Ye Olde Vox Kensington Amp Quandry

[g1]

So with your tracer you hear good signal at Q2 collector. You should hear same signal at both sides of C6 and R9, do you have that?

[fdesalvo]

G1, what’s up!

I will reconfirm my findings now that I’ve got my eyes and ears on. Since I no longer have time to work on instrumental music, I reconfigured my patch bay and interface and created a Logic session for amp work.

The patch bay has an audio probe input which routes to my apogee quartet and into a logic strip which has a clean amp model and frequency analyzer set up to receive and analyze the frequency band of any noise the probe sniffs out. There’s also an output setup, which will inject a sine wave from a variable oscillator (that’s setup on another channel strip in the same session) into the faulty amp’s input. Finally, there’s an input which will pass an amp’s speaker out through to my bench speaker, so there’s no more spaghetti on the desk.

Pretty excited to start hitting these types of amps head-on. I’ve had to pass up a lot of work due to being limited by what I was able to test, or in the case of solid state amps, understand. I literally quit focusing on school once we moved into solid state devices, which has proven to be to my peril.

I finally feel like I’m learning something again, so I can’t thank you guys enough. I usually just swim by here and get help, then move on due to being underwater with life in general, so all of your help has not only helped me grow as a technician, but it has helped me help others while providing financial help to my family. I have been living with some very mild spectrum disorder, which makes some things more challenging than others - namely getting lost on a printed circuit board when comparing its schematic, which I usually find moderately easy to metabolize by comparison. Damnit, Leo knew what he was doing.

cheers and update soon

[fdesalvo]

When I inject a 1khz sine into the input, the amp passes it all the way through loud and clear, with full tone control range. Swapping cables and playing my guitar into the same jack/cable yields no sound at the output. This guitar has zero issues.

Q2 collector has strong signal, but both sides of C6/R9 are very faint.

Edit: I've also run a sine sweep from 100hz - 6khz through at various db levels through with no issues. Just the instrument at the input causing this issue.

[g1]

What is the voltage level of your injected tone?
If it is low level, like a guitar, I suspect you may have a ground issue, and the test setup is getting around the problem.

[fdesalvo]

I haven't measured the output of my audio interface, so that's a great question. I swept the level from 0DB+ and the sound broke through around 1DB if that measure is useful at all. I also performed your test and the signal present at either side of c6/r9 are incredibly faint compared to whats at the q2 collector. I have the test tone panned hard left and the probe's hard right. It's just barely audible.

[g1]

Check resistance from Q2 collector to side of C6 it is supposed to connect to. Sounds like maybe a damaged trace or bad solder joint.

[fdesalvo]

Check resistance from Q2 collector to side of C6 it is supposed to connect to. Sounds like maybe a damaged trace or bad solder joint.

It's in the 5m ohm+ range

[g1]

Then it is not connected. Are you sure about the EBC pinout of Q2 ? You should be able to follow a trace from one leg of Q2 to C6.

[fdesalvo]

Edit - the EBC diagram doesn't reflect the state of Q2 on the circuit board. R7 is connected to the base, as opposed to the collector. Q2 base to c2 has continuity. C2 measures 3.5uf according to my meter and has great ESR measurement, as well.

[Helmholtz]

Then it is not connected. .

??
5 mOhm is essentially a short.

OP probably means MOhm. Difference is a factor of 1000000000.

[g1]

??
5 mOhm is essentially a short.

OP probably means MOhm. Difference is a factor of 1000000000.

Yes, I took it that he meant Meg ohms. Most meters do not have milli ohm ranges or readings.

[fdesalvo]

??
5 mOhm is essentially a short.

OP probably means MOhm. Difference is a factor of 1000000000.

The EBC diagram included on the schematic didn’t illustrate the actual pinout. That previous measurement was based upon the diagram and not the actual layout on the board itself.

[g1]

Did you mean 5 Meg?

[g1]

I don't think you should go by the pinout shown on the drawing, go with the schematic connections. C2 connects to Q1 base but also Q2 collector.

Is it possible when you were tracing signal there may have been signal at the base and you thought it was at the collector, if pinout drawing is wrong?

[fdesalvo]

I don't think you should go by the pinout shown on the drawing, go with the schematic connections. C2 connects to Q1 base but also Q2 collector.

Is it possible when you were tracing signal there may have been signal at the base and you thought it was at the collector, if pinout drawing is wrong?

I did mean Meg originally. Pardon. The schematic doesn’t represent what’s going on on the board. The base/middle leg is connected to the 18v B+ node, for example. In any case. There’s extremely weak signal on either side of c6/r9.