[HBR] Yet ANOTHER HBR Project -- Preface

[hbr@mailman.qth.net]

In a message dated 10/26/03 12:06:41 PM Eastern Standard Time, 
waltah@earthlink.net writes:


> Yeah, I gotta do it.   A premixed receiver but with a push-pull front 
> end per G2DAF Mk II.  No, of course I'm not addicted ... I can quit 
> ANY time.   Really.   I might decide to quit next Mon..., umm, 
> Monday after next.


Ya think yer hooked now, wait'll ya build an rx with that cap. But I'm 
gettin' ahead o' meself....

> 
> I got a copy of the SSB for the Radio Amateur book dated 1970 so I 
> could read the article on the W5OMX receiver on paper rather than 
> with the only microfiche viewer I could afford ten years ago.   It's an 
> interesting job and certainly represents a bold approach.   The 
> author's technical skills and experience seem to have been modest, 
> but he charged ahead, got help from friends, fixed and redid as 
> necessary and seems to have come up with a receiver that looks like 
> one of the better performers built on this side of the Atlantic.   
> 

And probably the last time a multiband hollowstate rx was fully described in 
QST. 

> There are a few points where it could have been improved.   He's right 
> that a significant 'birdy' in every band (and 6 on 10M) is excessive;  a 
> good hamband-only design ought to have essentially nothing in-band 
> below 10 and not much, there.   

I still can't figure out why he didn't just use the basic VFO output on 80 
and 20. 



The pentode half of a 6AZ8 isn't 
> 
> close to 1st choice as an RF stage -- in fact, with a 7360 mixer and 
> 3 IF stages, he didn't need an RF stage (at least not one with net 
> gain) and would have gotten even better performance without it.   Like 
> everyone else he used single-ended rather than balanced oscillator 
> drive to the 7360 and single-ended output, as well.  (Making the 
> birdies stronger and increasing IF feedthrough, tho he did not remark 
> a problem with the latter.) 

I think the RF amp and 2 high Q tuned circuits ahead of the mixer covers up a 
lot of problems.

The biggest booboo was the simple premixer and its single tuned circuit 
output. All kinds of crud hitting the mixer with that setup.

 The Miller MD-7 (36:1 and 6:1 ratios, ball 
> 
> planetary) is about the minimum acceptable dial.  But overall this is a 
> fine job, probably second only to the G2DAF Mk II design in overall 
> performance.
> 

If only it had a CW filter...


> Didn't the AF use SS-1Rs?   Wonder if that was where he got the 
> idea?   
> 


I don;t see how the AF would use such a limited-coverage ham rx.

> As the first step toward actual construction of Yet Another ... (and 
> recollecting Jim's comments about the BC-221) I pulled apart an LM-
> 20 frequency meter parts unit and extracted the reduction gear and 
> dial.  This is a 100:1 anti-backlash worm drive with a cast frame and 
> shafts coming out both ends.   (Frame is sand cast from a two piece 
> pattern and machined -- gee ...)   Four sealed ball bearings, preload 
> adjustable with shims.  The left end has a drum type dial, 0-50 
> revolutions of the knob shaft.   The right end has a 1/4" shaft and four 
> posts for coupling and mounting of the capacitor.  

There are two basic styles of the LM/BC-221 cap. One mounts by means of four 
tapped holes on the "bottom", the other mounts by three tapped holes on the 
"front". My experience is with the latter style.

There were some caps like this on eBay a day or two ago.


> 
> The knob shaft has a dial graduated 0-100 with a vernier reading in 
> tenths.  If used to drive a linear oscillator tuning a 500 kcs range, this 
> would give a dial graduated in units of 100 cps with 10 cps steps 
> readable on the vernier. The tuning rate would be 10 kcs/knob 
> revolution which is about ideal for SSB, in my opinion.   
> 

You're just getting to the good stuff...


> The linear oscillator is of course rather a challenge -- it's tough to get 
> better than about 200 cps linearity.   However, with careful work you 
> can make the error change fairly slowly, meaning that if the dial is 
> calibrated somewhere near the frequency in use you can use the 100 
> cps units, and the 10's are available for relative measurements.    I 
> never had a receiver that would do that ... 
> 

Me neither! I like a bit of overrun at the bandedges, which you won't get of 
you try for true 0-500 operation.


> It will require a 10 kcs calibrator to get the full value of the 
> mechanism.
> 
> This is about the only commonly available dial mechanism that has 
> significantly better resolution than the FT-101.  The -101 dial reads 
> directly to 1 kcs and has a reasonable tuning rate, but there are 
> several gears that run in sleeve bearings and a ball drive is used for 
> the knob reduction, so you get way too much backlash (100-200 cps 
> minimum) for serious precision.    
> 

I used these caps in the Southgate Type 5 receiver (last rx before I went to 
transceivers) and the Type 7 transceiver. Kept the built in capacitor and made 
a new dial drum 6" in diameter. That meant an extension on the drive shaft, 
but it's 1/4" - no problem. Used a BIG solid bakelite fluted skirted knob for 
tuning. Cap mounts with the shaft vertical and the drum above the unit 
oscillator box. Box for the Type 5 is a cut-down cast aluminum ARP-4 tuning unit. Box 
for the Type 7 is 3/16" aluminum plate from old rack panels and other scraps, 
held together with 3/4" architectural angle aluminum and about 100 6-32 
screws. Building that box with just a hacksaw, flat file, vise, drills and hand 
tapping is a story for another time.

Instead of pulling plates, I used a series cap to cut down the delta C and 
played with various parallel L and C combos to get the range I wanted. Works. 


> I don't relish the thought of trying to use a randomly chosen tuning 
> cap (even one of excellent quality) to get linear tuning.   For one 
> thing, most military units are not straight-line capacitance -- they're 
> SLF for a fairly large frequency range.   Unless I find a surprise in the 
> archives, the FT-101 tuning cap will get the nod.   It's small, dual 
> section (allowing push-pull operation), and face mounting (rather than 
> base mounting) is an option so it will go right on a plate mounted on 
> the dial drive posts -- mechanically it's a good bet for the job.
> 

The shaft coupling is a real challenge, though


> Building an oscillator that's up to that dial should be interesting.  
> Tube heat has to be kept completely away so the tube has to go ... 
> where?   The dial mechanism is tall, so I'm not sure the tube can go 
> above.   Then there's the problem of where to put the premixer 
> ('nuther heatmaking tube) and how to get the signal there.  But we'll 
> see.   This front end clearly has to be done as a subassembly.   

Unit oscillator, shaft vertical, dial drum horizontal, tube vertical next to 
the drum. 


> 
> A serious problem is that the dial is 0-50 over 180 degrees.   But end 
> effects will make any 180 degree tuning cap non-linear near the 
> ends.   Either I'll have to accept that -- it probably would only affect 
> the 1st and last 25 kcs of the band, or so -- or offset the dial slightly 
> and cover only about the first 450 kcs of each band.  That would only 
> affect 80 and 10.
> 

You need multiple segments to cover 10 anyway


Or say the heck with pure linearity and just do the drum. 

> An alternative would be to use only 250 kcs of the tuning range and 
> jigger the 1.75 Mcs oscillator to take care of it.  That probably 
> requires another tube or two, another bandswitch section and more 
> positions, and another set of coils -- or going to individual overtone 
> crystals for each band and pitching the Hahnel oscillator.   
> 

You can see why W5OMX did what he did...


> You could do using something like the regenerative frequency divider 
> in the ART-13.   In that case a 200 kcs crystal was used to control a 
> 50 kcs oscillator, but output also was available at 150 kcs, i.e., (n-
> 1)/n x f.   Two tubes are used ... this was going to be simple, right?
> 

it just gets worse

> Probably the first thing should be to see how bad the non-linearity 
> problem is.   If it's not off by more than a kc or two within 25 kcs of 
> the ends, I'd probably swallow it -- I rarely tune there anyway.   A 
> frequency counter on the output of an FT-101 VFO should do for 
> a test.
> 
The  osc in the Type 7 covers the lowest 25 kHz in 4-7/8 turns and the 
highest 25 kHz in 5-1/4 turns - and I didn't worry too much about linearity when it 
was under construction.



> I remember my first LM -- in 1961 I was delighted to get a unit that 
> was complete except for the dial, calibration book, case, and power 
> supply for only $35.   Actually, it must have been a BC-221 ...   I got 
> the dial somewhere else for $5 and built a case (another plywood job) 
> and a power supply.  Sometime '64-'67 I wrote my first higher 
> language computer program -- in ALGOL -- and used it to print a 
> calibration book from calibration points on an IBM 7094.   I believe I 
> still have that unit, somewhere in the archives ... 
> 
> Going prices for complete units with a calibration book matching 
> serial number (but you had to check under the s/n plate to be sure it 
> really *did* match) were $85 up, as I recall.   What would that be -- 
> $400 now?   It was about 10 days pay for me, at the time.   Now you 
> find them under hamfest tables, $5 max.
> 
> 

I've got 3 LMs (one has the book) and 2 221 carcasses somewhere. 

Fair Radio used to sell the NOS caps for $3.95 - that's where I got mine. 
Sigh.

Do not overlook the cap in the front of the ARC-5 xmtrs - almost as nice if 
greased up and fitted with a proper knob.

73 de Jim, N2EY


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