[HBR] That General Coverage HBR Project -- 2

[Ed Swynar]

Hi Walt et al,

Just a minor point of interest here, but one well worth noting in the future
re. working with those oh-so-fragile feedthrough capacitors...

Before installing them, "beef up" the body of the capacitor right at its
fragile point of exit for the one lead with a good, evenly distributed
build-up of slow cure epoxy glue. Let it dry at least a day, & then go head
& install it (you still have to be careful, of course, but not all THAT
careful, if you know what I mean).

I got this seemingly long forgotten idea out of a tip in "Hints & Kinks"
eons ago. It works like a charm, & I have yet to bust up any feedthough
capacitor since invoking the epoxy rule here at VE3CUI - VE3XZ!

~73~ Eddy VE3CUI - VE3XZ




----- Original Message -----
From: "Walt Hutchens" <waltah at ntelos.net>
To: "HBR Receiver List" <hbr at mailman.qth.net>
Sent: Sunday, October 08, 2006 1:13 PM
Subject: Re: [HBR] That General Coverage HBR Project -- 2


> Jim said:
> > C = .00000022 F or 0.22 uF
> >
> > If there's a dozen or more bypass caps of .01 each, plus some other
> > leakage - SNAP!!
>
> You guys are terrific!  Don't ask me why, but it never occured to me to
> try to calculate that.
>
> As it turned out, the problem was a bad (cracked) feedthrough cap on the
> filament line going into the 2nd mixer assembly.  Starting at the hot
> end I went down the filament line pulling tubes in turn until it
> switched from 'problem goes away' to 'problem is still there' meaning
> that whatever component was the source, now had voltage on it.  Sure
> enough, when I looked closely at the feedthrough going into that box, I
> could see a hairline crack.  Swapping it made the problem go away.
>
> I really hate using those things -- they break at a careless look, or if
> you even wave a small wrench past them while still in the packaging.
> Mine are all salvage so I have several chances to break them --
> attrition runs around 25%.  Then there's the risk of soldering ...
>
> Anyway, now I know how to think about bypassing in the set.  The
> filaments aren't quite so bad as they seem because the voltage declines
> as you go along -- only the first tube has the full 120 VAC on its
> filament bypass.  Other bypasses are on lines that are nominally DC-only
> -- the currents would be just stray signals, probably mostly in the
> microampere range.  The HV filter caps are huge and carry high currents
> but they go hot line to neutral line -- there shouldn't be any current
> in the ground wire.
>
> I have, however, had trouble in the past with a bypass from the neutral
> line to ground.  One factor is that there can be a significant voltage
> between neutral and ground; I just measured 4 mV in this receiver with
> the switch off and 0.75 VAC with it on.  Since that represents a current
> flow of 0.3A (filament current), there's a roughly 2.5 ohm resistance in
> the neutral line between the point where the pole pig is grounded and
> the entrance to the set where I made the measurement.
>
> There is considerable trash on the neutral line -- I have looked at it
> with a scope in the past and there's all manner of stuff.  Plenty of
> reason for heavy bypassing -- but you have to be careful.  If you use
> solid state rectifiers for HV supply then unless there's a series
> impedance you get very large current spikes in the neutral and if you
> observe that those VOLTAGE spikes (remember the 2.5 ohms) are a source
> of a buzz in your audio, then you might like to put a really big cap --
> say 1 mfd or more -- from the neutral to ground.
>
> You can help the buzz that way, but you are likely to get GFI problems
> -- been there, done that, on previous sets.  It's better to add some
> impedance in series with your rectifiers.  Either resistance -- using
> vacuum rectifiers as in this set gives you that automatically -- or a
> suitable inductance.  The latter would be better for a transmitter or
> other high-current supply and I guess you'd want a reactance in the
> ballpark of a hundred ohms at 60 or 120 cps, depending on the rectifier
> circuit.  That would considerably flatten the current peaks and greatly
> reduce the resulting noise pulses on the neutral line.
>
> Not too many people build 'transformerless' transmitters these days, but
> it can be done reasonably easily and with advantages similar to those
> for receivers at modest power levels.  The killer at higher levels is
> the need for a voltage multiplier to get the necessary HV; at the
> currents typical of a transmitter the power supply filter (and series)
> caps begin to compare to a plate transformer in size.  I build an AM
> transmitter that ran around 40 watts output and worked okay, but I
> wouldn't want to try to go much higher.  I think that one was parallel
> 22JF6's modulated by P-P 22JF6's with two or three more tubes as crystal
> oscillator and audio.  0.45A filaments.
>
> The Rx audio stages are working okay for now and I'm trying to get the
> VXO BFO working correctly.  None of the circuits I have are exactly
> right and as usual I don't know exactly how to think about the theory ...
>
> All things in good time.  It won't be possible to finish the VXO until
> the frequency range of the ladder filter is known.
>
> Bob said:
>
> > metric HW?
> > mdmetric.com
>
> Thanks -- bookmarked!
>
> Walt
> KJ4KV
>
>
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