SB-104A audio problem?

[JF]

Alexander N. Gerli wrote:
> I've noticed something odd about my SB-104A. Radio works very well, but in
> order to get proper TX audio out, I need to run the mic gain almost full on.
> The audio appears fine on the scope with the mic gain up that far, but
> something's not right. I would expect the audio gain to be 50% or less for
> typical full output. Using a standard EV-638 (HDP-121) Heath/EV mic. And,
> that mic works FB with my SB-102.
> On tuneup, the radio puts out full power with the mic gain less than 50%.
> Now, that's correct.
> Anyone have any idea which board to start with?
> 73,  Sandy AC1Y 


First, check the mic output with a scope to see if it's putting
out enough. As has been noted before, the SB-104 series was designed
for use with high output unamplified HI-Z mics of around 50K. For
example, the older Shure 444 works very well with the 104A with its
audio output of up to about 200 mv peak-to-peak; it drives my 104A
to full RF output with the mic gain set at about 65-70% of full
clockwise rotation. In tune mode full 104A power output is reached
at about 55%. Mics with less output are not suitable for the 104A
unless they have a preamp boost.

If the mic is good, then circuit board B (TX Audio/Regulator) gets
the mic input and gain control connections. From there TX audio
goes to board E (Carrier Generator).

You should perhaps go through the Tests and Adjustments section of
the Operation Manual.

73
John   KE5ZB

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