Collins R-188/51J-3 S-Meter Problem and Cure
 

I recently aquired an R-388. Its in good condition but
I found the S-meter read very low. From the entries in the
archives of the Collins Collector's List this seems fairly
common. I found that the problem was R-163, a 160ohm
resistor in series with the meter, it had increased in value
to over 200ohms. After replacing it with a good 160ohm film
resistor the meter works normally. There is also a 100 ohm
resistor, r-170, between the meter + side and ground. The
voltage to ground from the screens of two IF stages through
this resistor establishes the reference point for the meter.
I changed this resistor also but the old one checked good.
Evidently this resistor is critical because even though the
difference between the two resistors was only about five
ohms the zero adjustment had to be reset a fair amount.
The meter now goes up to about 95 or higher on maximum
signals and off scale with manual gain.
I am posting this in case it proves of help to someone
else with a similar problem.
FWIW, the meter on the 51J series reads decibels above
the AVC threshold. This is not an absolute level but is
approximately 2.5 microvolts on all but Band-1 (the BC band
where the receiver is delibrately desensitized). This is
different than the meter on the Hammarlund Sp-600-JX, which
reads carrier level and is calibrated in db above and below
one microvolt. When these receivers are working right the
meter is surprizingly accurate.

--
Richard Knoppow
Los Angeles
WB6KBL

Collins R-188/51J-3 S-Meter Problem and Cure
 

Thanks for the tip, Richard. I will look for this issue next time my
R-388 has its turn at the main listening position.

73,
Joe


In article ,
"Richard Knoppow" wrote:

I recently aquired an R-388. Its in good condition but
I found the S-meter read very low. From the entries in the
archives of the Collins Collector's List this seems fairly
common. I found that the problem was R-163, a 160ohm
resistor in series with the meter, it had increased in value
to over 200ohms. After replacing it with a good 160ohm film

(snip)

Collins R-188/51J-3 S-Meter Problem and Cure
 

"Joe" wrote in message
...
Thanks for the tip, Richard. I will look for this issue
next time my
R-388 has its turn at the main listening position.

73,
Joe

It was interesting that the problem turned out to be so
simple after some tail chasing.
I've checked the meter performance since replacing the
resistor. The R-388 meter does not have an absolute
reference, rather it indicates the level above the AVC
threshold. The handbook states this as being about 3 uV on
all bands except No.1 where it is higher. I think this
probably varies somewhat with the particular tubes in the
set and also probably from band to band. On my test, made at
about 4 mhz and checked at about 10 mhz, I found the
threshold to be about 2.5uV
On my receiver the scale is fairly linear up to about
60db and above that is compressed but the meter will go off
scale with enough input. The saturation point seems to be
about 300 to 500 milivolts on bands higher than No.1. Didn't
check it there.
I found that there is a rough correspondence between the
audio decibel scale and S-units assuming S-9 = 50uv
0 RF = about S-4.5
Audio scale
S-7 = -1db
S-8 = -4db
S-9 = -2db
S-9+10db = 0db
S-9+20 = +2db
S-9+30 = +3db
S-9+40 = +4db

At least gives you a sort of reference since S units are
not very exact anyway. Note that the audio level scale is
not co-incident with the RF scale so the level differences
in db don't match. The numbers are simply easier to spot
than those on the RF scale plus the audio scale is uppermost
for some reason.




--
Richard Knoppow
Los Angeles
WB6KBL