[HBR] Another one-week (okay, probably not!) HBR -- Part 2

[Dave And Merrijoy]

Walt, you are a craftsman, there is NO doubt!!
Dave w9ocm


----- Original Message ----- 
From: "Walt Hutchens" <waltah at earthlink.net>
To: <hbr at mailman.qth.net>
Sent: Wednesday, October 08, 2008 2:12 PM
Subject: Re[3]: [HBR] Another one-week (okay, probably not!) HBR -- Part 2


> The metalwork and the 'first mounting of parts' (as Heathkit would
> say) are done. Pictures are at:
>
> http://picasaweb.google.com/timbreblue/Walt#5254833299643465026
> http://picasaweb.google.com/timbreblue/Walt#5254833820737451154
> http://picasaweb.google.com/timbreblue/Walt#5254833430924088786
> http://picasaweb.google.com/timbreblue/Walt#5254833472562276914
> http://picasaweb.google.com/timbreblue/Walt#5254833653676698194
>
> In the front view, the controls along the bottom are (L to R) BFO
> tuning, ON/OFF switch, AUDIO GAIN, TUNING, and CAL (upper)/AGC
> THRESHOLD (lower). The big hole on the left is for a 3" speaker and to
> the right of the S-meter, switches for STDBY/OPERATE, BFO ON/OFF, and
> CAL ON/OFF. Holes under the handles are for latches to hold the set in
> the (to be built) cabinet.
>
> Hinged top AND a drawer under ... now THAT will be a project.
>
> The shield on top of the chassis is the upper half of a 3" x 4" x 5"
> minibox.
>
>>From the rear, the antenna coil is in the shield on the left, the RF
> stage is the tube just to the right of there, and the mixer grid coil
> is next toward the panel. The mixer is made up of the triode halves of
> the two 19JN8's for which the pentode sections are the RF and
> oscillator stages; the oscillator tube is behind the antenna coil in
> the shield and behind that, the 1st IFT. There is a shield plate under
> the chassis separating the antenna coil and RF stage from the
> oscillator/mixer tube and mixer grid coil.
>
> The oscillator coil is forward of the shield, next to the 1st IF
> stage. The coil is sub-mounted to reduce heating from the chassis and
> allow air flow around the coil -- there will be a way for air to come
> in under the chassis. The two stages of IF run to the right from the
> oscillator coil, ending at the 1st audio and plate detector tube. The
> 2nd audio and 3500 kcs marker oscillator stage is to the rear from the
> detector tube. The 9-pin socket nearest the panel at the right is the
> BFO and bias rectifier; the empty octal socket is for the 117Z6
> rectifier.
>
> The mounting of the dial and tuning capacitor is never easy but it is
> especially tedious for the Eddystone 898 because exact alignment has
> to be obtained with the capacitor mounted above the chassis. Laying
> this out is a challenge. The shield holes on the chassis are slightly
> elongated left/right to allow lateral adjustment; the holes in the
> mounting ears on the tuning cap are elongated (by the mfr.) to permit
> up/down tweaking.
>
> The most obvious weakness of this layout is that the leads from the
> oscillator tube to the coil are about 5" long. Long tuned circuit
> leads are always an issue with plug in coil designs but I could have
> done better by moving the first IFT out of the above-chassis shield
> and moving the oscillator (and 1/2 mixer) tube closer to the coil.
>
> It'll be a few days before there's anything else to report.
>
> Peter said:
>> I've been looking at the Star SR-700A receiver schematic
>
> Thanks for that; I didn't know of this very refined design. I'll watch
> for one on Ebay ... would be interesting to put one on the air!
>
> Two striking features: The use of cascaded 55 kcs IF filter coils that
> are detuned by switching capacitors in order to vary the selectivity
> without moving the carrier edge of the passband. THAT is the way to do
> adjustable selectivity if you're going to use a low IF to get narrow
> bandpass; you could even apply it to use of the 85 kcs command set
> IFTs.
>
> (But the 'without moving the carrier edge' trick is only useful if you
> also arrange for both LSB and USB to be LSB at this point; most
> probably by switching the final local oscillator between above/below
> the previous IF to select the sideband.)
>
> And the Vackar local oscillator. While there is no magic 'stable
> circuit,' the Vackar gives the best control over coupling between the
> tube and tuned circuit of any of them, thus simplifying the job of
> minimizing the effect of tube characteristics on output frequency when
> using high gain (modern) tubes.
>
> In a well-designed Vackar circuit at moderate frequencies, the only
> drift you have to worry about is in the tank circuit parts: plugging
> in a cold tube gives you the same frequency as the warmed-up one did.
> So if the coil and capacitors are protected from temperature changes
> and/or compensated, you get a very stable oscillator.
>
> One improvement to the circuit shown is to tap the coil at the point
> of zero RF voltage and feed the plate voltage through a resistor at
> that point with no bypass cap. This eliminates the plate choke which
> is effectively part of the tank circuit and thus a source of drift.
>
> Push-pull oscillators also offer great advantages, starting with twice
> the gain for the same (effective) tube capacitance.  However they're
> best used with a push-pull mixer.
>
> In the tickler feedback circuit of the W6TC designs, coupling to the
> plate (actually, screen) circuit can be varied by adjusting the number
> of turns and spacing of the plate coil, but the full grid capacitance
> appears across the tank.
>
> Using the screen of a pentode as the oscillator plate (as in the ECO
> circuit) gives you a low gain oscillator tube anyhow, so the amount of
> decoupling that can be done is probably quite limited. The ECO gives
> up this advantage in exchange for the simplicity of a built-in buffer
> and (in the W6TC designs) 2nd harmonic generator.
>
> Since the major source of drift in these designs is likely to be the
> plug-in coils, it's probably not worthwhile to spend a lot of time on
> the oscillator circuit.  Good coils and a chassis layout that keeps
> the oscillator coil as cool as possible are likely to be the most
> useful focus.
>
> Finally, I did find out why I can't post to this list using my regular
> e-mail program. QTH.NET assumes that anyone using The BAT (my mailer)
> is a spammer and rejects all such postings with a meaningless message.
> I can post normally by using Outlook Express, or probably (with a bit
> more work) by zapping the code that labels the mail files, as I'm sure
> the serious spammers do.
>
> Go figure.
>
> Walt
> KJ4KV
>
>
>
> ************************************
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> construction notes...... via http://www.qsl.net/k5bcq/
> there is also a mirror (faster response)at http://k5bcq.edebris.com/
>
>
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>
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