INTRODUCTION
The ATB is a small HF transmitter designed for US Navy aircraft, in WW2.
It can produce 25 watts CW, or 20 watts AM or MCW. It covers the frequency
range 2.3 to 9.05 mHz, and provides 2 channels in this range. It has two plug-in
frequency tuning bays, and each can contain a frequency tuning unit. The transmitter
can switch between the 2 bays, and therefore can change between two different
frequencies.
There are two different types of plug-in tuning units. One plug-in covers 2.3 to 4.2 mHz, and the other covers 3.0 to 9.05 mHz. The manual states that there are normally two of the 9.05 mHz plug-in units fitted. If required, a 2.3 mHz plug-in can be fitted into either bay. The transmitter is in one main box. It has an external dynamotor power supply that uses 28 volts DC, and requires about 12 amps. This also serves as a junction box. There is a remote control, for pilot operation.
The transmitter contains 6 valves, an oscillator (type 1625), a power amplifier (type 815), a speech amplifier (type 6N7), an audio driver (type 6N7), a modulator (type 815) and a voltage regulator (type VR150).
Picture 1: ATB front
It is used with the ARB receiver, being about the same size, and also made by RCA. The ATB is 7 and 3/16 inches high, 10 and 3/8 inches wide, 15 and 1/2 inches deep and weighs 35 pounds. It was intended to replace the GF transmitter and the RU receiver. The size and shape is the same as GF plus 4” coil pull out space. The GF had the coil box on the right hand side, and it was pulled out sideways.
About 30,000 ARB and about 5,000 ATB were made in the 1940s, so it is quite rare. This example is serial number 944, with the 2 mHz coil box serial number 812, and the 9 mHz coil box serial number 4650. One reference suggests that perhaps it is a reboxed ACT-50 RCA aircraft transmitter.
AIRCRAFT
The document “Radio characteristics, US Navy Airplanes 1943“ indicates that
it was fitted to VF class US Navy Aircraft, such as the F6F-1 Hellcat and F7F-1 Tigercat.
At the commencement of WW2, Australia found itself in need of more aircraft of varying
types, and an order was placed for 18 Catalina (PBY-5) flying boats, VPB class.
Due to the neutrality of the United States, these aircraft were flown out from the
USA to Australia by civilian crews, manned by QANTAS pilots. It is believed that
this ATB came to Australia on this flight.
Picture 2: Catalina Flying Boat (Type PBY-5)
Picture 3: ATB left hand side view
Picture 4: ATB right hand side view
Picture 5: ATB underneath view
| ITEM | NUMBER |
|---|---|
| ATB Transmitter | CRV-52233 |
| Tuning Unit 2.3 – 4.2 mHz | CRV-47191 |
| Tuning Unit 3.0 – 9.05 mHz | CRV-47192 |
| Metering Kit | CRV-60025 |
| Dynamotor | CRV-21724 |
| Mounting (right) | CRV-10082 |
| Mounting (left) | CRV-10083 |
| Mounting (dynamotor) | CRV-10084 |
| Tuning Unit (carrying case) | CRV-10085 |
| Pilot’s control box (single place) | CRV-23258 |
| Operator’s control box (dual place) | CRV-23313 |
| Pilot’s control box (dual place) | CRV-23314 |
CONTROLS
When fitted to single place aircraft, it used the CRV-23258 pilot’s control box.
When fitted to dual place aircraft, it used the CRV-23314 pilot’s control box
and CRV-23313 operator’s control box. The dual place control boxes are very different
to single place boxes. The dual place pilot’s and operator’s control boxes,
have “telephone switchboard” type switches, which are a momentary contact type,
and activate relays within the operator’s box. There are “indicator lenses”
to show the transmitter state. There is no picture in the manual of the dual
place control boxes, just a circuit and a sketch.
Picture 6: ATB Control Box (single place)
The single place pilot’s control box has simple controls. The main switch turns the transmitter OFF, and then selects either of the 2 channels, CHAN-1 or CHAN-2 (the coil boxes) and turns the transmitter ON. A toggle switch selects VOICE or CODE. A button on the top is the morse KEY. The bottom has a MIC socket, and the DYNAMOTOR connector.
The front of the transmitter is covered by knobs, and switches. All the knobs are on the coil boxes. The left side of the transmitter has several switches, and 3 terminals. Along the bottom is a row of six phone jacks. There is a TUNE and OPERATE toggle switch. When in the TUNE position, the keying relay is disabled, and the oscillator is turned on, to allow netting with a receiver. The CW and MCW toggle switch selects code using just a carrier, or modulated with a tone. There is a TEST KEY toggle switch (spring loaded in the off position) to turn the transmitter on for tuning. There is an antenna terminal, a receiver antenna terminal, and a terminal for connection to a frequency meter.
A microphone, can be plugged into the left hand phone jack MIC (or the control box). It can have a press to talk function (PTT). The next phone jack is labeled THROTTLE and can be used for a throttle mounted press to talk switch. The next 2 phone jacks are SIDETONE outputs. The next phone jack is used for a morse KEY. The right most phone jack is used for monitoring the PA PLATE current. A meter can be plugged in here for the PA (power amplifier) current measurement.
Each plug-in coil box has four knobs for tuning labeled C, D, E, and F. Each knob has a lock. The coil box also has a small diameter phone jack on it. A meter can be plugged in here to measure antenna current (labeled ANT. METER). The microphone and antenna current phone jacks are a small diameter phone plug, so that headphones or a key, cannot be accidentally plugged into them. There is a hidden phone plug on the back of the transmitter. It is normally covered by the case. When the case is removed for servicing, the PA grid current can be measured. There are no controls on the dynamotor.
MECHANICAL DESIGN
The chassis is constructed with metal panels and ribs, all spot welded and silver
soldered together. The frame is stainless steel, copper and nickel plated. The case
is aluminium, and held on by DZUS fasteners. The front section is taken up by the
plug in frequency units, the two of them using up most of the front area. There is
a small part of the front panel (at the left side) that has 3 toggle switches,
3 terminals, and the main power and control connector. The panel below the plug-in
units has 6 phone jacks and an Earth terminal. There is one phone jack on the back
panel. There is one phone jack on each coil box.
The rear one third of the transmitter contains the six valves, and the transformers. There is so little space in the transmitter, that the meters are added externally, called a “METERING KIT”. Also the chassis is low and occupies the space between the shock mounts. The chassis and spacing is so tight, that you need to remove the left coil unit, so that you can reach inside, and set the antenna loading coil links! The dynamotor is external, and the junction box and dynamotor have been made into one unit. The single place pilot’s control box is very simple, just containing 3 switches.
Each coil box, contains 2 ceramic coils, and, 3 roller inductors. The bottom of the coil unit has a pull out chart, so that you can record the tuning details. The top of the coil unit has a graph on it, to give you the approximate tuning calibration, as the VFO is not calibrated in frequency. When all the covers are removed, the top view looks like it is full of ceramic inductors.
Picture 7: ATB Roller Inductors (these are normally covered by panels)
There are four epicyclic reduction drives on each tuning unit. There are many specialized ceramic parts used. Of the four relays in the transmitter, two are special ceramic types. The power resistors are clamped to the chassis is several places, and tucked away in places that are sometimes hard to find. The transmitter mount and dynamotor mount are rare and difficult to find.
Picture 8: ATB Transmitter Mount CRV-10082 and CRV-10083
Picture 9: ATB Dynamotor Mount CRV-10084
ELECTRICAL DESIGN
The transmitter is a MOPA design (Master Oscillator and Power Amplifier),
using two valves. The modulator uses four valves. The valve heaters are connected
in a series parallel configuration. Each 815 is connected in series with a 6N7
for 12 volt operation. These two heater strings are in parallel. They are fed
from a tapped resistor in the dynamotor enclosure. The 1625 is fed from a different
tapped resistor also in the dynamotor enclosure. There is a resistor in the negative
HT line, which provides a bias used for keying. It is applied to the PA final,
the modulator, and audio oscillator.
Picture 10: ATB Valves and Meter Kit
The transmitter uses a VFO rather than a crystal oscillator. The oscillator is a 1625 valve (this is a 12 volt version of the 807), which is a beam tetrode. It is a Colpitts oscillator circuit, with the screen serving as the oscillator plate. It oscillates at half the transmit frequency, which allows the use of larger capacitors in the tuned circuit, and thus the variation in valve capacitance has less effect. The tapped oscillator coil is contained in the plug-in coil box.
Picture 11: ATB Simplified RF Circuit (showing lettered LINKS and DIALS)
The output from the oscillator plate, is capacitor coupled, to another tuned circuit, tuned to the transmit frequency. It is designed in a similar manner to the oscillator coil. When the oscillator is tuned, this coil tracks the oscillator coil frequency, which is a clever design. There is no DC on this coil. It is then coupled to the grids of the 815 dual tetrode power amplifier.
The tetrodes are connected in parallel. There is a key jack to measure the grid current. There is also a wire terminated NEXT to grid resistor, but not connected to it. It relies on the proximity to the grid resistor to capacitively pick up some RF. The wire is joined to the front panel CFI terminal, called the CRYSTAL FREQUENCY INDICATOR. A frequency meter can be attached here to measure the transmit frequency, probably an LM type or similar.
Picture 12: The CFI Wire (not connected to anything)
The anode voltage for the PA is supplied through an RFC. The PA not neutralized. A capacitor connects the anode tuned circuit, which is contained in the plug-in coil box. The tuned circuit consists of a fixed capacitor across two roller inductors, in series. The large roller inductor is used for the frequency tuning of the tank coil. The small roller inductor acts as the loading tap on the tank coil. There is a phone jack on the front panel, which measures the cathode current of the PA. The METER KIT can be plugged in here, to allow tuning of the anode circuit. The PA meter has two scales. When plugged into the front phone jack (PA PLATE), the 0 – 250 mA scale is used. When plugged into the rear phone jack (GRID CURRENT), the 0 – 10 mA scale is used. The PA jack has a suitable shunt across it, to allow the same meter to be used for both.
The roller inductor loading tap, is connected to the output antenna terminal, through antenna matching components. External to the coil boxes, is further inductor. This can be switched fully in circuit, half in circuit, or switched out, for each channel. This allows for matching a large range of antennas. The practicalities of electrically short aircraft antennas is recognized. The power output is quoted in paragraph 1.5 as 25 watts CW, with the antenna is defined as 8 ohms and 300 pf. The power output is also quoted in paragraph 1.4 as 3.5 watts CW, when the antenna is defined as 1 ohms and 60 pf.
There is a thermocouple unit in the plug-in coil box to allow antenna current measurement. It has a jack on the front of the plug-in so that the METER KIT can be plugged in. The jack is a small diameter so that the PA current and ANT current meters cannot be wrongly connected. The thermocouple is unusual. It is in a discrete Bakelite block. This means that it is protected from the elements, and can be changed easily. It is also shorted out by a wire! Antenna current still flows through the thermocouple, as this arrangement merely appears as two low resistances in parallel. This gives a “relative” indication of antenna current. When 3 A actually flows through the thermocouple, this indicates FSD on the meter. However, the thermocouple is shunted by the wire, so this is not the actual antenna current. The manual states that this is a feature to provide output antenna current, even if the thermocouple is burnt out or fails.
Picture 13: Thermocouple
MODULATOR
The modulator applies high level anode and screen modulation, using three valves.
The microphone can be a dynamic or a carbon type. A wire can be changed inside the
transmitter to suit. The microphone can be plugged into the front of the transmitter
or into the pilot’s control box. A metal twin triode (6N7) is configured as a normal
two stage Class A voltage amplifier. A second metal twin triode (6N7) is used as another amplifier, but the triodes
are strapped in parallel. There is some negative feedback on this driver stage.
It has an anode transformer with balanced output, to drive the final modulator. The final modulator is a twin tetrode (815) arranged in class AB2 push pull, driving a transformer, to modulate the RF power amplifier plates. To improve modulator efficiency, the modulator screen is voltage regulated using a VR150 valve. The cathode has a fixed positive voltage from a tapped resistor in the dynamotor enclosure.
There is a secondary winding on the modulation transformer, to provide sidetone output. This is available at two front panel phone jacks. The sidetone level can be set using two internal preset potentiometers to two different levels. This can be used for either VOICE or CODE, or this also allows for two receivers if required. The second twin triode is configured as a 1kHz audio oscillator when switched to CODE operation. This provides a sidetone for CW operation, and also a modulation signal for MCW operation. The modulation transformer secondary is switched to modulate the RF final amplifier, for VOICE and MCW, but not CW operation. When switched to CW, the secondary is unloaded, so the screen voltage is reduced.
RELAYS
There are four relays in the transmitter, and two in the dynamotor box. Two are a special design.
There is a simplified control diagram in the manual which makes the operation clear.
The CW and MCW/VOICE relay switches the modulator secondary out or in, for the PA final. It is operated from the CW/MCW toggle switch. It is also operated from the VOICE/CODE relay. It switches in a resistor to reduce the modulator screen voltage for CW, as the secondary is unloaded.
The EMISSON relay selects VOICE or CODE. It is operated from the toggle switch on the front panel, or the pilots control box. For CODE, the dynamotor runs continuously. It configures the audio amplifier as an oscillator. It removes the modulation input. It selects the appropriate sidetone level. It removes the voltage from the carbon microphone. The oscillator HT is removed for key-up When the Key is down, the bias is removed, and the oscillator has HT.
The CHANNEL relay is a unique relay using special ceramic parts. This is operated from the pilots control box. It selects either of the two plugin tuning units. It is a low capacitance type, and it can also handle high RF voltages. It has two coils. They are both used for pull in, and then a resistor is switched in for holding at a reduced current drain. The relay requires 2 amps, but when in, a resistor in circuit reduces this to 0.3 amps, and it has a capacitor across it.
The KEYING relay is also a unique relay using special ceramic parts. It is operated from the morse KEY plugged into the transmitter, or the KEY on the pilots control box, or the PTT button on the microphone, or the THROTTLE switch, or the TEST KEY toggle switch. The dynamotor only runs when the KEYING relay is activated. It changes the antenna from receive to transmit. It also removes the keying bias from the PA final and the modulator final. The KEYING relay uses 2 amps to pull in, which opens a contact, reducing the current to 0.3 amps. One reference suggests that this relay is capable of keying at 20 WPM.
The TUNE OPERATE switch disables all the relays. When in the TUNE position, it powers the oscillator only.
DYNAMOTOR
The dynamotor supplies 425 volts DC at 320 mA. There is a separate box,
which carries the dynamotor and serves as a junction box. It has three connectors
on it. One 3 pin male connector is used for the 24 volts DC input. One 16 pin connector
is used to connect to the transmitter. Another identical 16 pin male connector
is used for the pilots control box. The Control connector is keyed differently,
to prevent miss connection. The junction box cover has 4 snap slides, so it can
be easily removed for maintenance.
The junction box contains fuses for the DC input and the high voltage DC output, and some filtering for the supplies. There is a large start relay (K201) for the dynamotor. There is a tapped dropping resistor (R201) for the modulator cathode (V105). There is another tapped resistor (R202) for all the valve filaments. There is a further tap on this resistor that is switched into circuit by the relay K202. This relay is voltage sensitive, and will switch the resistor in series, if the DC input voltage rises above 24 volts. This under voltage relay is provided to insure transmission “up until almost final exhaustion of the batteries when the plane’s motor is dead”. To prevent this “under voltage” filament relay from latching, there is a contact on the dynamotor start relay, which momentarily, interrupts it. The dynamotor came with the mount, which was lucky.
Picture 14: Dynamotor
PLUG IN COIL UNITS
Picture 15: Coil Front panels
There are two types of coil unit, both being very similar in design and operation. One covers the frequency range 2.3 to 4.3 mHz and the other covers 3.0 to 9.05 mHz. They are a very compact design, and a little too cramped, as it is difficult to service them. The tuned circuits are isolated from the HT voltage by capacitors, so there are no DC voltages inside the coil units. The coils are made of thick silver and copper plated Invar wire wound on ceramic formers. There are temperature compensating capacitors in the tuned circuits to ensure frequency stability. The oscillator capacitors are the silver plated ceramic tube types, and mounted vertically so they will be cooled.
Each plugin coil unit is divided into three sections. The lower section is for the oscillator. The top right section is for the PA. The top left section for the antenna matching. The rear pins on each coil unit are labeled A to E on the schematic, which is not to be confused with the tuning knobs and switches, which are also labeled alphabetically. The tuning adjustments are labeled with the letters A to H, which make understanding them easier. The frequency range or BAND is set by the links “A” and “B” which are inside the coil boxes. The variable frequency tuning is set by knobs “C” and “D” which are on the front of the coil box. The loading is set by adjustments “E” and “F” which are on the front of the coil box. The links “G” and “H” are inside the transmitter and used to add additional fixed inductance or capacitance for antenna matching.
OSCILLATOR
The oscillator section contains two coils on ceramic formers. They appear
to be separate, but they are not. The rear coil section is the tapped coil for
the 1625 oscillator. This only has one connection to the oscillator valve, the
oscillator grid (pin E). There is also an earth connection (pin D). Inside this
coil is another coil, which is connected in anti phase, so that it reduces the
main coil inductance. As the oscillator knob MO TUNING (labeled “C”) is rotated,
a “micrometer” like screw mechanism moves inside the rear coil to change the inductance.
There is a sliding switch on the coil side, to connect different capacitor combinations.
This is effectively the BAND switch, and has 6 positions. The switch is labeled “A”.
There is a chart on top of the coil box (and in the manual) which shows the
calibration curves for the coil unit.
Picture 16: Coil Tuning Chart (9 mHz plugin)
This shows the BAND and the DIAL SETTING. The manual says “The calibration charts supplied with each tuning unit are accurate for settings within an audible frequency range of the correct frequency.” and “The master oscillator reset accuracy is within 3kc.” It also says “Coil units may be tuned on the test bench, then transferred to an aircraft, with only slight retuning required.” This implies a high quality and stable coil box.
In the same section as the oscillator coil, is an almost identical coil, (but with fewer turns), on the same axis. It also has an internal anti phase coil for the inductance adjustment. There is also a sliding BAND switch, with 6 positions, labeled “B”. There is only one connection to this tuned circuit, (pin F), but it also relies on the earth connection. This tuned circuit is the coil between the oscillator plate and the power amplifier grid. As the oscillator knob (labeled “C”) is rotated, the anti phase coil moves inside the front coil to change the inductance. The oscillator knob tunes both coils simultaneously, and they are designed to track together. There are capacitor trimmers to adjust the tracking. The front coil is tuned to the transmitter frequency, and the rear coil is tuned to half the transmitter frequency.
The top right hand section of the plugin coil box contains a roller inductor with 37 turns. This is the tuned circuit for the PA anode. It has a fixed capacitor to produce resonance, and the roller coil is adjusted to change the inductance using the “D” knob (PA TUNING).
There is another roller inductor on the same axis, but it only has 5 turns. This is in series with the main roller inductor, and acts like a coil tap. It is adjusted using the “E” thumbwheel (ANT. COUP.).
The roller inductor at the top left hand side, is the antenna loading coil. It has 75 turns, and can adjusted to match the antenna, using the “F” knob (ANT. TUNING). At the rear of this inductor is a link (“G”) which can add additional inductance (in the 2.3 to 4.3 mHz plugin) or additional capacitance (in the 3.0 to 9.05 mHz plugin).
Picture 17: Coil A and B range switches
When using a trailing antenna, the manual says that control “F” is not used (set it to 70). Then wind the trailing antenna in or out, using the antenna length instead of control “F” for tuning.
KNOBS AND GEARBOX
The three knobs on each plug in unit, protrude from the front panel.
Each knob has an epicyclic reduction gearbox between the knob and the front panel.
This gearbox (round in shape) provides the reduction drive, from the knob to the
shaft behind, which connects to the tuning component. There is also a scale on the
front of the gearbox, and a pointer to indicate where you are tuned to. The gearbox
also provides a positive stop at each end of travel. The knobs protrude different
distances from the front panel. There are locks on coil knobs.
Picture 18: Side View of Knobs
KNOB “F” (ANT. TUNING)
This is has the simplest arrangement, and protrudes out from the front panel,
the least. There are 2 concentric shafts, one inside the other. The outer shaft is
connected to the gearbox, and serves to provide: a tuning scale, a position indicator,
and a positive stop mechanism. The inner shaft goes straight through the gearbox and
the front panel, and drives the roller inductor in the coil box. The knob (labeled “F”)
has five (5) grub-screws! There are 2 in the smooth part of the knob, and these are
used for the outer shaft and gearbox. There are 3 in the knurled part of the knob,
and spaced at 120 degrees, not the normal 90 degree spacing. These are for the inner
shaft and drive the roller inductor.
The order of grub-screw tightening is important. For servicing, loosen the 5 grub-screws. Remove the knob. Clean and lubricate the gearbox and shafts. Clean and lubricate the roller inductor. Put the knob back on, tighten the 2 rear grub-screws only, as these contact the outer shaft. Use the knob to rotate the gearbox. It should turn freely, and it should stop at each end of the scale. The roller inductor should not turn at all. Rotate the roller inductor by hand. It should turn freely, but may not stop at each end. Ensure the follower travels properly. Gently tighten the 3 front grub-screws, until they contact the inner shaft. Then tighten them in order, one at a time, a little at a time. Rotate the knob. It should turn freely. If you have tightened one of the 3 grub-screws too much, the inner concentric shaft will bind on the outer shaft and the knob will be stiff to turn. Slack off one of the three grub-screws a tiny amount and tighten one of the others a tiny amount. Keep on doing this, until the knob turns freely and the grub-screws are tight. The scale and the roller inductor should now turn freely.
To align the roller inductor follower position with the scale reading, there are 2 ways. You can start the grub-screw tightening procedure, with the scale at one end of travel, and the roller inductor at one end of the travel. Alternatively, you can finish the tightening procedure, then lift the roller inductor follower and slide it to the correct position. This is easiest done if the scale is fully at one end. The scale on the front of the round gearbox is graduated from 0 to 68. The gearbox will stop at each end of the scale. The knob will rotate 41 turns only, and is connected to the roller inductor directly (with the 3 grub screws). The roller inductor has 43 turns, so position the follower 1 turn from each end.
KNOB “C” (MO. TUNING)
This the next simplest tuning control, and is the oscillator control (labeled “C”).
This is not calibrated in frequency, as the “A” switch can select different BANDs.
The chart on top of the plugin coil unit is required to estimate the frequency.
The knob shaft passes directly through the gearbox, and turns the lead screw in the
oscillator and grid coils, thus moving the anti-phase coil and changing the inductance.
The knob is held on by only two grub-screws. It requires a spline wrench (Bristo key)
which is attached to the plug in unit outer panel. This has a scale 0 to 70 affixed
to the round gearbox, and a scale 0 to 99 on the knob, with a pointer.
KNOB “D” (PA. TUNING)
The PA tuning knob (labeled “D”) protrudes the largest distance from the front panel.
It has a similar gearbox, but with a scale 2.3 to 4.3 (or 3 to 9) megacycles affixed to
the front. It has an outer and inner concentric shaft, so tightening of the 5 grubs screws
is important. The outer shaft drives the gearbox and the scale, while the inner shaft
drives the 38 turn roller inductor.
THUMB WHEEL “E” (ANT. COUP.)
Under the “D” knob is an extra gearbox. It drives the small 6 turn roller
inductor used for tapping (coupling). It has a scale that only shows 0 to 8.
Because there are 2 gearboxes on the control, the knob protrudes the furthest
from the front panel.
RESTORATION
I was clearing out a shed a long time ago, and in the dim recesses, under all
the dust and rubbish was a panel full of knobs. I pulled it out and was amazed at
what I saw. It sat at home in a cardboard box for several years, until I eventually
found the: manual, the dynamotor, the meter kit, the plugs, and the remote control.
Only then could I start restoration.
It required a thorough clean. It was sprayed with degreaser and hosed off on the lawn, and dried in the hot sun. This was done twice, as it was so filthy. The valves and coil boxes were removed and put in a cardboard box. The chassis was examined on the bench. It was still dirty and slightly corroded, and many modifications were apparent.
RELAYS
There are four relays in the ATB transmitter proper, two are common types,
and two are very special and unique. They are very important to the function of
the transmitter, and required a lot of care. The relays were seized and broken,
and one was missing.
The EMISSION relay is an ordinary multi contact relay. It was cleaned, contacts burnished, and tested. It did not pull in. The bearings were lubricated, and it was exercised with 24 VDC until it was operating properly, and all the contacts opened and made correctly.
Picture 19: Emission Relay
The CODE/VOICE relay was actually missing, the wires were just soldered together. The manual showed a photograph. It is an unusual shape, but a brand new one was found in my junk box of relays! It was fitted, and tested, but not wired in just yet.
Picture 20: CW VOICE Relay (missing)
Picture 21: CW VOICE Relay
The KEYING RELAY was a very special shape, size and used unique ceramic parts. It was seized, the antenna changeover contacts were soldered closed and the contact arm tied down with cloth insulation tape. The wires were marked, it was removed, and it was cleaned on the bench.
Picture 22: Keying Relay (contacts taped closed with cloth tape)
The solenoid was cleaned, de-rusted, freed up, and lubricated. The contacts were cleaned, the solder and tape removed. It was operated by hand till it moved freely and positively. Power was applied, and it banged in, quite loudly! This was done many times, till it appeared to be functioning properly.
Picture 23: Keying Relay Antenna Changeover Contacts (receive)
Picture 24: Keying Relay Antenna Changeover Contacts (operated for transmit)
This relay has two coils, a “pull in” coil and a “holding” coil. Both are used to actuate it, but one is switched out when it closes, and then the “holding” coil is just used. The contacts are affixed to ceramic bars, with nuts and screws, but not riveted on, so they can work loose and move. But also they can be easily adjusted. All the contacts are normal, bar one. It is a special tungsten contact. The manual states that when the contacts open the “pull in” coil, the tungsten contact must break first. Upon close inspection, this was not happening, and the contact was visibly burnt. I adjusted the contact to operate correctly. The “pull in” was measured at 1.7 Amps. The “holding” coil was measured at 0.3 Amps.
Picture 25: Keying Relay Changeover Contacts (unpowered)
Picture 26: Keying Relay Changeover Contacts (operated)
Picture 27: Keying Relay Burnt Contact
Paragraphs 6.4 to 6.7 in the manual cover this relay, and recommend cleaning with “carbon tetrachloride”, which is not available nowadays. They describe the adjustment. But if two tungsten contacts were fitted instead of just one, then the adjustment procedure and the paragraphs in the manual, would not be necessary.
The FREQUENCY SELECT relay was working, but some contacts were broken off. Similar contacts press against the plug in coil units and these were broken too. There are 6 spring contacts in each bay, and each one presses against the contacts on the back of the plug in coil box. The left bay was fine, but in the right bay, they were all broken. I found a large relay, and removed some of its contacts and then bent them to shape. They were soldered to the old broken contact springs, and then adjusted to contact the coil plug in properly. The contacts on the FREQUENCY relay were repaired in a similar way.
Picture 28: Broken Contacts
Picture 29: Broken Contacts
RESISTORS
There was evidence of several modifications. Some power resistors were wrapped
in cloth tape, others were hanging out. These were simply removed. Many resistors
were a ceramic type and some were broken. These were replaced with a similar type.
Picture 30: Resistor Modifications
Picture 31: Broken Resistors
Once all the obvious problems were fixed, it was time to start wiring. The circuit was printed out and wires were traced one by one, and marked off the circuit diagram with a red pen. Wires were put where they should be located, missing wires were added using period cloth covered wire. Resistors and capacitors were checked for value but few required replacement. Modifications were removed. The heaters were rewired for 28 volt operation. The microphone was linked for a carbon microphone.
The RF loading coil was missing, and it would be impossible to find one because of the transmitter rareity. I dug through my box of ceramic coils and roller inductors, looking for something similar. I found one that looked close, and when I bolted it in place, it fitted perfectly. I realized it was the correct coil! I must have salvaged the coil at the same time as saving the transmitter. I wired it in circuit, with heavy tinned copper wire, and covered it with ceramic beads, as per the photographs in the manual.
DYNAMOTOR
When I obtained the dynamotor, I was told it was faulty. I cleaned it and the
junction box, and greased the bearings. I applied 28 volts, and surprisingly,
it span up. But all was not well, it was obviously struggling, input current was
very high, and the output voltage was low. It was groaning and getting very hot.
It had shorted turns. I did a visual inspection and discovered someone had cut
some wires on the commutator, attempting to remove the short circuit. I tried
as well, but to no avail. I found an electric motor rewinder, but he was reluctant
to take on the project. After many calls, a year had gone by, and he admitted he
had not touched it, and did not want to. I went and got it back. While I was waiting
for the dynamotor, I made up the cables, from the connectors and wire that I had.
Picture 32: Cables
I dug through my dynamotor box, looking for some dynamotor that would be suitable. I found a dynamotor that was identical. What luck was that! I cleaned it, and greased the bearings. I applied 28 volts, and it span up, and happily produced the correct 425 volts. I mounted it on the junction box, and checked the wiring and the resistors and capacitors. One broken ceramic resistor was replaced.
Picture 33: Dynamotor
It has fairly simple wiring. The huge start solenoid operated correctly. The two heater dropping resistors were good. There is a voltage sensitive relay, which switches a series heater resistor in or out, depending on the supply voltage. I tested this on a variable voltage supply, raising and lowering the input voltage. The relay switched the resistor in circuit at 24.5 volts and switched it out at 23.5 volts.
Picture 34: Dynamotor Inside
TRANSMITTER
I checked the wiring of the Pilots Control box, and marked each wire off the circuit
in red pen. I connected up the transmitter, with all valves removed, coil boxes removed,
and all fuses removed. Using the Control box, I verified that the front panel switches,
and the control box switches worked.
The FREQUENCY relay changed properly.
The EMISSION relay changed properly.
The VOICE/CODE relay changed properly.
The KEYING relay changed properly.
The dynamotor started when it should.
The heater voltages were checked. The 1625 was plugged in, and the transmitter was powered up. The heater voltage was correct. The HT fuse was inserted. A coil box was plugged in. It was powered up again, and the dynamotor started, and the voltages were correct. It could see some RF at the oscillator output on an oscilloscope. A receiver could hear the oscillator. The frequency appeared stable, and the coil box knob tuned from end to end, and produced the correct frequencies.
The PA final valve and one of the audio amplifiers were fitted. The audio amplifier was needed as the heaters were in series. It was powered up again, voltages were checked, and I could see some RF at the PA anode. I turned the roller inductor PA tuning but it was stiff and the PA current was all over the place. So I stopped at this point and refurbished the coil boxes and the meter kit.
METER KIT
This had been modified, but only minimally. The cables and plugs had been cut off,
and plastic wires attached. I found the correct plugs and cables and wired them in.
They tested correctly. I applied a small voltage and the meters moved without sticking.
The ANTENNA current plug is a small diameter plug like the microphone plug, and it
can be accidentally inserted in the microphone jack. The PA current plug is a normal
diameter plug, and can be accidentally inserted in the THROTTLE, SIDETONE or CW jacks.
Picture 35: Meter Kit (inside before restoration)
Picture 36: Meter Kit
COIL BOXES
The coil boxes were extremely dirty. They were cleaned with water, degreased,
hosed and thoroughly dried. Then the bearings were oiled and freed up. The roller
inductors were cleaned with a non metallic scouring pad, until they were clean
and shiny. Two were still intermittent. The follower was not a wheel, but a slider.
The shaft that it slid on was removed and cleaned on all sides. All the followers
were re-tensioned. The roller inductors are held together with nuts and bolts,
so these were adjusted and tightened. In several places the broken ceramic was
re-glued. Two were completely taken out for repair. The PA TUNING had its rear
bearing broken away. It was glued back in. The ANT TUNING had a broken front
ceramic frame, which was re-glued. The ceramic plates on the rear of the boxes,
that carry the 5 coil contacts, were cracked, but these appeared to function
properly. They were cleaned and re-glued.
Picture 37: Broken Roller Inductor (PA TUNING)
Picture 38: Broken Roller Inductor (ANT LOADING)
After reassembly of the roller inductors and gearboxes, they did not function properly. The gearbox would only allow one turn before stopping. The manual discusses gearbox design in paragraphs 6.23 to 6.35, and knob and gearbox adjustment. My knob and gearbox design is slightly different. It has angled (or ramp ends) on the lift plate, not tabs shown in the manual as Figure 9. For knob removal, paragraph 6.29 to 6.31 say, turn the knob until it hits the stop, then remove knob. When I do this, it snags and bends the lift plate. With the design fitted to this ATB, another method is required. Back the knob off at least 3 turns from the stop, before removing the knob, thus clearing the lift plate, so as not to bend it! I straightened the bent lift plate, then reinstalled the knobs (but 3 turns away from the stop), and then they functioned correctly.
Picture 39: Gearbox diss-assembled
With clean coil boxes installed, it tuned up much better. I used the TUNE switch and CFI output to set the oscillator frequency. The PA roller inductor was adjusted, and the meter dipped at the correct place. I set the PA meter to about 30 to 50 mA. A 50 ohm dummy load was attached, and the coupling and loading inductors were adjusted, until there was a maximum of 0.75 amps reading on the ANT meter. The dummy load indicated 17 watts. This was a little low, but since the transmitter is designed for a 10 ohm load, it was considered reasonable. There was no ANT current indication on one coil box. This was traced to an open circuit thermocouple block. I attempted to repair it but could not, without re-welding or completely replacing the thermocouple element. I replaced entire thermocouple block as I had a spare.
I plugged in the other audio valves, and a carbon microphone. The oscilloscope indicated reasonable AM modulation, and I could reach 100% fairly easily.
PERFORMANCE
The transmitter was a little tricky to tune into a 50 ohm dipole. Part of the
problem was the small too-close together knobs (or my big fingers). Adjustment wearing
gloves would be difficult. I tuned one coil box the 80 meter band and the other
to the 40 meter band. Using the CFI output and a frequency counter, made setting
the oscillator easy. Tuning the PA was easy. But matching that aerial took some time.
Once set up, I could change frequency easily. I have made a few contacts so far.
One gentleman said that I sounded like “bomber command” when using a T-17 microphone.
Plugging in the antenna current meter does change the tuning slightly, and the meter
lead does get warm.
MANUAL
The manual has serial number 669, and is titled PRELIIMNARY. It contains many errors,
and at the end, it has 3 pages of corrections. The manual a bit disjointed and it is
not easy to find things. It does have some useful theory of oscillator function,
and antenna characteristics.
The diagrams show a different gearbox mechanism to the one fitted. The picture of the pilots control box (Fig 23) shows that it is actually fitted into an ARB case, so it must be an early photograph. The main circuit (Figure 31) shows the circuit for the tuning units. Tuning Unit No.3 has the earth connection wrongly labeled E whereas it should be the earth symbol. The manual is wrong as it says R106 is used for “key bias” in the negative HT line, but it is actually R108.
After the parts list is Table VI, which shows the code of all the manufacturers used to supply the parts. The first one is CRV which is RCA Manufacturing Company. There is a section on fault finding and remedies. There are valve data sheets at rear.
There are 29 paragraphs (4.1 to 4.19) describing the Installation, drilling, mounting and testing of the transmitter. In paragraph 4.8 for installation, the manual discusses the spiral “lay” of the multiple wires in the cables which were provided for installation in the aircraft, and says that the pin connections of the plug were planned for the easiest connection with the least number of cross overs.
There are 4 paragraphs describing oscillator theory (3.18 to 3.26). Then another 5 paragraphs talk about how this oscillator is designed. The paragraphs 4.32 to 4.33 discuss the PA tuning. Then paragraphs 4.34 to 4.48 discuss antenna matching using the E, F, G, and H controls. Then the following paragraphs 4.49 to 4.55 discuss how to match the antenna.
CONCLUSION
The transmitter performs well, and is easy to operate, once tuned. However,
it is hard to tune. The knobs are difficult to adjust due to their size and closeness
together. Tuning to a new frequency is slow. The tuning units contain several ceramic
roller inductors, and there are many unique ceramic parts inside. This must have been
expensive to manufacture, and has too many unique and fragile parts. Perhaps this
is why it was not produced on a large scale, and why it is not common. The power
output of 20 watts is not enough for its function as a “liason” transmitter (long range).
The high amplitude modulation is efficient. The design shows plenty of careful
design and thought has gone into it. It also appears that the transmitter was still
being developed, and the next model may have been better. I think it would have been
better if the gearboxes were behind the front panel, or maybe flush with it.
But there is not much space available. Despite its short comings, it is nice to use.
(It is always good to have complete junk box.)
REFERENCES
Aircraft Radio Transmitter Model ATB SNo. 669, Preliminary Instructions, IB-318143, RCA Manufacturing Company Inc, Camden, NJ, U.S.A.
The ATB Aircraft Transmitter, Walt Hutchens KJ4KV, Electric Radio Issue 21, Pages 4-10
ATB/ARB Aircraft Transmitting and Receiving Equipment, Navy Radio & Sound Bulletin No. 8, 1 October 1942
Catalina History http://www.aarg.com.au/consolidated-pby-catalina.html
RADIO CHARACTERISTICS – U.S. NAVY SERVICE AIRPLANES, 1943
Picture 40: ATB Transmitter Circuit
Picture 41: ATB Dynamotor Circuit
Copyright
Ray Robinson