[Elecraft] K2 RF Gain Control Quit

[N2EY@aol.com]

Update to previous message:

Lots of strange voltage readings in the gain control circuits. Replaced Q12 
with a junkbox MPS3904 (I KNEW I kept that old 386 motherboard around for a 
reason!) Working much better. Going to fool with the threshold adjustment and 
realign before buttoning up. And of course stick a 2N2222 in there...

73 de Jim, N2EY 

In a message dated 5/13/03 1:18:42 PM Eastern Daylight Time, N2EY@aol.com 
writes:


> K2 #2084 (Rev. A, no mods) has developed some strange symptoms over the past 
> few days....
> 
> When first turned on, only the very strongest signals can be heard, S meter 
> is fullscale and RF gain control makes no difference. Rx acts as if RF gain 
> is turned way down. Turning AGC on or off doesn't help.
> 
> Tap the key and after each dit or dah the gain jumps up to normal for a 
> couple of milliseconds, then drops back to very low gain.
> 
> After some time, rx begins to act normally, but the response to the RF gain 
> control is very slow (turn the knob and the RFG goes up or down very 
> slowly).
> 
> Symptoms come and go.
> 
> Looking at the schematic, it appears to me that the PN2222 transistor, 
> 1N4148 diode and 3.3 meg resistor in the AGC circuit are prime suspects. 
> 
> As I understand the circuit, the two op amps (U2) have pretty close to 
> unity gain, and are used mostly for isolation and to drive the various 
> control lines. The PN2222 is an emitter follower which charges the AGC time 
> constant capacitors and provides T/R control. The 3.3 meg resistor gives 
> the AGC time constant caps a predictable decay time.
> 
> Any hints or suggestions before I start poking about? Firstordr of business 
> seems to be voltage checks.
> 
> 73 de Jim, N2EY
> 
> 



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