[Collins] 75S-1 Problem

[Robert & Linda McGraw K4TAX]

A few comments along the same lines.

First, the 50 ohm series resistor is to assure that the source Z from the 
generator is indeed ~50 ohms.  Most generators have an actual source Z of 
<10 ohms or so.  In these cases, these are designed to feed a load of 50 
ohms or higher without introducing significant errors in output level.  I 
find a lot of mis-understanding between source impedance and load impedance 
in many discussions.

>From my manual "To set IF gain control, a 50 ohm calibrated generator is 
required.  Connect the signal generator to the receiver ANT jack and adjust 
to 2.0 microvolt output at 14.3 mc.  Tune the receiver to the generator 
signal and adjust R57 to the point which produces a just-perceptible 
increase above the no-signal reading on the S-meter.  Do not make this 
adjustment until receiver alignment has been completed."

Regarding S-meter calibration, again from the Collins manual: "Approximately 
S-9 with 100 microvolt antenna input signal."  Note the word 
"approximately".   I find that most of today's modern radios use 50 
microvolt for S-9 but I find that there is no de-facto standard.  My 75S-3B 
does produce a S-9 reading with 100 mV at the antenna terminal.

Perhaps someone along the way has changed stage gain or substituted tube 
with higher gain thus the errors.

73
Bob, K4TAX




----- Original Message ----- 
From: "DrGerald Johnson" <geraldj at ispwest.com>
To: <GBabin73 at aol.com>
Cc: <Collins at mailman.qth.net>
Sent: Thursday, February 10, 2005 10:28 AM
Subject: Re: [Collins] 75S-1 Problem


> My 75S-1 manual says nothing about setting S-9 at 100 microvolts. It does
> demand a 50 ohm resistor in series with the signal generator. That will
> reduce the signal to the receiver. Then it says to apply 1.5 microvolts 
> and
> set the IF gain to show a just detectable increase in AGC voltage when
> monitored using a VTVM.
>
> I don't have a great trust in the output calibration of the URM-25. I have
> one. It has many black beauties.
>
> The receiver could have high gain if there was something besides the
> original tubes in RF or IF stages, like 6GM6 in the RF tube socket or if 
> the
> AGC bypass or time constant capacitors were leaky. IF tubes with grid
> emission also counteract AGC voltage, but they tend to make the S-
> meter zero setting drift negative as the tubes heat up. Receiver gain
> might be high and the RF gain control relatively ineffective if the bias
> rectifier (CR-3 a selenium device) or its filter capacitor are bad.
>
> 73, Jerry, K0CQ
>
> -- 
> Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
> Reproduction by permission only.
>
>
>
>
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