OK Dave. So why do you then continue CC: to the reflector if you are
uncomfortable?
I'm not afraid to admit a mistake or have an open discussion. Gaining
knowledge takes a high precedent over getting my ego bruised. So lets
stick to one subject at a time in this msg....RX front end related.
Anyway, a tube preamp is obsolete and offers no advantage over a good FET
style amp...in my opinion. If you have one by all means use it, but then
try an A:B comparison with say a FET device as used in the RF amp of
the best performing rigs. Those 2SC or 3SC type devices are pretty cheap
and available.
Adding an external BP filter is an excellent suggestion but it is only a
single band affair and does nothing to help all the in-band signals that
still drive the RX diodes nuts. For only one band operation it is
probably ideal. For 80 and 40 other filters would have to be built or
purchased and switched, adding to expense and complexity. The
multi-station contesters do this all the time and commercial products are
available. As noted by others previously, the BC Band crud bothers many
on 80M as well as 160M and all of the megawatt SW signals in and around
40M (anywhere within the RX internal BP filter range) can bring many rigs
down on their knees with 2nd and 3rd order products. Ham band only and RX
only BP filters with steep skirts and a few dB of insertion loss (too
much loss for TX) would be the ideal setup for many; but I know of no
readily available sources.
A misconception held by many is the location of the RF Gain control on
some rigs. They are actually located on (via the leads from the front
panel pot), and control, IF board circuits; it is really an IF AGC
control. In the TS940 there is an internal trimpot on the RF board that
controls the RF AGC, I have not tried to remote it as a true RF Gain
control. The basic concept holds true with an IC751A also, the only other
radio that I have a schematic for. I will stick my neck out somewhat and
say that this general design probably holds true with the majority of JA
produced rigs.
So what you wind up with is a front panel AGC switch with various
selections which is really the RF AGC control and allows you no true
variable selection of the front end gain. Which in turn means that any
external preamp is still going to always go full bore into the RF amp.
The next level is the IF AGC (RF and IF AGC ckts are separate and
distinct) control which is actually called the RF Gain control. By this
time the signal is way down into the food chain of the radio and nibbling
away at even more diodes. Does reducing this pseudo RF Gain pot help?
Yes, No, Maybe; take your pick. I have a stock 940 here that I just ran
a test on; the RF pot does nothing to help BC crud at this QTH. Once I
open it up to do the mods for the customer I will try adjusting that
internal trimpot. Gut feeling says it will do nothing for the BC crud but
might be useful for basic in-band overload. I wonder if anyone else has
tried this?
BTW, the TS940 has a true IF Gain control buried on the IF Board. It is
trimpot VR1 and is at the input to the 3rd IF amp, right in the signal
path! I have found that turning the gain up a bit to be helpful. The
caveat is that this same pot is used to set S-Meter calibration. On many
of the 940's that come thru the shop that pot is set too low anyway for
proper meter calibration. I try to find a balance between an overly
liberal S-Meter and good copy with the narrowest crystal filter / PBT
settings.
Things are a lot more complex than the RF Gain pot in the RF Amp cathode
of an old tube Boatanchor. I own 5 Boatanchors, from an HQ-120X to a
51J4, and each has its place here.
If you wish to continue this particular subject, I would prefer to do it
"live" since others may have very valuable contributions. Other issues
may be off-line if you wish. I'll follow YOUR lead.
73......Carl
SNIP
>
>I am not comfortable discussing this subject and others raised in your
>"Whoa Hoss!" message in this public forum. I have some differing
>opinions but don't want talk about them publicly. I suggest we stop
>the broadcasts and send messages direct.
>
>Merry Christmas--73, Dave N4SU
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