|Using a base-collector strapped transistor as AM detector, sharper knee for better weak signal demodulation|
The emitter on such an NPN transistor would be the cathode, and the base tied to the collector is the anode Here I used a 2SC2785 salvaged from a VCR or such. If the transistor is a PNP, its emitter would be the anode, and its base tied to its collector the cathode. Try various transistors. I found that 680K seems to hit the sweet spot in this radio, but it's not a critical adjustment on a trimpot (which you can use to quickly find the value you'd want in a fixed resistor). I had thought that using a transistor this way was the mark of a sloppy company (did they run out of diodes?), but I got the best weak station detection performance with this. Oh, check strong stations too, which worked well too.
Some simulations of a diode, a base-collector strapped transistor,
and such a transistor with a bias current.
I'm not sure how much, if any, knee can be below 0V here. Conduction on the other side of 0V would probably mess up the detection. IIRC, the FCC requires that 10% be the minimium amplitude of an AM station's carrier, which should permit some knee curvature of stations that aren't too weak (else signal to noise would make them unlistenable anyway).
You know the old loose bipolar NPN or PNP transistor test method of checking the PN junctions (with
your DVM in diode mode), like
from the base to the emitter, and then base to collector. You can figure out if it's an NPN or a PNP
transistor (compare to a diode). And additionally, you can see some transistor activity by doing this
following test: First, having
one test lead on the emitter and the other on the base (but not the collector), in the direction you see a diode drop voltage,
typically you'd see about 0.68V on a silicon transistor. Now while keeping the same test lead on the emitter,
touch at the same time with the other lead both the base and the collector. You should see a lower
voltage like 0.59V (around 85% lower). This test can also tell you, with surface mount devices, if you
really have a transistor, or a device with two diodes, one with a common anode, or a common cathode on what would
look like the base pin.