Originally Posted by K5Doom
From left to right (jack to potentiometer) , I get
1672mV (left pin), 27V on the center pin and 2.23 V on the right one
No, No. Measure output offset from Ground to the tabs on the headphone jack. I don't think it matters, though. If you were able to bias everything correctly, then the signal itself is blocked. I suspect FallenAngel is correct and the relay is not firing.
Unfortunately, the most common mistake is installing the BD139 backwards - but yours looks fine. The QM2 looks a little weird, though - is that the MPSA14? It will not work without that transistor - and it's not a normal transistor - it's a Darlington.
Also, it looks like your input wiring on the pot terminal block is hanging by a thread - maybe those are not making good contact?
Be patient ... your biasing measurements look fine ... it has to be something very simple that's wrong. One sure way to check that the issue is with the relay is to connect some jumpers to Ground, and to the leads on RB14L and RB14R, then to your headphone jack or directly to your headphone plug. This will bypass the relay circuit. The tab closest to the hole is ground on the jack, or the long part of the shank on the headphone plug. It doesn't matter which jumpers of RB14L and RB14R you connect to the other two positions - all we want to confirm is whether you have sound.