[CQ-Contest] 2nd Question

VR2BrettGraham vr2bg at harts.org.hk
Mon Dec 3 06:26:04 EST 2001

N5RP asked:

>So, here is question @#2; a bit harder.
>KM5M came over last night with a new (old) MM3 keyer as once upon a time
>manufactured by AEA under his arm, and it does not key.
>One output keying jack shows no activity; the other shows dead short.
>I have no schematic to look at, but on blindly opening up the enclosure, I
>see two transistors mounted on the PC board that are located close to
>those+ and - output keying  jacks.
>Are these the keying transistors, and if so, what is the nominal type or
>value of these transistors ?

For the positive output, an NPN transistor with sufficient rating to handle
the open state voltage of the radio connected & current when not open state.

For the negative output, a PNP transistor of the same flavor.

They also have a diode across them to protect the transistor from being
connected to a radio of polarity type wrong, so you may be seeing a shot
diode instead of transistor.

Something like a 2N3904 or 2N3906 (respectively) will get you up & running
in the meantime, but these transistors are not quite as hefty as the originals.

Sheesh - I must qualify for QCWA now if I can't remember stuff like this, 
I don't recall having to change them frequently.  For some reason, MPSA51
rings a bell for one, but don't trust me on that!

73, VR2BrettGraham

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