Posts: 1,162
Threads: 26
Joined: Aug 2005
I have an ascii-art schematic, which I have (hopefully) privately e-mailed to you. I can't think of a sane way to include it in this post.
The constant current circuit is 4 components. 2 resistors and 2 transistors. The idea is that when the load current increases, the voltage drop across the 13R (Ohm) resistor increases, thus turning the small-signal
transistor on harder. This reduces the bias on the power transistor (normally applied by the 4k7 resistor to ground), thus reducing the load current again.
It's not going to be hard to debug. Check the transistors (remove them, and at least check them as a pair of diodes with a multimeter). Most likely the power transistor is open-circuit on at least one junction. I don't think these
transistors have exact modern equivalents, but they're not exactly critcal, and any PNP small power transistor would work. Similarly, for the small-signal transistor, anything will probably do. I'd try a 2N3906 first, as I've got those by the dozen. But I haven't _actually_ tried it.