ASP Gain Modification
Many thanks to Paul G4DCV for the following modification.
"I've been looking for an ASP for a while and recently found one on
ebay. It worked OK and I've been getting nice reports when using it but
was clearly tired. The main problem was that it was a bit insensitive &
I found I had to talk louder than I wanted to get the OK LED to stay lit
rather than just flash on and off. OK for casual QSOs but not much fun
when I wanted to save my voice in a long contest or speak quietly late
at night without disturbing the family. I tried it with 3 dynamic mics I
have here, a Yaesu MH-1b8, Shure WH20 (my headset mic) and just out of
curiosity, a Beyerdynamic M58 broadcast reporter's stick mic. All gave
similar results.
So I did a mod thanks to the circuit diagram on your web page. TR1
(BC182) is the mic amp and on the low Z setting the 22k emitter
resistor R5 is bypassed with a 10uF electrolytic (C2) to gnd to increase
the gain. The second, un-bypassed. emitter resistor R4 (3k3) sets the
stage gain and if my sums are right Datong set it at 17x (25dB).
Given the age of the Datong ASP I figured that C2 might have dried up a
bit & if the ESR had increased it would lower the stage gain. So I
replaced it. While I was at it I replaced all the electrolytics on the
board, there are only 5 of them so it was a 10 minute job. But for belt
and braces I removed R4 and R5 and replaced them with a small preset pot
so I could adjust the gain. The new preset pot is set so that there's
just enough gain for the ASP to opearte properly at comfortable speaking
level but no more.
It now works great. I can speak quietly and the OK LED comes on and
hangs there as described in the manual."