[R-390] OT: Pass Transistor Question

[ews265]

Don,

Thanks for the info on the MJ15003s.  I've already purchased a handful 
or 2N3773s since they were cheap and available so I'll give them a shot 
first.  They are Brand X by the way.  Could be one of the thousands of 
processes Motorala must of spun off to the 2nd line fab houses.  If I 
end up with flight at/near current limit I won't be too concerned since 
I don't plan to run the amp anywhere near there anyway.  And yes I am 
the curious type.  Sounds like this could turn into an interesting exercise.

So how did Crown Audio get the kind of recognition they received?  Given 
the parts available at the time, dealing with basic design problems is 
one thing.  The packaging  however is such a disappointment.  I think I 
could have done better in my garage.

Regards,
Jon  WA3MVM



2002tii wrote:
> Jon wrote:
>
>> 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?
>
> In all honesty, unless you are just obstinately curious, if you 
> haven't put the effort into it yet -- don't bother.  After hassling 
> with the worst mechanical design you've ever experienced in an 
> electronic product, you will have a thoroughly mediocre amplifier (at 
> best).  Sell it on eBay and buy an Ashly FET-200/2000 or a Bryston 4B.
>
> If you insist on repairing it, use MJ15003 output transistors (real 
> Motorola ones) instead of the original 2N5631s.  If the TO-66 drivers 
> are broken, good luck -- very few manufacturers make TO-66 packages 
> these days.  You can make a TO-220 transistor fit, but being 
> plastic-cased transistors, they don't last.  Whatever you do 
> (including using stock parts), be prepared to adjust the compensation 
> to maintain stability.  (Note that Crown provides two schematics, one 
> for each brand of output transistors they used.)
>
> Run it up to clipping without a load, with an 8 ohm load, and with a 4 
> ohm load, using any convenient sine wave in the 500 Hz to 2 kHz 
> range.  Look for bursts of oscillation as you do this (it will appear 
> at the same spot each cycle -- check particularly as the amp enters 
> and leaves clipping).  Don't push it very far into clipping without a 
> load.
>
> Do not try to reach full power above 10 kHz unless you are willing to 
> fix it again.  I generally ask power amplifiers to drive 2 kHz square 
> waves into an 8 ohm load resistor paralleled by a variety of 
> capacitors from 0.001 uF to 1 uF, but Crowns will not tolerate this, 
> either.
>
> Good luck!
>
> Don
>
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