[R-390] OT: Pass Transistor Question

[ews265]

Don,

This is an old beast that I just happened upon that has one bad 
channel.  Right now I'm resisting working on it just for the mechanical 
reasons you describe.  Sounds like I don't need to worry about 
matched/selected parts and your comments do explain the parasitic 
suppression components.  Also sounds like the older transistors may have 
had a lot of phase shift at their upper limits with the potential for 
creating havoc with the amp's feedback loop.

The mechanical packaging really is pretty bad.  I was unpleasantly 
surprised.  Documentation's nothing to write home about either.  Digging 
in to the docs still leaves ???  Anyway I'll be sure to sweep the output 
for any signs of life after I get it back together.  Any special 
conditions that's apt to make it go into flight; level, freq etc?

Jon

2002tii wrote:
> 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
>
>
>
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