Help me compare these two power amp transistors

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Tom Whitworth:
I managed to fry (I think) a power amp the original part is MJE521

Data Sheet :

I selected a MJ200STU from mouser

Data Sheet :

They look similar except the replacement seems a little smaller in wattage.

My transistor checker cant deal with the big  power amp transistors so I don't really know if its cooked or not but I do know the bias resistors got a good cooking  >:(

Clark McDonald:
That really isn't a good sub here.  A good sub should meet or exceed the 3 main specs, and this one does not. 

The 521 is 40, 40 and 5

The 200 is 40, 25 and 8 

The 25V for the Vceo is lower than the other and that means this transistor would likely not survive in that circuit. 

Use your DMM set to Diodes scale or VTVM/VOM set to Low Ohms to check the three leads of the transistor against each other.  If any are shorted, bad power transistor. 

MCM Electronics lists the exact part you need, at twenty seven cents ea.

While there, pick up the Emitter resistors that opened as well: 

Use these two .47 ohm to replace the original .51 ohm.  Not a problem that they are slightly off of the original, what's important here is that both are in the ballpark and matched. 

Is there a Trimpot or two located on the driver board for this amp?  This could be important. 


Tom Whitworth:
No trimpot, there is a zenior in there I need to check as well.

The rig is a SB303 that up until this little screw up worked fine.

Here is a link to the schematic unfortunately it has a captcha thing on it so I could not directly link it.

Clark McDonald:
Likely all that got hit were the output transistors and the resistors. 

In the ss tlo power amp, diodes only need to be replaced is shorted, which is very rare but can happen.  I don't see a zener in there, incidentally. 

And not likely anything could be damaged further back due to the coupling cap between driver stage and outputs. 


Tom Whitworth:
Your right it is not a zenior, my bad. still its an odd thing evidently adjusts bias to compensate for thermal drift.

It checked out okay though.


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