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

[Glen Zook]

Here is the reply to this that I sent yesterday:

Some people do use RG58/U cables for the interconnection.  However, that often requires retuning of the equipment as well as, sometimes, not enough injection, especially from the heterodyne oscillator into the transmitter.

It is the capacity of the interconnect cables that affect the tuning of the circuits.

If additional circuitry was used in the receivers (SB-300, SB-301, and SB-303), then the price of the kit would have to be increased.  Heath furnished the RG62/U cable in the receiver kit.  Therefore, amateur radio operators did not have to obtain cable.  Remember, Heath had no idea that the SB-30X receivers were going to be used over 40-years later and, as such, they didn't even consider using RG58/U cables.

Using RG59/U cable for the interconnect cables does result in less capacitance for the assemblies and, sometimes, that works OK on the 10-meter band.  But, for best operation, use the 24-inch RG62/U cables.



You might add an amplifier to each of the 3-outlets in the SB-303 to match the output of the signal without having problems with using cable other than the RG62/U.  Doing so would require some work and expense.  However, at least to me, since I am a firm believer in the principles of the conservation of my finances (you can say "cheap and lazy" if you wish) doing so is a lot of work when RG62/U is still readily available.
 
Glen, K9STH 

Website:  http://k9sth.net


On Tuesday, August 19, 2014 10:20 AM, Robert Myers <rsmyers at rogers.com> 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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