Checking bias after transistor sub

[nashvillebill]

All the ancient gear I work on has obsolete transistors. So I use Keen's Laws (thanks R.G. ) and sub in a readily available part instead. I haven't run into any major hassles yet, other than needing to always check pinouts. (A shout-out to the Peak DCA55 transistor checker, it figures out the pinouts for me!)

After soldering in the substitute, I've always just run the most basic tests: measuring voltages around the new device in-circuit and checking against the schematic, then looking at a sine wave to see if there's any obvious problems. Do you guys go any further to check if the sub is biased relatively correctly? (I'm talking about BJT's since that's more typically what I've had to sub)

[dwmorrin]

Checking to see if the circuit is "biased" correctly is more about looking at the whole circuit and figuring out how it's supposed to work.
I would jot down "expected" voltages on the schematic, if the schematic doesn't already have them, and then judge the replacement based on that. Also do a little ohms law to determine maximum current you'd expect, and make sure your replacement is robust enough (datasheet: "absolute maximums") to handle the circuit.

I would highly recommend this old book called "Solid-State Servicing" by William Sloot. I found it used on Amazon for very little $ after a top NYC tech recommend it to me. It cleared up all my questions regarding transistor troubleshooting.

[J M Fahey]

agree and add:

in low power transistors (preamp) biasing is not that critical, as long as the gain stage is reasonably centered; but in high power ones (driver and output) biasing must be carefully measured, adjusted if necessary and checked that it's stable (leaving amp on for at least 10 minutes and checking it stays normal) to avoid thermal problems (overheating, runaway).

[nashvillebill]

Yes, I should've clarified, small-signal transistors, not driver or output devices.