All tests perfect - no audio (SOLVED)

[jamesnorth]

Hi Tim/Everyone,

I've built 2 J99's as well as a bunch of other SCA stuff and I've enjoyed it all.

I have one perfectly working J99 and one non-working J99.

The non-working one tests voltages perfectly on all the recommended areas and I adjusted the Op Amp voltages to run at +/- 18V which both J99's run at without any fuss.

Phantom works, the switch and fader work fine as well, and through the output, I can actually hear the gain going up and down as well as the normal small amount of hiss associated with turning knobs up and down. It is no more or less noisy than my other J99 when doing this same process.

When I plug a mic in, it pops etc and the XLR @ CONN1 seems to be continuous with nearby links in the circuit. The Jumpers are the same on both the working J99 and the non-working.

However, I hear no signal. For all intents and purposes the amplification and switches seem to be working, but no signal is passing through the pre-amp.

It must be something pretty basic that I'm missing, but I am a musician and producer before I am an electrical expert, so wonder what I should test out to check these parts of the circuit?

Thanks in advance!

James

[tpryan]

With power on and gain about half way up, what happens if you touch U1, pin 1 with a bare finger?

[jamesnorth]

With power on and gain about half way up, what happens if you touch U1, pin 1 with a bare finger?

Tim,

Thanks for replying ...

Absolutely nothing happens! Same as my working J99

Rechecked everything today - still nothing too obvious...

James

[jamesnorth]

With power on and gain about half way up, what happens if you touch U1, pin 1 with a bare finger?

Tim,

I think I misunderstood what you were wanting me to do ...

With the pre plugged in to a source, when I touch pin 1 on U1 and U2 it makes a buzzing noise - both SC99's make the same level output noise.

So ... I guess that might help?

James

[tpryan]

With power off, measure the resistance between pins 2 and 3 of the input XLR. Is it the same for both modules?

[jamesnorth]

With power off, measure the resistance between pins 2 and 3 of the input XLR. Is it the same for both modules?

Thanks again Tim,

It seems that both modules are the same, yes. They both show in excess of 20MOhm on the female XLR ... either really high or infinite I guess. My DMM only goes to 20MOhm.

I've tried reflowing the XLRs, jumper, SC99 and transformer points just to be sure as well.

I know this one isn't far off working ... and it kind of does seem linked to that input section. I found the J99 build to be a lot of fun and the working one sounds killer!

James

[tpryan]

You're not making the measurement correctly, or there are open connections on both boards.

[jamesnorth]

You're not making the measurement correctly, or there are open connections on both boards.

Managed to work out that I can only get a reading measuring the pins above the circuit on the back of the XLR itself.

Working XLR: 122 Ohm
Non-Working XLR: 120 Ohm

Any better?

Thanks again Tim

James

[tpryan]

What resistance do you measure between the cathodes (banded end) of D3 and D4?

[jamesnorth]

What resistance do you measure between the cathodes (banded end) of D3 and D4?

Hi again Tim,

I measure 10.2Ohm between the banded ends of these two.

Not sure if that helps?

James

[tpryan]

Sounds like pin 6 of the transformer is shorted to ground, possibly at the transformer itself or elsewhere in the circuit.

[jamesnorth]

Sounds like pin 6 of the transformer is shorted to ground, possibly at the transformer itself or elsewhere in the circuit.

Tim - spot on!!

I traced pin 6 along the schematic and circuit board and found the tiniest bit of solder was shorting two of the resistors over on the left hand side of the PCB. I had to use the strongest magnifying lens I had, but I could see a small drop.

I cleaned it up and bam - sound came through.

Really happy it's sorted and thanks heaps for your testing advice and help!

James

[tpryan]

Good job