Collins R390A modifications  [ham radio magazine, 11/75, pp. 66-67]

Several simple modifications for the R-390A which can considerably 
improve performance

Alexander MacLean, WA2SUT, 18 Indian Spring Trail, Denville, New Jersey 
07834 [address in 1975]

With the R390A receiver, it sometimes pays to work around a built-in 
problem or known trouble, rather than make extensive repairs.  Here are 
some simple and somewhat complex modifications that may help, depending 
on the trouble you have.

Audio section.  if you have audio problems, it may be due to a mismatch.  
All of the audio outputs are 600 ohm.  A pair of high-impedance (3000 
ohm) headphones will work as is.  For low-impedance (8 ohm) speaker or 
phone use, the output transformer from a small tube type receiver or one 
of the universal replacement types will provide a reasonable match.

If there is still trouble, the entire audio section can be completely 
bypassed by using the diode load terminal at the back of the set.  Leave 
the jumper connected and couple the signal to an outboard amplifier and 
speaker through a suitable blocking capacitor (fig. 1).  A hi-fi 
amplifier used with the R-390A will give you beautiful shortwave 
broadcast-band listening.  The added clarity will help amateur band 
reception too.

[Figure 1 shows a jumper between terminals 14 and 15.  There is a .05 600
volt capacitor between terminal 15 and the external amplifier.  The other
lead to the external amplifier is connected to terminal 16.]

I-f section.  Ssb reception with the R-390A has a mushy audio quality 
because of the envelope detector and the low bfo-to-signal ratio.  One 
solution is to rewire the detector as a product detector.  [Footnote here 
refers to Eugene A. Hubbell, W7DI, "Improving the R390A Product 
Detector," ham radio, July, 1974, page 12.]

The set can also be used with a companion ssb adapter fed by the i-f 
output jack.  There are advantages to building an adapter for the set 
rather than converting the existing circuit:  you will have more room to 
work with; you can choose your own parts layout; and, you can build as 
elaborately as you want.  More important, you should be able to get 
better performance from a totally outboard unit than by piece-meal 
modifications to the set.

Originally these sets were stagger tuned to improve the bandwidth 
characteristic.  For amateur use the i-f stages can be retuned to the 
same frequency, which noticeably increases gain.

Rf section.  When you increase the gain you also increase the noise.  The 
rf gain control works in both the rf and i-f sections.  As the set is now 
there is no way to vary the i-f gain without adversely affecting the rf 
stage.

The rf stage determines the overall sensitivity and noise level of the 
set.  Removing the rf amplifier cathode circuit from the rf gain control 
and grounding it directly lets the stage work at its maximum gain and 
sensitivity.  The cathode resistor (fig. 2) [not reproduced] runs from 
the tube socket to a nearby standoff insulator where it connects with the 
rf gain control wiring.

Remove the wire from the standoff and tape it out of the way so it can't 
short.  Then run a short wire from the resistor end on the standoff to a 
convenient ground lug.  It would be a bit fussy, but you could run a 
shielded cable to a switch on the front panel and make the modification 
optional.

While the modifications are simple to make, you will need the manual to 
safely disassemble and reassemble the rf deck.  Without it, it is too 
easy to damage the set or misalign the tuning mechanism.

It is possible to position the rf deck on its side in the main chassis so 
that the cables will just reach and you will be able to get at the bottom 
of the subsection chassis for testing or trying modifications with the 
set in operation.

Be careful when doing this as there is almost no slack in the cables and 
it is very easy to break one or damage some other part.  Replacing one of 
the coax cables would try the patience of a saint.

Low sensitivity.  If your R-390 [sic] seems to have lower sensitivity 
and higher noise below 8 Mhz, and no fault can be found, try bridging 
either C281 or C282 (first mixer output coupling capacitors) with a 
higher value; the gain may come right up.  The value probably isn't 
critical; I replaced both capacitors with 100 pF.

Antenna matching.  The unbalanced antenna jack, J103, was intended for a 
whip antenna with a very short lead-in or a random length of wire.  If 
you are using a longer length of coaxial cable you may be losing most of 
the signal.

A UG-970/U adapter, used with balanced input jack J104, makes the 
necessary changes with a substantial improvement.  The following 
modification, originally issued by the Navy as a field change, does much 
the same thing.

1.  Disconnect plugs P205 and P206 from the antenna box inside the set 
and reverse them; P205 to J106 and P206 to J105.

2.  Connect a shorting plug to J104.

3.  Connect the antenna to J103 which, because of the internal changes, 
provides a much better match to the antenna.

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