[HBR] GC-HBR Project -- Was 'Navy guy ...'

[N2EY at aol.com]

In a message dated 7/7/06 10:51:53 PM Eastern Daylight Time, 
waltah at earthlink.net writes:


> I said:
> > ... I have yet to hear anything about why it won't work!    
> > LOL ...  
> 
> Jim's excu ... I mean, reason:
> > Two words: Field Day. 
> > 
> > FD #1 for me was 1967 IIRC and I haven't missed one since.
> 
> This is actually very cool. 

I am amazed how much some things have changed - and how much some things have 
*not* changed. Everyone should read T.O.M.'s "Rotten Radio" series - most of 
them could have a few technical words changed and they'd look like they were 
written yesterday.

 I have occasionally worked the Electric 
> 
> Radio vintage field day but the last couple of years have been so 
> busy I can barely keep up with the traffic on this list, let alone 
> participate in an event.
> 

I've been lucky for 40 FDs now...

> 
> > DEVIL'S ADVOCATE MODE = ON
> 
> SHOOT -- you mean yours has a switch?   I wondered what was 
> supposed to be where that snap hole plug is.   The front panel on 
> mine is stencilled "MFP June 1944" which I understand and "Auto 
> operation only" which I didn't figure out until now.  
> 
> Ah well -- on to the serious stuff.  
> 
> > The plan is a bandswitch with 9 positions, used in three 
> > groups.   Each group of three selects the same front end tuning 
> > range, roughly 1.75-5.5-15-30.5 Mcs.   The mixer input and 
> > synthesizer coils are switched in those ranges and must be 
> > manually tuned.   
> 
> Jim comments:
> > Interesting! But such a switch is going to have a *lot* of sections! 
> 
> Well, not THAT many -- not more than required for a typical ham 
> transceiver of the time period.   'Parts only' FT-101's are pretty 
> cheap when you look at all the stuff that's in there.   I haven't 
> actually done the counting but I'm pretty sure the number of switch 
> sections is managable -- something like 8, I think, counting 
> switching of the crystals.   
> 

All depends what you can get. Plus your design will have several 
bandswitches!

> > And getting the isolation between sections isn't easy. 
> 
> This is an issue all right, and I haven't worked out all the details 
> yet.   But a word about the layout of the receiver is in order.
> 
> The 1st mixer and calibrator will be in their own 4" x 5" box at the 
> left rear corner under the main chassis.   Everything will be in there 
> -- tuning cap, bandswitch sections, tube sockets, coils, and the 
> antenna connector. All signal grounds can go to the frame of the 
> cap.   The tuning cap mounts to the front of the box rather than the 
> chassis and since input is push-pull to the mixer grids (no RF 
> stage) there's pretty good isolation from chassis currents.   Power 
> goes in via feed through caps.
> 
> The synthesizer is directly in front of the mixer box.  It's an open 
> layout.   Bandswitch, sockets for six internal and one external 
> crystal, synthesizer tuning cap and coils, the two crystal 
> oscillators, multipliers, and mixer are all there.   Syn. signal flow 
> front to rear, starting with crystals near the panel and ending with 
> the syn. mixer right in front of the front end box.   The syn. output 
> signal goes into the box via a couple of turn link on the syn. mixer 
> plate coil.   Drive push pull to the signal mixer cathodes.   
> 
> The two bandswitches share a shaft but there should be plenty of 
> room for an insulated coupling to prevent the shaft taking 
> synthesizer trash into the front end.   That will require using two 
> bushings to support the shaft -- one on the panel and one on the 
> front of the front end box.     
> 
> To the right of the synthesizer -- basically on the centerline of the 
> chassis -- is a second box containing the tunable LO and 2nd 
> mixer.   This gets the output of the first mixer via a coaxial cable 
> between the boxes.   The 2nd mixer is similar to the first -- signal 
> goes to the grids in push-pull, LO input is p-p on the cathodes.  I 
> was pleased and a bit amazed to discover (with the R8040A 
> design) that you can run a high-C oscillator right into a mixer 
> cathode without a buffer AND without a trace of anything bad.   You 
> can even apply AGC to the mixer.   It'll be a while before I do an LO 
> for higher than 2 Mcs, there's such an advantage to being able to 
> truly swamp the tube capacitances.  
> 
> Output from the 2nd mixer is again in parallel from the plates of the 
> mixer tube; it goes over the shield box via a crystal ladder filter 
> located on top of the chassis to the IF stages which run front to 
> rear on the right side of the chassis.   The LO tuning cap (main 
> tuning) is mounted on the chassis and again, all grounds can go to 
> the capacitor frame.   That's important for this one, because it 
> develops some serious power and chassis currents WILL lead to 
> coupling to other places.   But again, the balanced design helps -- 
> only the imbalance between the two halves leads to a current in the 
> capacitor rotor contact and frame.   There is some 2nd harmonic 
> current there.
> 
> The rear central part of the chassis contains the detectors, the two 
> audio stages, and two 17H3 rectifier tubes.   Since I need two 
> rectifiers anyhow I may use two completely separate HV supplies, 
> one for the oscillators and other possibly voltage sensitive loads 
> and the other for the IF stages and audio output that tend to make 
> the voltage vary.   There's plenty of room for a couple of command 
> set 3H chokes and associated filter caps.
> 
> Separate HV supplies is probably an unnecessary refinement.  The 
> LO frequency variation in the R-8040A from S0 to meter pegged is 
> barely detectable with the ear, so not enough to cause distortion of 
> SSB signals.  The GC-HBR will have even higher C due to narrower 
> tuning range and slightly lower operating frequency.
> 
> Enough of that.  There are large unwanted signals in that chassis 
> and pretty good defenses (layout, circuit, and tuned circuit) against 
> most of them.   Some will be detectable and possibly worse.  Let's 
> consider the details.
> 
> > The problem I see is that the multiplier/mixer setup can generate
> > a *lot* of spurious injection signals for the first mixer. Weak
> > harmonics of the multiplied LO mix with the 50.7 and all sorts of
> > hilarty ensues. 
> 
> > This may not seem a bother on paper ...
> 
> But of course in reality, it WOULD be a bother, size to be 
> determined.  Let's try to see how big. 
> 
> The strongest spurious signals generated will be the harmonics of 
> the MF crystal oscillator -- 5.825-8.675 Mcs, whichever crystal is in 
> use for the band.   Since the bands are only 250 kcs wide but the 
> crystals are all higher than 5.8 Mcs few bands will contain one of 
> these harmonics but the exact sequence of harmonics depends on 
> the band in use.   
> 
> The 2nd harmonic is above the band in use until 14.00-14.250 Mcs 
> where the 7.050 crystal gives you a harmonic at 14.100 Mcs.  
> That's relatively ugly and it cannot be moved out of the band with a 
> simple jiggering of the numbers without either ending the receiver's 
> coverage below 30 Mcs or getting into other kinds of trouble.   
> 
> For example dropping another two crystals to start with 5875 kcs 
> (Channel 278) thus ending the receiver's coverage at 30 Mcs would 
> use the 7100 crystal for the band 14.000-14.250 and place this 
> signal at 14.200.  Going the other way with the crystals you could 
> solve this problem but at the price of putting crystal fundamentals 
> in the high (dial) end of EVERY band because those frequences 
> overlap the tunable IF -- 5.550-5.800 Mcs. 
> 
> The spurious signal at 14.100 Mcs has to get through three tuned 
> circuits to reach the mixer, but the 7.050 fundamental will be a volt 
> or more so I think this one will be heard.   A trap will help, though it 
> might require another bandswitch wafer.
> 
> (The band crystal frequency formula is: (lower band edge + 5.8 
> Mcs + 50.7 Mcs) / 10  )
> 
> Above the 20M band the 2nd harmonic is below the tuning range for 
> the band.   The 3rd harmonic isn't a problem below the band 24.00-
> 24.25 Mcs.   This band uses an 8.050 Mcs crystal and the 3rd 
> harmonic is 24.15 Mcs.  The 4th and higher harmonics are above 
> the receiver's range of coverage.  
> 
> No crystal frequency or harmonic falls within the tunable IF, 5.550-
> 5.800 Mcs -- we just went through fixing that design glitch.  
> 
> Now we get to the 10X(crystal frequency)-50.7 Mcs mixer.   The 
> 50.7 Mcs signal is from an overtone crystal at that frequency so it's 
> pure except for multiples of that -- 101.40, etc.   The other signal -- 
> 58.250 to 86.750 Mcs by 250 kcs steps -- contains all the 
> harmonics of whatever is the chosen crystal frequency, with the 
> desired harmonic probably 20 db or so above the strongest of the 
> others.  Thus for the lowest band (1.75-2.00) we intend 58.250-50.7 
> = 7.550 Mcs but we will also produce significant signals at 7.550+/-
> 5.825, namely at 13.375 Mcs and 1.725 Mcs.   Whew ... dodged 
> THAT bullet, didn't we?  25 kcs below the end of the band is pretty 
> close. 
> 
> On the next band up (2.0-2.25 Mcs) the frequencies are 7.800 
> desired and that +/- 5.850 = 1.95 and 13.65 Mcs, unwanted.  For 
> 2.25-2.50, spurious frequencies are 8.050+/- 5.875 = 13.925 and 
> 2.175 Mcs. It doesn't appear that these spurious signals will be an 
> issue, as they flank the band all the way up.  Pure luck ...
> 
> Yet another sequence of unwanted signals is produced by 101.4 
> Mcs (second harmonic of 50.7) - 10X crystal frequency.   For 
> example at 29.75-30.0 Mcs, that would be 101.4-86.25 = 15.15 
> Mcs.  That one is way out of the band, but we dodge another bullet 
> at 22.25-22.50 where the same calculation leads to a spur at 22.65 
> and 22.50-22.75 where it's 22.4 Mcs.  In this case the spurious 
> signal jumps right past the band in question and will not be an 
> issue.
> 
> There are yet other beats that could possibly matter:  101.4 - 9X 
> and 11X crystal frequency for example, and all the other such 
> multiples.  These are of course much weaker than the 10X one.  I'd 
> have to guess that some of these will fall in a working band but 
> whether they'll be audible or not I can't say.  All of these unwanted 
> signals can be reduced by cleaning up the 50.7. Mcs signal with a 
> second tuned circuit or 101.4 Mcs trap; since this is a fixed 
> frequency that's not a big problem.   
> 
> Harmonics of the LO at 1.354-1.704 are a potential source of 
> spurs.  They are the reason both the LO and front end are in 
> boxes.  About the only way for these signals to be heard is via 
> coupling from the LO/2nd mixer box back to the 1st mixer box 
> along the coax between the two.   Careful tweeking of the oscillator 
> circuit to minimize harmonic production can make 10's of dB 
> difference in the strength of these signals.   If audible at all, I 
> believe these will be very weak, they tune backward, the higher 
> harmonics are going fast, and they don't fall in any of the real ham 
> bands.
> 
> Wait -- there's even more!  These were just the spurious SIGNALS 
> coming from the synthesizer.  But spurious frequencies that are 
> not heard can hetrodyne with strong wanted signals from the 
> antenna to produce yet another whole set of unwanted responses.   
> A receiver that is silent when tuned end to end with the antenna 
> shorted may be very far from 'clean' when the antenna pours in 100 
> mV signals from 41M religious broadcast stations.   
> 
> (Gotta love 'World Harvest Radio.'   At least if you're serious about 
> testing receivers ...  I swear there are winter nights when you could 
> run a lightbulb from your feedline by cutting an antenna for that 
> one.)
> 
> The worst offender here is likely to be unwanted multiples of the 
> band crystal reaching the 1st mixer and the defense is better 
> purification of that signal.   But that's tougher than it may seem 
> when 2x, 3x, 4x (band crystal) is close to the synthesizer output 
> frequency, 7.550-35.550 Mcs.  
> 
> For example on the band 6.75-7.00, the synthesizer output is 
> 12.55 Mcs and the crystal frequency is 6.325 Mcs with a second 
> harmonic at 12.65 Mcs.   On 7.00-7.25, the synthesizer output is 
> 12.80 Mcs and the crystal is 6.350 Mcs, second harmonic 12.70 
> Mcs.  Now that is UGLY.  Rejecting a frequency that's <1% from 
> the wanted frequency isn't going to happen in one or two practical 
> tuned circuits.  It's especially ugly because the band is 41M -- 
> home of some of the strongest unwanted signals ever heard on the 
> planet.   
> 


This is the kind of thing that my concern is based on. What a mess!

> The unwanted second harmonic has to be killed before it reaches 
> the synthesizer mixer output because it cannot be eliminated 
> there.  Well ... Uh, I was going to use a balanced mixer for the 
> synthesizer anyway.   Yeah, balanced mixer.   And probably an 
> extra tuned circuit ahead of that mixer to hold down the unwanted 
> harmonics.  Yeah, I was going to do that.
> 
> I'll consider the design of those circuits specifically to minimize 
> second harmonic generation.  A trap might also have a place in the 
> scheme.   
> 
> > Big coffee can for the xtals. 
> Actually, that's a drawer under the receiver in the usual plywood 
> cabinet.   The cabinet will differ only in detail from the 'One month 
> HBR' that used the command receiver plug in tuning units and 
> stored the spares in a drawer under.
> 
> I have the crystals in original military cases with felt blocks 
> including a junk case that I don't mind sacrificing.  I haven't worked 
> out the details but I plan to transfer the felt blocks to the drawer.   
> 120 = 10 x 12, so that should work out okay.
> 
> > Also you have the problem that when you are tuning a range right
> > next to or on top of the tunable IF, the feedthrough can be a real
> > headache. 
> Not necessarily when the mixer input is push-pull and the IF is 
> single ended.   I have that scheme in the R8040A and it's perfectly 
> stable and quiet when you tune the front end through the IF. 

But the R8040 IF isn't tunable.

The 
> 
> GC project will have somewhat more gain:  It is possible that 
> there'll be instability at or near the 1st IF, but that's a very common 
> failing of somewhat simplified designs.  Even the S-line has a 
> forbidden zone, does it not?  The frequences 5.550-5.800 (tuning in 
> the reverse direction) aren't that big a deal in any case.
> 
> > Seems to me that what you want is a general-coverage (160 through
> > 10 with all the spaces in between) rx that will make best use of
> > that barrel of FT243s in the 4-9 Mc range. 
> 
> > The simplest way to do this I know of is to build a basic receiver
> > and put a converter in front of it. Say we built a basic 1.75 to
> > 2.25 receiver with IF in the 455 range (actually more like 465, to
> > use some of those bazillion FT-241s). Such a receiver could use
> > the tuning gang from a BC-454, so we'd have a slow tuning rate
> > (10-12 kHz per turn or so), integral dial and ganged sections for
> > tracking. 
> 
> Followed by a design for a receiver using harmonics of a smaller 
> number of crystals, mostly from the same set but WITHOUT 
> mixing with another crystal frequency, covering that range in 500 
> kcs segments.  
> 

Yup. This is how the Drake 2-ABC receivers work, except they use 80 as the 
basic range. 


> This would have to be run through the same wringer as above and 
> this is already a very long post.  The 500 kcs bands could lead to 
> problems.  One of the main reasons I chose 250 kcs bands was 
> that I found it much harder to avoid in-band spurious frequencies 
> with the wider range.  

That's because your setup was so different. But if you want, try it with a 
250 kc range. 

1.75 to 2.0 gives you 160. 
5000 xtal gives you 3000 to 3250.
5250 xtal gives you 3250 to 3500

etc. with xtals spaced 250 kc until

8750 xtal gives you 6500 to 6750

Then,

5000 xtal gives you 6750 to 7000
5250 xtal gives you 7000 to 7250

etc. with xtals spaced 250 kc until

8750 xtal gives you 10500 to 10750

Second harmonics take you from 

6375 xtal x2 gives you 10750 to 11000

with xtals spaced 125 kc 

8750 xtal x2 gives you 19250 to 19500

Etc. However, I prefer the 500 kc range because it avoids splitting the wider 
ham bands (except 10 meters). Some receivers like the Drake 2ABC and the 
Mosley CM-1 used 600 kc ranges, but I think that was to save a 10 meter xtal. 500 
is easier in a lot of ways.

 

 However ethis is a differnt set of numbers 
> 
> and the details would be different.
> 

Try e'm out!



> Time again to nod in the direction of the W6TC designs:  His 
> genius was in popularizing a design that delivered good usable 
> performance while being home-buildable.   It has plug-in coils and 
> drift is an issue on the higher bands, but any ham with modest 
> construction skills could build it and beat the selectivity, crossmod 
> performance, and sensitivity of anything he could buy for the same 
> money six ways from Sunday.  

I disagree! See below.

In other words, he delivered a good 
> 
> COMPROMISE design.   

All designs are compromises. Even if cost and complexity are not issues, the 
result is a big heavy powerhungry receiver. It's just a matter of choosing 
what compromises you want.

W6TC's receivers chose a particular set of compromises:

- Gave up bandswitching to permit the use of really good front end coils, and 
to avoid the complexity, layout compromises,  and cost of a bandswitch.

- Used only current-manufacture parts and avoided surplus completely, even 
though surplus was all over the place and inexpensive in that time period

- Used a few single-source parts and a layout that didn't have a lot of spare 
room. This worked fine at the time, but makes substitution 
difficult-to-impossible if the specified parts aren't available. 

- Used the basic double conversion idea  (two fixed IFs, one about 2 Mc to 
control images, one about 50-100 kc for selectivity) of the early-mid 1950s. The 
S-76, SX-101, SX-100, SX-96, NC-300, NC-303, and a bunch of others used this 
same idea. The compromise is that the tunable LO is way up there on the higher 
bands. 'TC made an additional compromise by using a single selectivity 
setting. 

- Put all the parts in one box, which saves some money on chassis and parts 
but puts the power supply heat in with the RF circuits.

- Gave up general coverage to get really good ham band performance.

- Put three active stages ahead of the selectivity knothole. (Actually it's 
more than three because the first 100 kc IFT isn't all the selectivity, like a 
crystal filter would be). This isn't the best design for crossmod performance, 
as was pointed out by W1DX in the classic 1957 QST article "What's Wrong With 
Our Present Receivers?".


HBRs are great receivers if you agree with his compromises. But for a lot of 
uses they are kinda weak. For example, the selectivity is much too wide for 
serious CW operating, or even casual operating in a crowded band. 

If it were 1960 again, or even 1970, and I had the surplus availability, I'd 
build a receiver like this:

Basic 80/40 band-imaging receiver with ~1700 kc IF. Use surplus DC-34 xtals 
for sharp CW filter. Tunable oscillator would be built on a cut-down ARC-5 tx 
chassis, in unit fashion. External power supply. No RF stage - instead, use a 
lownoise mixer like a cathode-injected 6AH6 with double-tuned input stage. 
(dare I say the P-word?)

For 20/15/10, a simple crystal controlled converter. Only bandswitching is 
the heterodyne oscillator. 

Such a receiver would be more stable if the tunable osc was well built. It 
would be much more CW-selective, and if 'phone reception was wanted, a wide 
filter made from the surplus xtals could be used. The crossmod performance would 
be at least as good on the higher bands, and better on 80/40. Breaking the 
project up into pieces would cost a bit in parts but would make it more buildable. 
Total cost could be less than an all-new-parts HBR. Of course it would depend 
heavily on surplus for those savings.

The "silver receiver" (Southgate Type 4) on the HBR website was a similar 
approach. It was limited by the fact that, at the time, I hadn't yet encountered 
the cut-down ARC-5 TX idea, nor the DC-34 filter idea. So it uses FT-241s and 
covers 80 only, with an external converter for other bands. (If I knew then 
what I know now....)

Or here's another HBR alternative: a "Poor Man's G2DAF". Build a basic 
receiver for 5.0 to 5.5, using a BC-455 tuning cap and dial, with FT-241 filters for 
the ~455 kc IF. Then precede it by an allband converter. Using surplus xtals 
and some multiplying (as G2DAF did) would cut down the quartz cost and 
complexity.  

> 
> That's also my goal with the GC-HBR, but for the somewhat 
> different function of 'station general coverage receiver.'  No 
> compromise very quickly takes you out of 'you can build it' territory. 
> The very first thing that would happen is you'd need specialized 
> tuning caps -- four gangs instead of two in the front end, six rather 
> than four for the LO, maybe more than three for the synthesizer.   I 
> have no idea where these parts would come from.  
> 
> IF this paper set ever becomes solder and aluminum and IF it 
> works, then I hope others will improve on it.  But the designs I've 
> found most successful (both mine and those of others) were also 
> those that were the frankest about their own limitations.   This one 
> will doubtless have a few spurious internally generated signals and 
> be less than 100% bulletproof against spurious responses to the 
> very strongest unwanted signals.
> 

I suppose the biggest issue is 'what compromises do *you* want?" The HBR 
compromises suit some, your HBR-GC compromises suit others, my Southgate Type X 
compromises suit others.



> Thanks Jim!
> 

You're welcome!

73 de Jim, N2EY