Help wanted - R.G Neovibe with GGG PCB - no sound

[rustycage]

Hi, wondering if I can get some assistance? Neovibe build using a PCB from General Guitar Gadgets. No sound, no light. True bypass works OK.

Schematic: http://www.geofex.com/FX_images/vibeschm.pdf

Transistors Q1 - Q13 all 2N3904.

Powered by a 24V 1.25A wallwart; voltage after the rectifier reads 23.2V.

Voltages coming...

[rustycage]

Q1   C=2.4   B=0.9   E=0.4
Q2   C=2.4   B=2.4   E=1.8
Q3   C=13.1   B=2.4   E=1.7
Q4   C=11.9   B=4.7   E=4.8
Q5   C=11.4   B=4.7   E=3.6
Q6   C=14.9   B=4.7   E=4.5
Q7   C=11.5   B=4.7   E=3.6
Q8   C=14.9   B=4.7   E=4.5
Q9   C=11.4   B=4.6   E=3.6
Q10   C=14.9   B=5.9   E=5.3
Q11   C=23.2   B, E oscillating
Q12   C=23.2   B, E oscillating
Q13   C=23.2   B=0   E=0

There seems to be a problem with Q4 - base should be higher than emitter, right? - but I'm not sure where to look for the cause. I'm going to resolder R12 and check again but any help is gladly appreciated. Thanks!

[R.G.]

You're well on your way to doing your own debug. That's the first thing I'd have spotted.

If the base is right and the emitter's too high, then the emitter is either open or being pulled up. Check the emitter resistor to ground for correct value and proper soldering.

[rustycage]

Thanks R.G.

I've re-flowed the joints and checked component values for the following: R13 (100k), R12 (47k), R17 (4k7) (Q4); R19 (100k), R21 (47k), R23 (4k7) (Q6); R25 (100k), R24 (47k), R31 (4k7) (Q8)

Joints look good and the values are correct - at least according to the parts list. Still no change. Any suggestions?

[R.G.]

Yep. Grind harder.

When you measured the voltages, did you measure on the component pin or the soldered pad?

[rustycage]

I've now checked both pin leads and solder pads - all consistent. I also checked the resistors with the multimeter and they are in the ballpark. I'm stumped.

[R.G.]

Upon some more reflection, I think I missed another problem and that this may be a false one. We've run into this before. It's hard to measure the base, then the emitter accurately on some transistors in this circuit because of the high impedances involved. Measure Q4 with your red probe on the base and your black probe on the emitter.

While you're at it, redo those same tests on Q1, 2, and 3. I notice that I overlooked your listing Q2's collector and base being at 2.4V, and Q3's emitter being low. I may have missed the real problem.

[R.G.]

I did a little measuring. You should see close to these:

		C		B		E
Q1		2.18		1.51		0.96
Q2		4.24		2.18		1.55
Q3		11.3		4.24		3.6

[rustycage]

Thanks for your help R.G.

Q4 measures 0V from base to emitter. I reflowed the joints on R1 - 10 and measured the voltages on Q1 - 3:

Q1   2.2   1.0   0.7
Q2   3.2   2.2   1.6
Q3  12.1  3.2   2.6

So Q2 is looking a bit closer to ideal but Q1 B-E is still a problem. This measurement is with the 100k pot fully clockwise, at the other end the values are B=1.4 E=0.9.

[R.G.]

Q4 Base-emitter is shorted, either by a transistor failure, or by an external short.

Run a razor knife between base and emitter pads and all places where there could be a solder short or an almost invisible thread of copper left on the PCB material (it happens sometimes). If that does not fix it, remove Q4 and test for a base-emitter short outside the circuit; put a different transistor back into the board in any case. This may fix it.

[rustycage]

I desoldered Q4 and tested it - got an hfE reading of 186. I put it back in the circuit and measured 0.1 - 0.2 B to E.

I've got a show to get ready for tomorrow night so I won't have any time to look at it until the weekend. I'll have a closer look then.

[rustycage]

Dammit. I thought I had the solution - I'd installed a shorting jack at the input by mistake. But replacing it hasn't helped at all.

I think I need to razorblade between the pads and re-measure the voltages on the circuit.

[R.G.]

As Arthur Conan Doyle said through Sherlock "Eliminate the impossible, Watson, and whatever remains, however improbable, must be - the truth."

(They used a lot of commas back then.)