[HBR] Need help with noise

[Peter Bertini]

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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