[R-390] High Noise Floor in 0-1 MC Band

[Barry Scott]

I think I may have found a problem.  I have tried to run the STAGE
GAIN procedures and, right away, the first test (connecting the signal
generator to TP E206 and injecting a 4uv to 16uv signal) fails
miserably to achieve -7v at the DIODE LOAD.  It should produce that
voltage with 4uv to 16uv input but it takes about 40uv to produce -7v.
This is with the GAIN ADJ set to produce -7v with 150uv at J525.  I'm
thinking I have RF deck problems.

Thanks,
Barry - N4BUQ

On Mon, Oct 21, 2024 at 9:25 PM Barry Scott <72volkswagon at gmail.com> wrote:
>
> For the past few months, I've been trying to determine why the noise
> floor on the 0-1 MC band in my R390/URR is so high.  If I tune to a
> strong station, the noise disappears so, perhaps, "noise floor" might
> be the wrong term for it but in places where there's little-to-no
> signals, the noise is very noticeable and all but drowns out the weak
> signals.
>
> If I switch from 0-1 MC to 1-2 MC (or virtually any other band), the
> noise is reduced significantly.  The difference can be on the order of
> 15-20 dB on the carrier level meter.
>
> I presume this isn't normal but maybe it is.  I think someone
> mentioned that the noise on the bands under 8 MC are typically a bit
> higher than the other bands but that doesn't seem to account for why
> the lowest band is by far the worst of them all.
>
> There are only passives that change from 0-1 MC to the other lower
> bands and I'm wondering if it's even possible for those to be the
> problem but I don't know of anything else that could be causing it
> either.
>
> I think someone suggested that the 1st crystal oscillator's output
> might be low for that band but I seem to recall checking that on the
> scope and output for the 1st and 2nd positions seems to be very close
> to the same amplitude so I'm thinking that might not be the problem.
>
> Has anyone seen this and possibly have a cause for it?  Like I said, I
> presume this isn't normal but I don't know that for sure.
>
> Thanks,
> Barry - N4BUQ