On Sat, 26 Aug 2006 21:30:47 -0500, tim gorman
wrote:

Andrea Baldoni wrote:

....................................


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

Ok, have you checked the Dual Gate FET to insure that the bias supplied by
the RF gain control puts the device at maximum gain when the AGC is off?

Dual Gate FET's have transducer gain curves that are peaked curves.
Depending upon the Gate 2 voltage, the transducer gain can actually go down
as the Gate1 to Source voltage goes up.

I would still be interested in knowing *exactly* what the AGC voltage on the
gate of the DGFET is for 1) AGC on, no signal, RF gain wide open, and 2)
AGC off, no signal, RF gain wide open. It would also be interesting to know
what the Gate 1 bias voltage is for each state as well.

Tim,

Your hitting the specifics of what I refered to earlier. I'll repeat
it for emphasis.

What I have seen in some cases is where the no
signal resting point for gain control bias voltage is not correct and
the gain can go up a bit before going down. Often seen on oder
recievers where the large part of the radio is discrete devices
and the various setpoints have drifted from age or componenet
changes.

I have seen this on older radios where parts have been replaced or
the original parts used were at opposing ends of the allowable
tolerence. Occasionally a part like a zener doide can drift form
heating. The end result is the full gain voltage can be off or full
gain for a single stage can be off (high or low from optimum).
In one case it was a mechanical switch (agc/manual) causing
difficulty (leakage path). Other suspect components seen in
Japanese built radios are those commonly used ceramic disk
caps for bypasses, they can and do go leaky(high resistance),
or short and I've even seen microphonic. I have a reciever I
repaired where the DGfet developed a substrate to gate2 short
which casued all manner of unusual problems.

The worst case by far was one that the agc bias point had drifted
a bit high. When on manual agc the RX was hot. When agc was
enabled the RX sensitvity would drop noticeably. The problem
was the higher agc bias point had the IF and RF running harder and
producing more noise and when agc was turned on it would see the
noise and pull the agc voltage. That sounds ok save for the front
end was more agc sensitivve than the overbiased IF and the front
end would loose gain faster (it was 2 jfets cascode) rendering the
reciever less sensitive. The fix was repairing the internal voltage
regulator that fed 9V to most of the circuits (it was running at 11v
due to open zener).

Just a few examples of what can occur. I havent even gone into the
golden screwdriver problems when pots are tweeked for "more".


I'll bet you'll find an interesting interaction between the bias voltages
and the actual stage gain as the controls are manipulated.

I'd be inclined to agree.

Allison