[HBR] Need help with noise

[Ian Wilson]

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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