[Premium-Rx] continuing Cubic experiments

[John USA]

Hello to the list!!

Download and refer to the Cubic R3030/R3080 manual from the Premium-RX
web page.  If you have a R3030A, don't worry--the versions are almost the
same...biggest differences are filter load and rear panel connections.

I thought there might be some general interest in some studying I've been 
doing
lately.  My experimental platform is a Cubic 3080 (single side of a 3030 and
slightly differently equipped than a 3030A).  The techniques are valid on 
any
electronic gadget, not unlike our rx's in general.These modifications carry 
on
where my previously discussed mods left off.

The subjects I've been playing with are:

Bypass and decoupler caps usage and selection...difference between bypassing
and decoupling.

Retrofitting various current components into gear of 1980's/early 90's
vintage

Stopping EMI before it gets started--thus, less need for mechanical emi
shielding efforts

Various RX qualities such as low phase noise/power supply decoupling/dynamic
range/noise figure/etc.

Google search words of interest include:  multilayer ceramic capacitor, 
bypass
capacitor (using multilayer ceramics), decoupling capacitors, emi 
suppression.
Look for application notes and/or white papers...they present ideas very 
well.
With the properties of the multilayer caps, some new (to me anyway) 
techniques
in bypassing are now available and work incredibly well.  The techniques for
decoupling are presented with some new ideas also.  Also be sure to 
understand
the differences between bypass vs decouple.

None of these modifications are particularly earth shattering;  they are 
just new
ideas to me, and hopefully someone on the list.

I have just taken a stock 3080 receiver and given it my usual modifications, 
which
include:

a)  keypad and display shielding, headphones jack bypass, and renew
electrolytic caps in power supply and modules

b)  adjust AGC attack & decay times to more closely follow ARRL general
suggestions;  keeping the hang feature, one gets an interesting AGC with 
fast
attack, adjustable hang, and soft decay (while keeping the instant AGC dump
used by the CPU to move on to next frequency while scanning).

c)  meter recalibration-convert to logarithmic response-still in process.

d)  adjustable audio out for the 600 ohm line (if rx has the fixed gain
audio board) to interface with PC decoders and analyzers

e)  improved audio frequency flatness in the 350Hz to 5KHz range, which
passes digital signals with less influence from the rx.  Just getting gear
deployed for testing.

f)  redistribution of gain--namely the IF amp in the detector module is
throttled down to less than 50% gain (approx 20db) and resetting the
1st IF attenuator engage threshold.

While this all works fine, the rx has historically been left with
self-induced digital emi, although at significantly lowered levels and
various buzzes, burps and farts at avarying frequencies across the bands.

First, note the display board is not in a module of its own and there is
essentially no bypassing and only the slightest attempt at decoupling the
board from the motherboard supply rails.  My first modification was to
add the keypad and display shields, and improve the full periphery
contact around the display;  I did this by applying thin rubberized tape
that increases the pinch pressure on the 350 x 350 stainless mesh.
Next, Display Board U1, U2, U3, and U4 have been treated to multilayer
ceramic bypass caps of 0.1 and 0.01 uf.  The power lines coming into
the board is stripped of the long-leaded 330uf cap (which in later revs
was repositioned for shorter leads) and a short/no lead 470uf  35V
electrolytic capacitor.  Further bypassing may be worthwhile, pending
practical testing.

The audio board in the 3080 was the fixed gain version, and needed a
variable gain mod on the 600 ohm audio out.  The surgery is simple,
requiring the removal of a 100K resistor and replacement with a 100K
pot; cut a trace and solder the pot wiper to the trace-viola-variable
audio same method as the variable gain version of the audio board.

There is an issue of sharply defined burps/clicks associated with the 
optical
encoder...per my studies, I applied a 100uf cap (smaller would probably be
fine)
right on the encoder itself;  the theory is that the encoder is on a length
of small
gage wire and has to pull current down these wires (thus emi).  The cap
supplies
the short burst of current needed (without involving the wires) and the
cap is recharged between pulses...works
like a charm.  My excitement with this is because up until my recent
studies, I'd have been trying to bypass the wires at the motherboard or
trying to shield the wires.

On air testing reveals the emi issues are silenced!  Intuitively, I'd guess
there is an improvement in MDS, as well.

Continuing experiments will include:
a)  reworking bypassing of all 3 pin regulators and addition of diode
reverse current protection.

b)  reworking bypassing on all synthesizer - related module  chips.

c)  improve module decoupling from mother board power supply rails.

best regards

John Collins