[TenTec] hiss hiding - Some Corsair II Experiments

[Duane - N9DG]

--- Steve N4LQ <n4lq at iglou.com> wrote:

> Well if you turn down the "RF Gain" on a Corsair, virtually
> all the hiss 
> disappears leading me to point blame at the IF chain.

Actually there are two sources of hiss, one is indeed the
RF/IF stages, and the other is clearly the audio chain.

As an experiment turn the RF gain all the way down (with
50-ohm load or no antenna) and then the audio all the way up
there's plenty of audio hiss. Can be further demonstrated by
lifting connector 45 (RX IF in) to the IF/AF board and crank
the audio full up again, about the same level of noise -
though the RF gain setting does have some effect. Also if you
lift connector 76 (BFO in) you get a similar result, this
time the RF gain has no effect.

This set me to wondering if I could actually characterize the
amount of noise contributed by the PBT board in the Corsair
II a bit better. After studying the schematics some I saw
that it is quite easy to do. At least for a quick test. All
that I needed to do to bypass the PBT board was to take plug
75 from the PBT board and plug it into the position 73 of the
XTAL filter board. That simple change bypassed the PBT board
completely. As it turns out the PBT board adds a good share
of the overall RX noise, - more than I had originally
thought. 

Needless to say this jumper cable reconfiguration removes 8
poles of IF filtering at the 6.3 MHz IF and PBT goes bye bye
too. And there is only the 2.4 kHz bandwidth IF filter at
9MHz left, essentially makes the Corsair II into an Omni
A/D/C. Didn't study it any further to see what the
ramifications are to the TX side. So DO NOT necessarily
consider this to be a mod that you want to make. 

DISCLAIMER: EXPERIMENT WITH THIS AT YOUR OWN DISCRETION. I
take no responsibility for any problems that you may cause by
trying this yourself.

Duane
N9DG



		
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