[Amps] Resolved - Re: Lightning strike - Alpha 77Sx grid issue

Gary Smith Gary at ka1j.com
Sat Dec 21 02:30:32 EST 2019


Thanks for the on & off list replies. I'd 
checked out the push switch, resistors 
relating to the grid & ALC circuits. All 
were fine in the CB-2 Control board. All 
the caps were WNL. The diodes were all 
responding normally except D211 which 
offered a beep in each direction with the 
DVM so I was on the right track

Removed the diode and it was fine out of 
the circuit but it shouldn't have done 
that so looking at the 2N304 transistors, 
rather than test each one with the DVM, I 
just pulled and replaced them. Since I had 
the board out, I replaced Q202 (MJE802) 
using a liquid metal thermal compound for 
better heat transfer instead of the 
original paste compound), I'd already done 
that with the 24V regulator a few months 
back. 2nd step was replacing Q203 (2N3904) 
as that diode connects to 203's base, 
retested the diode and it now properly 
only passed in one direction. 

I assume that was the only problem but as 
mentioned, I replaced all the inexpensive 
2N3904 transistors related to ALC & Grid, 
put everything back, fired her up and now 
the ALC works as expected and the grid 
current is reading properly as well.

Yesterday when I did the sleuthing & 
repair it was my Birthday, so fixing it, 
'twas a nice present I gave myself. :)

Thanks again for those who offered 



> 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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