[Collins] RE: 75s-3 RF gain and muting problem

[Gerald]

On Wed, 2005-11-30 at 13:39 -0500, David Vest wrote:
> Thanks Jerry for the info, I forgot to mention that there is bias voltage,
> may be a little high at -95 volts and it is there all the way to the RF gain
> control, it is lost at the junction where the mute line and bias meet up.
> 
>  
> 
> Dave
> 
>  
It ought to be a few volts at the mute wire with the receiver in
standby. That point gets grounded to receive. Mute wire open, the
voltage on it should be about -24 volts. That cuts off the first mixer
directly and the RF and IF gain stages through the RF gain control and
the AGC circuit. With the receiver in Receive or calibrate the voltage
on the wiper of the RF gain control should vary from essentially zero to
about -30 volts. 

Your -65 is high. Perhaps this is a receiver originally built with a
selenium rectifier where R52 and R62 were 3900 and 10K and that selenium
has been replaced with the more modern silicon diode where R52 and R62
are 5600 and 6800, both 2 watt carbon composition resistors. My Yellow
book schematic for the 75S-1 shows the old values and my manual for my
75S-3B shows the new values. I don't know when there was that diode and
resistor change in production.

I see only three places the mute voltage can be shorted to ground by
components. Once at the stby, opr, cal switch, once by the bypass
capacitor on the feed to V3A which is C51, or by the mute jack bypass
capacitor. Which isn't to say the mute connector can't have broken down
as plastic fell apart with age. But you see 12K from mute to ground.


> I have run into a strange problem with my 75S-3, really two but I
> believe
> they are related.  The RX will not mute, and my first thought that
> there
> must be a short to ground in the mute line, what I found were 12.5 k
> ohms to
> ground which makes taking a look at the schematic.  I can make the
> receive
> mute by applying bias to pin 9 of the V3 (1st mixer), yes the rx is in
> STBY.
> Check the resistor values in the circuit and all appear to be within
> tolerance.  What it appears to be is the bias is never getting to V3,
> and
> checked the wiring and thinking about pulling one side R58 (330 ohm to
> ground where the mute jack and bias voltage meet up) wondering if this
> stands to reason and if anyone else has seen this type of issue in the
> 75s-3.
> 
>  
> 
> On the other problem if I turn the RF gain control (front panel) back
> about
> 25% the receiver goes into cutoff and the s-meter goes to the right
> hand
> peg, makes no difference if in STBY or OPR as well the AGC setting has
> no
> affect on either issue.
> 
The high bias voltage will reduce the working range of the RF gain
control making it act excessively quickly. I think you need to fix that
first but that you have two problems. With the bias supply voltage
reigned in, then we can look for the mute problem. Often a low bias
voltage inhibits muting. At the junction of the 12K and the 330 ohm
around RF gain control there should be about -24 volts in mute, 0 in
operate and cal. If there is and the receiver doesn't mute then there
needs to be a voltage on the wiper of the RF gain pot (which could be
open in some positions as a failure mode) that varies from -24 to -48 in
stby, but 0 to -24 in receive or calibrate. The voltage depends on the
RF gain control position.

It is possible that an RF or IF tube (one that is AGC controlled) can
have so much grid emission that it counter acts the cutoff bias through
the AGC circuit. In operate or calibrate mode it probably will drive the
S-meter negative, generally driving it more negative as the affected
tube warms up.

Your reading of 12K to ground from the mute line confuses things because
if there was a ground from a bad capacitor, switch, or jack on the mute
line you probably should see it with the ohmmeter. Check for voltage and
then chase where that voltage gets lost. There should be -24 volts on
the grid of V3A in stby. Its possible for a grid resistor to go open or
a solder connection to wire or component to have gone open but the rest
of the stages besides that mixer should have been cutoff through the AGC
circuit being driven negative through the RF gain control in the STBY
mode.

OK rereading. 12K to ground from the mute line. Says the mute wire isn't
grounded. No voltage on the mute line (you said "lost at the junction 
where the mute line and bias meet up." Sounds like an open RF gain pot. 
Check in stby for voltages on the circuit. Should be -65 at the filter capacitor.
Should be about 1/3 that or -24 volts at the mute line. Should be just under 
2/3 that at the high end of the RF gain pot or about -40 volts. Should be 
essentially -24 volts on the other end of the RF gain pot (not much drop 
in the 330 ohm resistor in this test). The wiper (center terminal) of the 
RF gain pot should change smoothly from -24 volts (maximum gain position) to
-40 volts as you turn the RF gain knob. What I think you will find is that there 
is -65 on one of the RF gain pot and nothing on the other though at some position
in the knob rotation there might appear that -24 and -40 if the wiper bridges a
break in the resistive element. Running 95 volts on the bias circuit may have 
sped up the failure of the pot by increasing the power dissipation in the element.

If the RF gain pot is open, you can restore the muting operation by bridging the pot 
with a 10K resistor. Then if you jump the wiper to the end by the 330 ohm resistor,
the receiver will receive but you won't have RF gain control, but AGC and muting
will work properly.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer