MPSA42 Transistor in Linear circuit (HX-1681)

[Steve Harrison]

At 05:17 PM 12/17/98 -0500, George Gleim wrote:
>Hello Green Guys,
>
>I have another question that I'd like to put to the group if you don't
>mind.  In the Heathkit HX-1681, there is a transistor (Q-411) that
>provides a path to ground and triggers the T/R relay in the Linear Amp.
>The transistor is a MPSA42.  I found out that this transistor can handle
>the relay in my Ameritron AL-811 Amp just fine (24 volts, 100 mAmps) but
>cannot handle the 130 volts 100 mAmps required by my Heathkit SB-230
>Amp.

My Motorola Small-Signal Transistor Data book (DL-126) calls the MPSA42 a
"high-voltage transistor" and rates it at 300 volts maximum
collector-to-emitter voltage to 500 mils maximum collector current. I think
it *should* have worked fine with your SB230's relay. Perhaps the original
constructor installed the kickback suppression diode across the 230's relay
backwards and it has been blown. I would try replacing the kickback diode
first; the lack of this diode will kill nearly any reasonable-sized keying
transistor.

As for a replacement for the MPSA42: Digikey does list the specs of some
vendor's transistors in the catalog and you can try looking for
"high-voltage" transistors (you want an NPN silicon, at least VCEO=300 VDC,
IC=500 mA, hFE over 25 will do fine). Alternatively, you can search the
Motorola web site for such transistors and look to see if they're in the
Digikey or Mouser catalog.

Also, don't forget to check Dan's Small Parts listing. Try doing a page
search for "high-voltage transistors" or words to that effect. You probably
won't be able to beat his prices. He has nearly everything else under the
sun: it's IMPOSSIBLE that he doesn't have at LEAST one HV transistor!

>I know that I can install a more  robust switch like the SoftKey
>modification that Harbach sells (www.harbach.com) but does anyone know
>if I can simply replace the MPSA42 with a different transistor to
>achieve the same result?

I really think the relay in the 230 kicked back and killed the 42; check
that diode across the 230's relay, or just replace it with a 1N4004 or
better (any power silicon diode rated at greater than, say, 200 PIV, note
that a 1N4001 is only rated at 50 PIV). Since there's 130VDC across the
230's relay, you can't use an ordinary low-voltage small-signal diode such
as the 1N4148 or 1N914! If you want, you can simply install a diode across
the MPSA42 inside the 1681. However, that will still allow an
externally-mounted relay to generate quite a voltage spike over the long
cable between the 1681 and the relay due to the cable's inductance. But you
would at least probably protect the MPSA42.

>And, does anyone know where I might find a replacement MPSA42
>transistor?  (Dan's Small Parts doesn't carry it, nor does SS of NE,
>Radio Shack...)  I may have damaged the currently installed transistor
>during my testing.

To test the new transistor, simply connect a resistor calculated to draw
something like 10 mils to, for example, +12V or the like. Key the radio
while measuring the collector voltage: it should drop from the +12V to
almost ground. Or plug it into your AL-811.

73, Steve Ko0U/1

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