| Subject |
R-390 - 33 tubes (1951-54 1) |
R-390A
- 26 tubes (1955-70/85 2) |
Comments |
| Power supply |
- Transformer: 115 & 115 volts primary, 570 & 25.2 volts secondary, both center tapped.
- Electronic, 2 tube rectifiers plus a 5 tube DC filtering circuit, hum balance control.
- All tubes share same filament voltage source, including rectifier tubes, this implies the filaments of operational
tubes must be connected in several groups in series in order to match with the voltage of 25.2 from secondary.
- Unit mounts on floor plate by 3 screws only, floating bushings are needed. |
- Transformer: 115 & 115 volts primary, secondary 570 volts center tapped & 25.2 volts for tube rectifier filaments (center-tap not used) & 6.1 volts to operational tubes.
- Standard, 2 tube rectifiers plus a choke input inductance/capacitance DC filtering.
- Separate source of filament voltage for rectifiers and operational tubes, parallel feed 6,3 volt line to filaments of
all tubes, except for VFO and BFO in series with the current regulator that are connected to the 25,2 volt line.
- Unit mounts on floor plate by 6 screws, secure and direct fastening. |
• R-390A has much better power supply design, and this is because of its
simplicity. The R-390 lab-style power supply is not very practical for
a military communications receiver precisely, sharing the feed line of
filament voltage to rectifier tubes along with filaments of operational
tubes is a high risk design (B+ current may flow to all tubes in the event
of a cathode to filament short in a rectifier tube). Filaments connected
in series may cause defficient voltage distribution, difference in tube
filaments cause some tubes to run higher than normal filament voltage and
others lower, some filaments are not connected to mass potential also.
• R-390A has better design with less tubes, works with less load at less temperature.
The power supply module is easier to remove. |
| FE RF stages |
- Two front end RF stages, 6AJ5 (2.5 mA/V) & 6BJ6 (3.8 mA/V) tubes. Best
for sensitivity and image rejection, worst for cross modulation and noise
figure, also exists some chance that stability of stages may be problematic
by erratic couplings when tuning due the implied high gain of a two-stage
RF amplifier. Standard coils with diferent type of cores.
- AGC line is not connected to the 6C4 tube mixers.
- Calibrator is an extra unit: 100 Kc multivibrator locked to a 1 Mc crystal. |
- One front end RF stage, 6DC6 (5.5 mA/V) tube, improved coils, higher permeability
ferrite core, higher Q. All RF coils uses same permeability cores. In this
case the best/worst comment at left must be reversed.
- Due the elimination of one RF stage
and two IF stages, the AGC line is now connected to the grids of the 6C4 mixers.
- Calibrator mounted on the RF unit:
100 Kc multivibrator locked to a 200 Kc crystal. |
• Debatable
matter, it is not clear the R-390A is superior on sensitivity due the elimination
of one RF stage and loss of two IF stages, however, it is clear on
RF performance. Elimination of one RF stage is compensated by the use of
an improved tube, and the loss of two IF stages by the flat response characteristic
of the IF chain, but a very detailed comparison about sensitivity would
be required. The most important considerations on designing a RF stage
are adequate gain, good AGC control for this gain, low noise, low cross
modulation. Probably we would have contradictory results here IMHO.
• It is considered best not to connect the AGC line to mixers, but no problem here because the oscillator outputs
are buffered by means of low-impedance links between the coupling transformers.
• R-390A calibrator has better stability and greater harmonic output. |
| IF RF stages |
- 6 IF stages; 16, 8, 4, 2 Kc bandwidths are obtained by means of the traditional
more-to-less LINK coupling using tuned interstage transformers, this varies
coupling between the primary and secondary windings increasing selectivity
when the coupling is being lowered, this also lowers the signal but it
is recovered by tube amplification; 1 and .1 bandwidth positions are obtained
including in front of the 2 Kc tuned passband chain a conventional one-crystal
filter. The interstage coupling transformers have the control over the
performance of the whole IF chain that is tuned to 455 Kc, this is critical
taking into account the involved high gain, and may cause instablility
due the 6 IF stages are adjusted to the same frecuency (oscillation may
occur as a result of coupling between stages). |
- 4 IF stages; 16, 8, 4, 2 Kc bandwidths are obtained by selection of a specific
mechanical filter for that passband after the first stage, interstage coupling
transformers are designed to give flat top over a wide frequency range
of more 16 Kc; 1 and .1 bandwidth positions are obtained including in front
of the 2 Kc mechanical filter a conventional one-crystal filter. Thus,
the bandwidth of the selected mechanical filter has the control over the
performance of the IF chain. The broadband response of the IF chain eliminates
troubles on adjusting, the IF tuning is not critical and do not require
frequent alignment, bandwidth of these transformers are sufficiently wide
to have negligible effect when the 16 Kc mechanical filter is selected,
but they provide enough attenuation at 8 Kc off center for preventing spurious
responses in the mechanical filters. |
• Adoption
of mechanical filters avoids the classical HI-Q tuned chain design, that
is, a lot of stages having weak coil coupling followed by high tube amplification
to compensate the loss of gain of the weak coupling, and having an AGC
with great efficiency to compensate differences in amplification with relationship
to the bandwidth selected. In this case the result is: IF stages can be
dramatically reduced, but a collateral consequence of this design is the AGC
line must be redesigned to maintain the smooth operation that provides
a long AGC chain, also inclusion of mechanical filters may introduce an
effect resembling as "metal audio"... but on the other hand the passband
shape is excellent, close to vertical with a flat plateau, much better than a
conventional HI-Q chain. R-390A would be clearly better on this respect.
It should never be forgotten that an R390A is a communications
receiver, not a SWL receiver, HI-FI evaluations regarding this type of receivers
are incongruous; the goals are sensitivity, selectivity and low noise,
the goal on audio quality is simply obtaining a frequency response that matches
the human voice. |
| Audio stage |
- WIDE AF band position combined with two audio response filters for 3500
cps (speech, MED) and 800 cps (morse code, SHARP).
- SQUELCH function incorporated from factory. |
- WIDE AF band position combined with 800 cps (morse code, SHARP) filter.
- SQUELCH facility is optional, wiring and squelch position in the function switch are retained
(this option is hidden under the knob by means of a washer-clamp behind
the hex nut), but the parts in the AF unit are missing (holes for 6C4 tube and
associated relay are covered
by a plate). The SQUELCH facility may be installed on the field. |
• As the IF employs mechanical filters for the 16, 8, 4, 2 bandwidth positions,
their flatter tops and steeper skirts cause the 3500 cps MED filter is
unnecessary (but I don't agree on this point*). In CW positions
(0.1 and 1) the R-390A uses the conventional one-crystal filter with
static phasing (this filter does not provide rectangular shape), after,
the signal passes through the 2 Kc mechanical filter that only cuts
skirt excess over 2 Kc, that's why the 800 cps AF filter is retained. In
fact, the R-390A "Audio Response" lettering on panel could be substitued
by a "CW AF FILTER - ON/OFF" decal. The R-390A does not have strictly an
"audio response" concept, because the audio response is determined by the
passband of the selected mechanical filter.
• SQUELCH use is infrequent taking
into account the R-390's frequency coverage (HF), changes in propagation
and fading characteristics on this spectrum of radio waves makes it inconvenient,
squelch use has more applications on VHF.
* A response like that of the R-390's MED filter is not necessary because it is the same as that of the 4 Kc mechanical filter, but mimicking the R-390A selective response approach for CW, an increased-step response audio filter should have been included (an audio clarifier filter; uneven skirts, smooth in the low range and abrupt in the high range). |
| Misc |
- Pilot lamp: #327 28 volt 0.04 A lamp.
- Has tube pullers & pin straighteners on the back.
- Has connectors for AC power line and remote control.
- Coaxial connectors used inside are standard BNC type.
- Gear train with Kc & Mc drive shafts supported under the floor plate and running apart from the body of the RF deck.
- Due to the above, a sync tool (the green wheel) is needed if the RF deck is removed (it must be properly
installed before removal).
- Tendency to final end-stop failure of the Kc & Mc shafts.
- The almost rectangular floor plate does not reach the front. Front panel is supported by sides of relatively thin aluminum, they will distort under lateral pressure or impact, then front panel shifts, resulting that Kc & Mc shafts are bent. |
- Pilot lamp: #328 6 volt 0.2 A lamp.
- No tube pullers & pin straighteners.
- No AC power line and remote control connectors.
- Coaxial connectors used inside are Mini-BNC type.
- Gear train simplified, and the Kc & Mc shafts are integrated in the RF deck.
- Due to the above, no synchronization tool (green wheel) is needed here.
- Has a new ten-turn stop for the Kc & Mc shafts having much greater strength than the old.
- The floor plate has a prominent 90º angle-shaped area in the corner which reach the front, this design prevents shifting of the front panel moving off center under lateral pressure or impact, also the front panel remains static because it is bolted to the floor plate by an angle bracket. |
• The 6 volt dial lamps are more rugged than the 28 volt types and cause less of replacement problem.
• The standard BNC connector type used in the R-390 is more rugged than the mini version used in the R-390A.
• R-390A modules are easier to mount and remove.
• R-390A has improved mechanics and reinforced main frame. |
| Schematics |
Part 1 ::::: Part 2 |
Part 1 ::::: Part 2 |
EAC Designed Product Detector for the R-390A |
Notes:
- (1) Year of of manufacture (usually same as the contract), the R-390 project ends in Sep-1953. Looks like the R-389/R-390 designs started in 1951.
- (2) Year of of manufacture (usually not same as the contract), the R-390A project ends in Feb-1956. Looks like the military soon complained and the cost reduction program started just as the R-390 project ended (so the R-390/R-391 'cheapening process' lasted about 2 and a half years, Sep-1953 to Feb-1956). The delayed release of the R-390 manual TM 11-856 in Jan-1955 could be a clue to the military's complaints which may affect the final features that both R-390 and R-390A should have. The R-390A manual TM 11-856A was issued in Jan-1956, so both manuals were issued within a relatively short time interval, but the serial production of the R-390 had begun several years earlier.
- During the development of the Cost Reduction Program (in fact, the R-390A project) some mods for that receiver were also incorporated to the R-390 production. Subchassis affected by a modification are stamped with the letters "MOD" and a number.
- See an example of this on an audio subchassis belonging to the R-390A SN 841 under contract 14214-PH-51. The unit is almost New Old Stock, from the first 1000 units that were built by Collins Radio, never repaired by the Army (so a subchassis swap is impossible). The sharp filter was made by Chicago Standard Transformer Co, it shows a date stamp February 20, 1956. This proves that the first R-390A units were manufactured from 1955 onwards and not before, which is consistent with the Jan-1956 date of the TM 11-856A manual. This unit also shows that the '51' on the contract award has nothing to do with the year of manufacture; there is a lot of confusion on contracts and manufacturing dates. And to further complicate matters, the manufacture of decorative labels with generic data during the "boatanchor review and collecting era" (1980-2000 → 2008 including eBay auctions), or authentic but swapped labels from destroyed units, further increased the confusion.
- The development of the R-390 receiver (non A)
was initially classified as 'restricted', but in 1955/56 the A and non A manuals were
released without restrictions. There are no serious references about these radios were
declared 'secret', that is probably an urban legend issued for/by Collins'
mythomaniacs; in fact, except mechanical filters, all of the rest is
50's standard technology.
- Perhaps R-390A should not have been
classified as a revision of the R-390, it has technical differences enough
to have been classified as a new model. All modules have strong modifications,
the whole receiver has simplified and updated design, it has a new 'electronic
philosophy' in the 455 Kcs IF chain, only frequency coverage remains unchanged.
- My opinion and conclusion is: The military ask for
modifications, and Collins give them a new radio. R-390A is the ultimate
on design applied to communications tube receivers; despite the limitations due to the cost reduction program, it could be considered to be the first modern communications receiver due to the use of a numeric dial and a wideband IF with input bandwidth selection.
- Source of photographs linked to the text: eBay listings.
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