[Amps] Lightning strike - Alpha 77Sx grid issue

[MU 4CX250B]

Gary,
If you’re sure the other multimeter functions are working, then check
the switch contacts, R214(1K) and R211 (4.7 ohms). There isn’t much
else that could go wrong. The grid current is measured by monitoring
the voltage drop across R211. If other multimeter functions (plate
voltsge, ALC, forward power, etc, are showing a problem, however, then
you’ll likely have to dig a bit deeper to find the bad component.
73,
Jim w8zr

Sent from my iPhone

> On Nov 30, 2019, at 5:46 PM, Gary Smith <Gary at ka1j.com> wrote:
>
> The day before Thanksgiving I was
> listening on 160 & lightning hit close to
> the antennas. Heard a sizzle in the shack
> and lost a few things. Guess Polyphasers
> in the coax and a good ground isn't enough
> to dodge the bullet.
>
> Among the things damaged were the LP-100A,
> the P3 Wattmeter, Computer BIOS, GPSDO and
> now I find the grid metering is not
> working on the Alpha. When putting the amp
> to the load/tune settings I have written
> down, it performs normally and my signal
> report was 59+ with excellent audio.
>
> All the other meter functions are working
> as before but not the grid metering. When
> I transmit the grid goes to .7 MA and
> stays there unless I increase the input
> power significantly and then it raises.
>
> Adjusting the load does not affect the
> meter but if I increase the load out of
> limits, the ALC meter does light and then
> goes out when I back off the load. It
> appears the ALC is still intact.
>
> I'm not enough of an engineer or tech to
> adequately understand the schematic, but
> looking at circuit board CB-2, the control
> board, I see Q202 & Q203 as well as a few
> diodes are in the metering path. I have no
> way to test the transistors but they are
> inexpensive enough I can just replace
> them.
>
> I made a 3 page PDF of the schematic for
> the push switch for grid and the control
> board.
>
> http://doctorgary.net/grid-circuit.pdf
>
>
> The entire pdf manual is here at Alpha RF
> systems
>
> https://tinyurl.com/sndumoa
>
>
> Any suggestions for what I might check
> would be greatly appreciated.
>
> 73,
>
> Gary
> KA1J
>
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