[Heath] SB-303 / SB-401 LMO, HET and BFO interconnection cables

[rabruner at aol.com]

At the time I built my Heathkit combo, they were specifying the RG-62 for
All three connections. Though tibia about the same size as RG-59, RG-62 is 91 ohm cable and it is made in such a way that most of the dielectric is air. These two features mean that it has very low capacitance per foot, giving it very low loss, which was probably the main concern. It was simply the most efficient cable commonly available at the time. And it was widely available. It was used in AM car radio antennas and it was a very common cable on LANs in that period, also because of the low loss. If you worked around a TV station, it was used to network Chyron keyboards and systems. 
Bob
W4TAJ

Sent from my iPhone


On Aug 20, 2014, at 3:27 PM, Dave Edwards <kd2e at comcast.net> wrote:

It is likely the cable that has the closest match to the designed circuit. 
When the rigs were made, pretty much everyone had plenty of that cable laying under the workbench, or shed, or garage.  So, it was not even a thought.  Fast-Forward 40 years or so, and some hams do not even own a soldering iron....different world!!
....Dave
> On 8/18/14, 3:25 PM, Robert Myers wrote:
> Question about the SB-303 / SB-401 interconnection cables *required* for the
> HFO, BFO and LMO.
> 
> As I am sure most are aware, the SB-303 and SB-401 manuals call for the
> following cables to be connected between the various SB-303 oscillator
> output sockets and the corresponding SB-401 oscillator input sockets:
> 
> SB-303 ‘BFO OUT’ -> 24” RG-62/U  -> SB-401 ‘RCVR BFO’
> SB-303 ‘HFO OUT’ -> 24” RG-62/U  -> SB-401 ‘HET OSC’
> SB-303 ‘LMO OUT’ -> 24” RG-174/U -> SB-401 ‘RCVR LMO’
> 
> It seems to me that the designers, for some reason, decided to incorporate
> the various cable capacitances (that result from their specified lengths)
> and their characteristic impedances, along with the input impedances of
> those various oscillator inputs of the SB-401, into what amounts to
> ‘essential operating circuitry’ of the corresponding oscillators based on
> the SB-303.  (But why?  To save money?)
> 
> Is the requirement for 'cable weirdness' due to the various oscillators’
> output circuitry within the ‘303, or is it due to the requirements of
> various oscillators’ input circuitry within the ‘401?  Or is it a
> combination of both?
> 
> I mean, if the ‘303 requires a particular capacitance ‘hanging off’ the
> LMO’s output socket (such being provided by exactly 24 inches of RG-174/U)
> in order to function properly, as well as a 50 Ohm load (inside the 401
> during transceiver), would it not have been better to do something like
> provide all those necessities *within* the ‘303 (right at the LMO
> oscillator) and follow that by an isolating buffer/voltage-follower (still
> inside the ‘303) with a 50 Ohm output impedance?  You could do the same for
> the HET, and BFO oscillators’ circuitry as well – thus allowing for
> arbitrary lengths of, say, RG-58/U cables (not different cables) for
> interconnection of the HET, LMO, and BFO sockets between the two units.
> 
> Is there a fix for this cable 'weirdness,' allowing the cables to be the
> same type (say, 50 Ohm RG-58 since you need that for the antenna
> connections, for sure), and their lengths to be not critical?
> 
> Thanks,
> Rob Myers, VE3JQL
> 
> 
> 
> 
> 
> 
> 
> 
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