[sdiy] Some help with a current mirror, please

[Jeff Farr]

I'm working on troubleshooting my x0xb0x and I can't for the life of me
understand how a certian current mirror is operating.
Here is a pic of the schematic:
http://www.ladyada.net/make/x0xb0x/files/mainboard_beta.png
I'm looking at Q1-Q4 in the VCA section and while taking measurements I've
found that turning the gate on puts .5v at the collector of Q3 and 12v at
the collector of Q1. However, reguardless of the 'gate state' the collector
of Q4 is always at 12v. It's my understanding that the collector of Q4
should be ~ .6v below the rail while current is flowing and 0v while current
is not flowing. Perhaps I just don't understand the mirror?
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