tim gorman wrote:

: A block diagram would be helpful. Do you know if the AGC is being derived
: from the audio chain or from a sampling circuit in the IF chain?

The AGC is derived sampling the RF level at the last IF. In the same point,
(using another diode, capacitor, etc.) there is the AM detector. The same RF
signal go to a diode balanced modulator along with BFO signal for CW and SSB
and to the input of FM IF amplifier (TA7061AP).

: Where are you turning off the AGC? In the FR-101? It sounds like you have a
: frequency converter feeding an HF receiver with an FM position. Is that
: correct? Could you just as easily tune in 2-m SSB as well as 2-m FM?

I have a FR-101 with the onboard 2m converter. Yes, I could tune 2m SSB but
it's difficult to find SSB on 2m here usually so I didn't any test. The FR-101
is CW/SSB/AM/Narrow AM in segments of the HF range, plus you could buy (as a
option) a onboard 2m converter, a onboard 6m converter and a onboard FM
detector. I have all boards installed, so I eventually could use FM on
HF as well as 2m or 6m.

: If you could kill the power to the converter you could probably test for
: this by just killing the power and seeing what happens to the noise out of
: the receiver speaker. If it goes down, then the noise factor of the
: receiver is irrelevant. If it doesn't change then the converter is
: contributing less noise than the receiver itself.

Turning on the 2m converter don't seem to change noise level, while instead
turning on the 6m converter seems to double the noise. Maybe it needs
realigning, I never use it so I don't know if it's working well.

: If turning off the AGC causes less noise output then my first guess would be
: to look at what "turning off the AGC" is actually doing. Is it actually

The AGC line is derived from a fixed voltage using a 9V zener, then the RF
GAIN pot permit to divide this voltage from 100% to ground and feed it (trough
a resistor) to the first RF amplifier of the HF receiver (a DG FET) as well as
the first RF amplifier of the 2m converter, and the same for 6m converter.
It is also fed to the last but one CA3053. Other amplifiers are fixed gain
I suppose.
Everything in the receiver needs to reduce gain, lower this voltage by more
or less shorting it to ground.
For instance, the standby button shorts it to ground, silencing the receiver
completely. The RF level at the last IF instead reduce it by means of common
emitter transistor: the AGC voltage from zener at the collector and the
rectified and filtered IF at the base.
When you disable AGC, you disconnect the collector of this transistor, thus
the signal is let alone to the level adjusted with RF gain pot (normally at
maximum, so it is 9V).

Ciao,
AB

.... Andrea Baldoni, 2002: messaggio non protetto da copyright.