Testing Darlington Transistors with PEAK Atlas Tester

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bedrock602

Member
Joined 2017
The sound from the right channel of my Sherwood SEL-400 amp dropped out suddenly and had a "hissy" fit, no smoke but I did smell something cooking when I opened up the amp and noticed that a very faint smell seemed to come from the Right channel output transistors.

The schematic indicates that output the transistors are SJ1936 & SJ1937, these are present in the Left Channel and they each have a "blue dot".

The Right Channel has Motorola SJ1902 & Texas Instrument SJ1903 which are obviously replacements so the amp must have had this problem before.

I pulled the outputs to test them and this is what the PEAK Atlas showed. (Three of them read what is indicated with an asterik*, the fourth does not). My guess is that the Right Channel transistor Texas Instrument SJ1903 is faulty since it is the odd man out. Can someone please confirm this?

Right Channel:

Motorola SJ1902

NPN Darlington Transistor
*Diode Protection between C-E
*Resistor Shunt between B-E
*hfe not accurate due to B-E res
Current gain hfe=27
Test Current Ic=2.50mA
Base-Emitter V Vbe=0.860V
Test Current Ib=4.096mA
Leakage Current Ic=0.000mA

Texas Instrument SJ1903

PNP Darlington Transistor
Current gain hfe=34
Test Current Ic=2.50mA
Base-Emitter V Vbe=1.172V
Test Current Ib=3.785mA
Leakage Current Ic=0.000mA



Left Channel:

Motorola SJ1936

NPN Darlington Transistor
*Diode Protection between C-E
*Resistor Shunt between B-E
*hfe not accurate due to B-E res
Current gain hfe=22
Test Current Ic=2.50mA
Base-Emitter V Vbe=0.807V
Test Current Ib=4.148mA
Leakage Current Ic=0.000mA

Motorola SJ1937

PNP Darlington Transistor
*Diode Protection between C-E
*Resistor Shunt between B-E
*hfe not accurate due to B-E res
Current gain hfe=23
Test Current Ic=2.50mA
Base-Emitter V Vbe=0.841V
Test Current Ib=4.113mA
Leakage Current Ic=0.000mA
 
Last edited:
Netlist

Netlist

Moderator
Joined 2003
Paid Member
The current gain values will probably be far off if measured with the PEAK.

Edit: the PEAK is good for small signal transistors due to it's voltage and current limitations.

Here is a thread proposing replacements, which seem OK to me:
https://audiokarma.org/forums/index...00-output-transistor-subs-equivilants.282181/

Mouser has the 2N6059 and the 2N6052 but they are utterly expensive.
BDW83C and BDW84C can be used too, but will be hard to find.
Furthermore, I recommend to only buy from reputable sources.

Hugo
 
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wg_ski

Member
Joined 2007
In the early days (before the internet) all the big distributors required a tax ID. Mouser and Digikey didn’t but they were small potatoes with limited stock in those days. The first one that carried real audio transistors who sold to anyone with a pulse was MCM (spin off of Newark, now Re-absorbed).
 
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wg_ski

Member
Joined 2007
Aside from just the basic ratings, which any TO-3 will do you need to pay attention to the secondary breakdown breakpoint. You need a transistor that is happy on +/-50 V supplies. IOW, it must handle a useful amount of current at 50V. MJ11016 is on the edge of useability, 2N6284 is not suitable. Either is fine on +/-40. MJ11022 handles full power to 50V, which is far safer. They are $9 and the PNP is out of stock at Mouser. All of these are available in TO-247 (MJH package), and I suspect the TO-3 packaging is on its way OUT (keep eyes peeled for EOL notices). They are getting stock from ON now, so you may be able to get the 11021 in your lifetime.

Do NOT mix families or brands between an NPN and PNP pair. Mixing voltage grades is fine. The B-E resistor values need to be the SAME between a pair unless you plan on running the bias hot enough to overcome differences and force high current into both outputs. Even then the distortion will be higher than if they had the same resistors.
 
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