From: Garey Barrell 
Subject: Re: [drakelist] DRAKE R-4C restoration - S-meter may be not deflecting
 sufficiently


Garey Barrell  made an utterence to the drakelist gang
----------------------------------------------------------------------
Steve -

Quite possibly either alignment or merely S-Meter adjustment.

Quick check...  If you turn RF Gain control full CCW, the S-Meter should 
read full scale.  If it doesn't, and can't be adjusted, then suspect 
leakage or resistor value change.  The adjustments for R32, R36 and R55 
all interact.  R-55 is the critical one, since it sets the AGC "delay" 
point and overall sensitivity for the receiver, and should be set to 
where the meter just lifts off the mechanical zero with no signal to the 
receiver.  This typically results in approximately -1.3V at the AGC test 
point. 

The AGC circuit is extremely high impedance and so is susceptible to 
capacitor leakage and other leakage paths.  I have seen units where 
leakage occurred from chassis mounted pot wipers to chassis, again 
probably caused by over enthusiastic application of "contact cleaner" 
spray.  One of the fun ones is where someone has sprayed "contact 
cleaner" all over the tube sockets and switches.  The
"lubricant" holds 
dust and soon forms leakage paths to ground.

The Noise Blanker has a gain adjustment which should be adjusted so that 
the calibrator signal reads the same when the NB plug is replaced by the 
shorting plug.

If you don't have access to lab quality generators or calibrated 
attenuators, you might want to look at the Elecraft XG1 calibrator kit, 
for US$39.  This little kit puts out precisely 1 uV and 50 uV signals at 
7040 kHz and is great for receiver alignment and/or checking.  
http://www.elecraft.com/>

All that aside, S-Meters are NOT consistent from unit to unit, much less 
brand to brand.  Some manufacturers set S-9 at 50 uV, others set it at 
30 uV.  Some use 5 dB per S Unit (Drake, Kenwood), others use 6 dB 
(Collins, Icom).   Since they are ALL actually measuring AGC voltage 
rather than signal strength there are too many variables to  _reliably_  
compensate.

What we REALLY care about is the weakest signal we can hear.  If you can 
clearly hear the 1 uV signal and it causes the S-Meter to rise an S-Unit 
or two above the "no-signal" meter level, I wouldn't worry about it.!    
Drake caused all kinds of consternation when they made the left-most 
mark on the S-Meter S-1 instead of S-0!!

73, Garey - K4OAH
Atlanta


stephane.mazzoni wrote:

>  
> Subject : DRAKE R-4C restoration - S-meter may be not deflecting 
> sufficiently
> Reference : S/N 18518 DRAKE R-4C "4-position mode switch"
>
> Dear friends,
> I've corrected the dial gear problem. here comes another one :
> I've investigated a lot on this receiver during this week-end and 
> would like an advise for diagnosing and correcting the S-meter 
> sensitivity which I found insufficient.
>
> Best 73s and thanks for your suggestions,
> Steve.
>
> ________________________________________________
>
> 1. Context
> This R-4C (4-position) is equipped with the AM filter, the two 250 and 
> 500 Hz CW filters and the 4-NB noise blanker. It is combined with a 
> T-4XC S/N =
> 28513 and an MN-2000 matching network.
> The antenna is a FRIETZEL Conrad-Windom wire antenna type FD-4.
> All wafer tracks and sockets as well as all pins have been fully cleaned.
> I do not have the chance to get access to any RF generator or lampmeter, =
> however I can make comparisons with a KENWOOD TS-930S.
>
> 2. What is observed :
> Even with the strongest signals heard on 3,5 and 7 MHz bands, the 
> S-meter would not go higher than S9 + 10 dBs on the strongest signals 
> (i.e. broadcasting stations on 40m).
> On the other hand, the actual sensitivity seems to be good on all 
> bands with respect to the TS-930S rx : i.e. the smallest signals that 
> can be heard on the TS-930S seems to be also audible on the R-4C.
> A difference of more than  20 dB on S-meter readings is experienced =

---