[HBR] HBR -- Part 9

[Peter Bertini]

Walt:

I'm loath to try a 6GU5 or 6EH7 for my RFA after reading your trials and
tribulations
trying to tame that RF stage !  It sure makes me appreciate the work that
was done
by Ted when he came up with virtually trouble free designs that could be so
easily duplicated.  I'm going to print your missives and try to learn from
your
mistakes and how you worked around those problems.

I did manage to pick up two pairs of Heath HR-10 half-lattice crystal
filters (1681kHz)
and I intend to use them in my HBR's 1st IF; and I've even managed to locate
the
correct 2nd LO xtals to get me down to the command rx IFTs for the 2nd 85kHz
IF.  Any suggestions on how best to implement two half-lattice filters?  I'm
not sure
if I should cascade them at the input of the 1st IFA stage, or divide them
up between
the input and output of the 1st IFA. I can see advantages to both
approaches.

Pete



On Sun, Nov 9, 2008 at 1:27 PM, Walt Hutchens <waltah at earthlink.net> wrote:

> Well ... THAT was exciting.
>
> The mixer worked straight off, and the half lattice crystal filter is
> fine.  Two slight humps, adjusting the 1st IFT (which feeds the
> half lattice) lets you balance them, and if I want to flatten it a bit
> I can increase the load resistor somewhat.  I think it's 33k now.
>
> No problems wiring the RF stage or winding the mixer coil.
>
> Then I wound the Antenna coil ... hummm ... no dip. After a few trials
> of this and that, it developed that the 1st gang of the tuning cap was
> shorted. This was a bit hard to believe because it's somewhere between
> double and triple spaced and the stators are VERY solidly mounted.
> However after a few minutes study I noticed that the rearmost of the
> six hairpin-type contacts that pinch the shaft to ground the rotor was
> touching the stator. Furthermore, I don't see how it could ever have
> worked on this cap OR been reliable on any of those caps: There simply
> isn't room enough between the stator and the capacitor end plate.
>
> Six contacts is more than enough: I unsoldered and removed the
> offending one. The antenna coil then dipped correctly. Yep, the
> MG/Jaguar experience -- with a radio.
>
> So with everything dipped, the RF stage oscillated.  MAN, did it
> oscillate.  I always hate that problem because you (or at least, I)
> can't just work X hours and fix it: It requires study and flashes of
> insight.
>
> It was obvious at the start that this wasn't a minor issue: the RF
> stage was producing VOLTS of RF.
>
> The first thing to look for is places where the grid circuit can 'see'
> the plate. I did find one that was obvious: the tuning cap stator for
> the grid (antenna) circuit was in view of the plate (mixer) coil. A
> tinplate shield over that gang of the tuning cap ended that. The
> oscillation got better, but was still strong.
>
> (Having the S-meter working is a big help with this testing because it
> tells you which way you're going with the strength of the
> oscillation.)
>
> The plate and cathode leads between the two halves of the two-triode
> mixer (which are the triode halves of the RF amp and oscillator
> 19JN8's) ran through the RF amp stage: There is certainly some RF at
> the plate of the mixer, so I ran these leads with RG-174/U cable. The
> change wasn't significant.
>
> A day or two passed, looking at the circuit and parts layout once in a
> while, and chewing on the situation. Then another Ah HA! moment: For
> physical convenience I had mounted shared cathode resistor for the two
> triodes of the mixer on the RF stage socket, returning it to the
> common bus lead of that socket. But that caused the RF stage ground
> (common) currents to flow in the same 4" of wire that carried the
> current from the mixer which is basically a cathode follower for the
> RF stage output.
>
> Gee, that seemed like a bad idea. I moved the resistor to the
> oscillator tube socket and the RF stage oscillation nearly
> disappeared.
>
> Writing this, I realize that I need to give that resistor its own
> ground bus lead back to the central tie point.  Returning a cathode
> follower ground lead in common with an oscillator ground lead means
> that the CF is part of the oscillator circuit and changes there will
> affect the osc. frequency.  That's probably not an issue on 80M, but
> on 10M it almost certainly would be.
>
> With a separate common line, a separate resistor/capacitor filter can
> be used. Only the mixer bias will be changed and that circuit is quite
> tolerant.
>
> This illustrates the main issue in designing 'transformerless' HF
> radios: Since the common line returns to the AC line directly, it must
> be isolated from the chassis. But avoiding feedback paths when doing
> this can be challenging, and the problems are not always obvious.
> Fixing the cathode resistor problem involved no change in the DC
> circuit: It was just moving a resistor that went between one end of
> two wires to the other end of the same two wires -- but the difference
> in results is dramatic.
>
> The same issue, of course, comes up in every set in the B+ circuit.
> But resistances there are much higher, so filtering is simpler.
> Cathode resistors for high-gain RF and IF amplifiers are often less
> than 100 ohms and the reactance of a .01 mfd cap at 80M is ~5 ohms so
> you have to use unusually large bypasses to get adequate attenuation.
>
> Additional complication: Modern ground fault current interrupter
> outlet circuits don't like to see ANY current on the neutral line. But
> if you bypass the neutral line (that's B- in transformerless sets) to
> the grounded chassis (always a 3-wire cord for these sets, with green
> directly to the chassis) then those capacitors will charge when the
> set is turned on and the GFCI will trip, at least sometimes. So it's
> necessary to keep the total bypass capacitance down: you cannot just
> fling a handful of ceramic disk caps at one of these sets.
>
> The way that has worked best for me is to put the B supply filtering
> near the center of the chassis and run a cap of 0.47 mfd or so from B-
> to chassis ground right from the last filter cap negative. Then common
> (B-) leads go in star fashion to each stage from that point.
>
> Each stage that has an RF signal has its B- bypassed at the stage:
> for RF and IF stages the cathode resistor and cathode bypass cap does
> this job.  The remarks above about the probable inadequacy of 0.01
> bypasses and the need to minimize total neutral-to-ground capacitance,
> apply.
>
> Another consideration is that you can't do an unbypassed cathode in
> order to get degeneration unless you're willing to add a separate R-C
> decoupling section after the cathode resistor. But that will add to
> the the cathode bias and reduce the AGC action.  A choke MIGHT be
> used.
>
> >From the handful of these sets I've built, I'd say that the problems
> can always be solved, but there's a lot more head scratching,
> compromise, and partial redesign than I really like. It is getting
> easier: On this set the RF stage is the only place I've had a problem.
>
> Then I noticed that the grid pin on the socket isn't all THAT far from
> the plate pin and plate wiring.  I made a 'U' of tinplate and soldered
> it on a ground lug so that it shielded the grid connection from
> everything else.  That DEFINITELY helped.
>
> Adding an 0.1 mfd cap in parallel with the 0.01 cathode bypass on the
> stage helped a bit more.
>
> At this point it was stable without a load on the antenna connection.
> So I lowered the cathode resistor from the 270 ohms I'd been using, to
> 150 ohms, bringing the oscillation back.
>
> Then I realized that the layout caused the ground currents in the two
> tuned circuits to go through the same section of chassis: that too is
> coupling. So I ran separate leads from the hairpin contacts on the RF
> stage grid and plate sections of the tuning cap to the ground end of
> the coils, rather than just grounding the coil end. Each of those
> helped a lot; it was now barely unstable even with no antenna load.
>
> There are more things to be tried: The chassis ground of the bypass of
> the common negative point probably should be closer to the RF stage,
> the mixer cathode resistor should be given a separate common lead, I
> should again test shielding of the oscillator/mixer tube (which is
> near the RF stage grid coil) and by the time I've done those things,
> I'll probably see others.
>
> I will try a choke in the cathode lead with no bypass at the cathode
> itself.  If that works (and doesn't give trouble with the parasitic
> diode) then it will improve RF stage linearity.
>
> HF receivers don't need much RF stage gain unless the mixer noise is
> very high -- say a 6BE6 or something like that.
>
> There's considerable hum on signals: that's from the audio detector
> cathode parasitic diode problem noted a few days ago which I haven't
> fixed yet. And there may be some noise from a heater-cathode partial
> short in the RF or mixer stage: there's been enough other stuff going on
> that mere noise wasn't worth trying to pin down.
>
> There's quite a bit to do, but I think it'll all shake out. And the
> BFO, band edge marker, and standby circuits have yet to be wired.
>
> Oh ... I don't have an HF antenna since the last ice storm last
> winter, so I need to spend a day re-stringing my 160 meter full-wave
> loop.
>
> Walt
> KJ4KV
>
>
>
>
>
>
>
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