[R-390] OT: Pass Transistor Question

[2002tii]

Jon wrote:

>Wow.  Collector current vs. Base Emitter voltage.  That would be 
>difficult to measure and more so to characterize.  Crown of course 
>doesn't specify what should be matched.  The expectation is that you 
>buy THEIR kit of matched parts.  My supposition was that for an 
>"equal" drive top and bottom, the output circuit should supply equal 
>currents to the load, top and bottom so that at max output, the 
>output stage both top and bottom would go into current limit at the same time.

Unless we hear objections, may as well stay on-list.  You DO play 
your 390 through it, don't you?  Besides, the Crown is an antiquated 
boatanchor in its own right, and all too often they act as MW 
transmitters, "chirping" at certain output voltage and current 
levels, so a radio list is an ironically appropriate place to discuss 
them.  If anybody wants to follow along, the service manual can be 
downloaded (in two parts -- the schematic is in Part 2) from: 
http://www.crownaudio.com/gen_htm/legacy/legacamp.htm

The current limiting doesn't depend on the output transistor match -- 
it is done by limiting the drive to the output stage.  The voltage 
across the output emitter resistors, R132/232 and 134/234, is 
measured by Q103/203 and Q104/204 (and adjusted for the instantaneous 
output voltage by some diodes and resistors), which turn on to steal 
drive from the predrivers (Q107/207 and Q112/212) and thereby limit 
the output current.  Page R-8 of the manual shows the resultant V-I 
limiting curve.  The threshold voltages of R132/232/134/234 and the 
conduction voltages of the diodes will affect the current limits a 
tiny little bit, but this is entirely negligible.

As I recall, Crown said their output transistors were *selected*, not 
*matched*.  I believe Crown tested them for high frequency gain, 
because of the aforementioned tendency of the amplifiers to 
oscillate.  I think they also tested for second breakdown.  (Remember 
that these amplifiers were designed at a time when silicon power 
devices were in their relative infancy, and some of their parameters 
were much inferior to the next generation(s) of devices.)  I fixed 
hundreds of Crown amps without doing any beta testing or matching, 
and they have worked to spec and reliably.  Since 1975 or so, I have 
always upgraded the output devices to more modern parts, which vastly 
improves reliability and cures much of the fussiness and proclivity 
to oscillate.

BTW, the Crowns are beastly, hateful nightmares to work on, from the 
standpoint of mechanical design.

Best regards,

Don