[HBR] Need help with noise

[Ian Wilson]

The RF stage was touchy but improving the dressing of the leads to the
grid/plate coils/tuning cap seems to have tamed it.

Incidentally I don't intend to run the first stage at more than 3-5x gain;
it
is really there to act as a buffer between the preselector stages. The
6EH7/6EJ7 have good intermod characteristics so I am hoping that the
final front end will be pretty rugged.

73, ian K3IMW


On Thu, Jan 16, 2014 at 6:01 PM, Peter Bertini <radioconnection at gmail.com>wrote:

> Did you check the RF stage for parasitic oscillations? That might be a
> possibility if things seem squirrely.
>
> Pete
>
>
> On Thu, Jan 16, 2014 at 7:12 PM, Ian Wilson <ianmwilson73 at gmail.com>
> wrote:
>
> > Disclaimer: this is not an HBR. However, there are some receiver-builder
> > greats on this list so I hope this is OK.
> >
> > My receiver design is a sort-of 75S3C knockoff with newer tube types
> > and a number of other changes forced by available parts. It is a dual
> > conversion scheme with fixed 2nd IF of 455kHz.
> >
> > 6EJ7 RF amplifier. 6DJ8 first mixer. 3.075MHz bandpass filter
> > (+-75kHz BW). 6EA8 second mixer. 455kHz mechanical filter (2.5kHz BW).
> > 2x6EH7 IF amplifier. Beam deflection product detector. AF stage
> > borrowed from the 75S3. Uses a PTO (actually Dubrow but very
> > similar to Collins) for main tuning.
> >
> > Anyway. This works pretty well from some points of view. It is stable
> > enough and the birdies are under control. However, there is a great
> > deal of 'impulse' noise on the audio. This must be coming in through
> > the front end stages. If I short the IF strip input I can hear a faint
> > hiss.
> > If I short the first mixer input I can hear some band noise but well
> > below the level with an antenna connected.
> >
> > With a (very poor) antenna connected I can resolve CW and SSB
> > signals but both are very noisy. Listening on another receiver (with
> > a much better antenna), the same signals are way above the band
> > noise level.
> >
> > I have tried shielding the input tuned circuits (there is a bandswitched
> > pair of inductors scavenged from an HR-10, and a dual 100pF capacitor)
> > with no discernible effect.
> >
> > The audio bandwidth is not excessive (5kHz perhaps).
> >
> > Would welcome any suggestions. I have noticed before with this setup that
> > some receivers handle the conditions well, and some have lots of
> > apparent band noise.
> >
> > 73, ian K3IMW
> >
> >
> >
> >
> > On Sun, Jan 5, 2014 at 7:56 PM, Walt Hutchens <waltah at earthlink.net>
> > wrote:
> >
> > > Ron said:
> > >
> > > > After some had scratching, I began to suspect that a VHF parasitic
> > > > oscillation was involved. I installed a 33 ohm resistor, in series
> > > > with the control grid lead, of the oscillator tube. The problem was
> > > > cured.
> > > >
> > > >  It was my guess that 3.6 Mhz was a sub harmonic of the frequency of
> > > > the undesired VHF parasitic oscillation, that was occurring,
> > > > simultaneously, with the desired HF oscillation.
> > > >
> > > >  After that experience, I began installing parasitic suppressors, as
> > > > a matter of routine.
> > >
> > > WOW ... The diagnosis sounds right: AT 3.6 mcs the parasite either
> > > sucked so much energy from the intended tank circuit that it killed
> > > oscillation or developed enough grid bias to do the same.
> > >
> > > Now that's one I've never had.
> > >
> > > Sometimes probing around with a frequency counter will turn up a
> > > strong signal. Another old trick is touching things with the point of
> > > a lead pencil (HV safety issue ...) which may kill a VHF oscillation
> > > and suddenly make the circuit behave normally even if its off
> > > frequency.
> > >
> > > I like to build in a test point for oscillator grid current -- like
> > > use a 47k grid resistor but the ground end goes to a 1k resistor from
> > > a test point to ground. A DVM will show the current which should
> > > behave smoothly as you tune, a frequency counter may work there as
> > > well.
> > >
> > > I too use parasitic suppressors as a matter of routine. The latest
> > > tubes are excellent, they can often save you a stage in a receiver,
> > > but you pay for the savings getting the thing tamed.
> > >
> > > Walt Hutchens
> > > KJ4KV
> > >
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