the issue... I think I accidentally bypassed the 100K resistors.
Voyager10
Well-known member
Ah, so it was Q1/Q2 bases shorted together 
I suppose the moral of the tale is to actually measure it rather than just looking.
I suppose the moral of the tale is to actually measure it rather than just looking.
Yeah, you were right. Fortunately I am now getting the correct VDC at the collector and the correct VAC at the base but no output on the emitter... I gotta diagnose that when I get back home from work though. Probably a short somewhere. But thanks very much for everyones help.Ah, so it was Q1/Q2 bases shorted together
I suppose the moral of the tale is to actually measure it rather than just looking.
Ok, so the mic is finally finished. The problem I found was that one of the 2N5087s was busted and shorting the collector to the emitter. I am now getting a really good output with seemingly minimal distortion, from what I can tell based on the janky single injector. I just have to clean it up and reassemble it. Thank you, everyone, so much for your help and now I know how to diagnose issues, especially for my next upcoming builds.
On one hand, 2N5087's are rated for 50v Vce; preeeeeetty marginal if you ask me, for 48v phantom power.
On the other hand, the original Schoeps circuit has antiparallel 6.2v zener diodes (D3 & D4) across the two PNP output transistors, precisely in order to clamp the max voltage present across those transistors.
https://groupdiy.com/threads/schoeps-cmc5-inductor-values.73321/
Shouldn't be a problem using the same 12v zeners you use for powering the JFET (no need to source another part / the zener voltage isn't that critical).
On the other-other hand, i guess there's always the possibility of static electricity zapping some silicon junctions, but... to my knowledge, I don't think i managed to do that (especially on something as insensitive as bipolar transistors) in my 20-odd years of dicking around with electronics. YMMV though...
Last edited:
Ah, I'll probably incorporate this. I might've wired something weird while messing around with the circuit and fried the transistor at some point...
that is a common fault mode in power transistors where over voltage causes punch through shorting collector to emitter.
[TMI warning] Back in the 1960s as a junior electronic technician one of my tasks troubleshooting blown power transistors in a switching power supply was to grind the tops off metal TO-3 transistors. Voltage punch through would generally leave visible evidence with a small sunken region of melted silicon around the dead short. [/TMI]
===
I don't know what kind of voltages these parts see in normal use, but phantom power is notorious for large negative voltage spikes if charged up phantom decoupling capacitors get accidentally shorted to ground (like in a patch bay). It is also possible to damage devices with static spark discharges but that is less common with bipolar devices like 2n5087s.
JR
