[HBR] Yet ANOTHER HBR Project -- Preface, Continued

[hbr@mailman.qth.net]

The bad news is I put the Heathkit counter on my HBR2K oscillator 
as I tuned it end-to-end.   Like the FT-101 which donated the tuning 
mechanism, it covers about 700 kcs (9300-8600 kcs) but uses only 
the middle 500 kcs.   Linearity in the middle is okay -- maximum 
errors after calibration are +/- 250 cps.   Outside this range things 
aren't so good.   At the maximum capacitance end it goes off by 
about 7 kcs too high -- that happens because some little tips on the 
rotor (of no apparent purpose) start to come back out of the stator so 
they're *lowering* capacitance for the last few degrees, countering 
what the rotation is doing.  You could fix that with a grinder and 
make that end okay.  At the minimum capacitance end, things are a 
great deal worse -- the oscillator goes 35 kcs low at the end point.   
What's happening there is that the backside of the rotor starts to 
approach the stator, *increasing* the capacitance.   I can't see any 
way to improve that, short of a total redesign.

So much for the idea that the cap could be used end-to-end by 
tolerating minor errors.   You could get about 450 kcs that way, but 
not a whole 500/180 degrees.   

However ... an archive 'dig' yielded something very interesting -- an 
extemely high quality split stator cap with a range of 12-43 mmf 
(stator to stator) *over 240 degrees*.   I *think* this is from an ARC-
27; if not it's another Collins UHF military set -- maybe the GRC-27 
or ???  Maybe someone recognizes this item?   Silver plated, ball 
bearings both ends, all ceramic structure including a ceramic shaft 
with shrunk-on sleeves were metal is needed.   The thing must have 
cost $100, assuming 1960 manufacture.

The reason it's interesting is that one could use 180 degrees of the 
240 and not have end effects.   There is the question of linearity, but 
it may not be that bad -- picking the high capacitance 180 degrees I 
get 17-26-43 at 0-90-180.   A straight line capacitance unit would be 
17-30-43 and of course the ideal cap would be a bit shy of SLC -- 
probably 17-29-43 or 17-28-43.   A series padder of maybe 75 mmf 
might bring it within the range of the slotted plates.   The classic 
three-point-fit problem ... a few coils to wind, a few caps, a few days 
...
The capacitor is larger than the FT-101 unit, but not unmanageable.   
Mounting won't be quite as easy but it can be done.   

So the first step -- when everything else gets wrapped up -- will have 
to be building the oscillator and seeing if it can be linearized.  I know 
it can be done as a general thing but a dial mechanism with 
calibration at the 100 cps marks and a 10 cps vernier is asking for a 
whole lot of linearity.   The high quality parts definitely will help, but 
... that's a *whole lot* of linearity.    

My general plan is to use a chassis large enough for the entire front 
end and a power supply for the same.   Build it up as an independent 
unit, with its own front panel (probably a temporary one), controls, 
and so on.   Take it as far as the crystal filter so it can be tested with 
any receiver covering 6.5536 Mcs as an IF.  Then (assuming 
success ... hummm ...) do another unit for the IFs, detectors, and 
the rest, and mount that to one side of the front end assembly.  

But I'll start with just the power supply and oscillator.   Anybody 
know anything about battery operated tubes in VFOs?   50 mA at 1.4 
volts is a heck of a lot nicer heatwise than 150 mA at 6.3 volts.   Say 
a pair of 1U4's in push-pull?   Probably triode connected because I 
want a good sine wave output.  

The 1MHBR is behaving well.   Drift on 20 is excessive but not off the 
ranch -- just need to finish compensating the lower bands (another 
round there made things still better) and then add something to the 
20M coil.   I made up coils for 160; that went off without incident.  

Comparing to the HBR2K over this contest weekend, the 1M skirt 
selectivity isn't as good, and looking at the parts and wiring (4-85 kcs 
IFTs in two pairs) I conclude that that's pretty much the way things 
are gonna be. The input and output sides of those transformers can 
'see' each other too well inside the cans, and there's no good direct 
way to fix that.   The command receiver designers didn't need (and 
may not even have wanted) 80-db-in-10-kcs skirts, and the 
transformers weren't built to do it.   The HBR2K, OTOH, has good 
quality crystal filters and I gave lots of attention to shielding, filtering, 
and routing the wiring so nothing goes around them.   

However you could certainly work contests with the 1M.   All the 
signals in the pileups are copyable if you can tell they're there 
through the QRN and QRM -- no distortion at all, no matter what's on 
the side.  

Walt 
KJ4KV