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Backmail #29 (14 pages) INDEX; PRC-68 QUESTIONS; PRC-70 QUESTIONS ANSWERED & OPERATION,BY JIM KARLOW RT-70, MORE THAN JUST A TANK RADIO TRC-77, GOOD RADIO, WRONG TIME. ANOTHER CLUE TO THE TRC-77's USE; From Steve Hill AN NUMBERS VERSAS RT TYPE NUMBERS,& URC QUESTIONS ************************************************************* PRC-68 QUESTIONS; Dennis, After firing up this little rig and playing with it for a short time several questions have arisen. The radio will xmit and receive on all ten channels as well as unsquelch and be unsquelched. (150 hz tone squelch, haven't tried with carrier operated equipment) However, when channel 2 and 8 are selected they remain unsquelched all the time. Is this normal? When squelched out, regardless of listening on speaker mic or H-250, with the volume set at 1/2 or higher a slight putt-putt-putt can be heard on all channels except 2 and 8 which remain unsquelched. Is this normal? Is the Squelch Disable position on the function switch used as a monitor button like on some of the newer commercial VHF/UHF HT radios? Have you ever measured current draw at nominal battery voltage during receive squelch ops? receive unsquelch ops? xmit? I don't have a maintenance manual or another radio to compare mine against to know if it is normal.Any information you can provide regarding the above questions would be much appreciated. Sincerely, Doug. DJM-AIRCAP@juno.com -------------------------------------------------------------------------------------- Doug, The unsquelched operation on both channels 2 & 8 is most likely the result of stray RF from some external source, such as computer, TV, etc. The PRC-68 does not have 150cps tone squelch. In receive it has standard carrier squelch, which is adjusted via an internal trimmer resistor. This adjustment is very voltage sensitive, I/E if you adjust it with a real good battery in it, then said battery drains a little bit, the squelch will break. The best setting is sometimes tedious to find. In transmit, it has a 150cps tone oscillator so that compatibility with later model equipment can be maintained. The Squelch Disable position on the pwr switch simply defeats the radios internal squelch control. The slight Put-Put that your hear is the radio's battery save function. Non eccential sections of the radio are pulsed on & off to reduce the duty cycle, thus save battery drain until a signal is received. Another Put -Put noise symptom is that the VCO is out of lock, but as you say that it transmits & receives OK, this is not the case here. Trying to measure the current drain of the radio during squelched/unsquelched operation will not do you much good because of the pulsed supply when squelched, also if tried with a digital meter, it will tell you nothing. Dennis ********************************************************* PRC-70 GUESTIONS ANSWERED; The following are some questions posted by Joe Pinner in response to his reading of the PRC-70 description in my publication,& his desire to know more about the radio(as the lucky SOB owns one). Replies are being made by JIm Karlow,the only known real authority on how these things work. He mentions an attachment that will be included in a future posting do to present incomparability of our systems.To add to Jim's excellent response,I'd like to include,that the PRC-70's design requirements actually preceded that of the PRC-77(circa 1965) & the design of the radio began at about that time. Problems incountered with the early radios where in large part do to the attempt to use technology that virtually did not exist at the time. It was to replace all the multiple radio set's required for Special forces troops,FAC Pathfinders etc,all in a single package. These radios include PRC-64,74,77,71,72,82,83 & others,note that most of these radios have higher numbers than the one ment to replace them. Perhaps the explanation of this will be in a future instalment. BTW we needs some more participation like this! Dennis - Thanks for your note regarding the PRC-70. The following might provide some background information regarding the radio. The PRC-70 was designed and built with broad frequency coverage (2.0000 to 76.0000 MHz 100 Hz steps) to replace then (1970's) existing military HF and VHF radios (PRC-74, PRC-25, PRC-77 and others). It included advanced circuitry to make it both downwardly and upwardly compatible with all military radios issued from World War 2 through 1980, including 150 Hertz tone generator for FM transmit and signal to noise type squelch for receive. Vehicle and aircraft remote control versions of the PRC-70 were also proposed. These aparently never went to production. It is interesting to note that you can tune this radio with a serial data word through two pins on the X-Mode connector. The PRC-70 has a simple operator interface, backed by reasonably sophisticated circuitry designed to make the unit work even with an untrained operator. The idea was to eliminate the trained "radio operator" previously required in each platoon. In this respect the production PRC-70 design had flaws. While the circuitry is quite good, there are certain conditions that the untrained operator could get into where damage could occur to the radio. An operator with a ham radio background would not get into most of these problems. The operators manual also contributed to problems by suggestions such as "if the antenna match is not achieved, go ahead and transmit anyway." This text caused more burned out drivers in PRC-70's, because operators just didn't bother to execute the tune mode. In later years the Army was to assign a trained operator to run the PRC-70 as well as to design some improved accessories, including the SORAK antenna. These measures helped out. I like the radio. I have several units that I use. I also repair these units and have fixed now nearly 100 units. I also stock parts for the '70 and can help others who need assistance. I also have a library of conversions for this radio. Attached please find an errata sheet I include with every repaired PRC-70 I send out. It identifies potential problem situations and how to avoid them. Jim Karlow jkarlow@takata.attmail.com ---------- From: Dennis R Starks To: JKARLOW Subject: "Joseph W. Pinner" |
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Please read prior to operating your RT-1133/PRC-70 Radio The PRC-70 packs
allot of capability and sophistication into a small package. The same mechanism
that allows this radio the broad coverage (2.0000 to 76.0000 MHz) and multimode
operation can occasionally get into conditions which, if not recognized
and properly cleared, can cause the radio to fail and need repair. Properly
setting up the radio, as well as recognizing certain situations and how
to resolve them will assure that your radio will provide you with trouble
free operation. Normal Operation Steps and comments: 1. Connect antenna
For short range point to point work, the radio will work with a 9 foot whip
antenna from 3-76 MHz. A 6 Foot whip will work from about 5 MHz to 76 MHz.
The original whip antenna made for the PRC-70 (AS-2974) is rare. The radio
seems to operate well with the PRC-25 long whip antenna (AT 271/PRC). The
PRC-25 rubber spring base (AB 591/PRC-25) is required to use the whip antenna
as it includes a projection from the threaded tip to actuate the microswitch
within the PRC-70 radio to allow signal to be passed to the whip antenna.
I think it is best to trim the antenna to a 9 foot total length, so it will
be the electrical equivalent of the original antenna. Two long range antennas
have recently been released through surplus channels that work will with
the PRC-70. These are the AS-2795/PRC-70, the factory supplied doublet antenna,
and the OE-452/PRC Special Operations Antenna Kit (SORAK). The SORAK is
the best of the long range antennas. The PRC-70 will also work with a dipole
antenna cut roughly to the frequency desired for transmitting. If the antenna
is too short the coupler will not be able to achieve a good match on transmit
and damage to the radio can occur. This is particularly important at lower
frequencies (below about 30 MHz) 2. Connect Power supply The nominal operating
voltage for the PRC-70 radio is 24 Volts DC. Positive connection is made
to Pin 5 of the power connector, negative goes to pin 1. The radio will
operate satisfactorily from about 20-30 volts, a robust enough power supply
is needed so that the voltage will not sag below 19.5 volts when in the
turn on, tune or transmit modes. Maximum current draw is 7.5 amps in high
power transmit. Current draw should never exceed 7.5 amps for a properly
working unit on transmit. Receiver current draw should be about 250 ma.
after initial turn on. I recommend using a voltage and current regulated
power supply when operating this radio. Under no circumstances should the
operating voltage fall below 19 volts or damage will occur to the radio.
Be careful if you use the PP 6184/U power supply with this radio that the
voltage and current ranges are properly set. You should not attempt to use
this supply in the high power transmit mode as the regulator can go out
of regulation and damage the radio. I would also recommend not clamping
the radio to the front of the power supply, but rather using a cable between
the power supply and the radio so that power supply status can be monitored.
3. Connect Handset Use the Military H-250 Handset. Several other series
have come surplus recently that were originally used in other applications
that look physically correct but don+t work or function poorly, due to wiring
differences and differences in earpiece and mike elements.. I have had several
units sent to me for service where the only problem was the handset caused
the radio not to function correctly. Substitution using the H-250 solved
the problem. 4. Select desired frequency 5. Rotate power switch to receive
only or low power When the power switch is switched from -OFF- to any on
position, the operating current should momentarily spike up to about 3 amps
and then drop to about 250 ma. An audible click of the power relay can also
be heard. If the current does not drop from 3 amps within about 2 seconds
after turn on, a problem has occurred and the radio should be immediately
turned off. Turn on can be repeated in about 10 seconds. If the current
again fails to drop, service is needed . Receive operation The PRC-70 radio
can be operated in the receive mode when the power switch is in the receive
only mode or in the low power or high power mode. Turn the radio on, select
the frequency and mode, set the squelch and volume and the radio will receive.
With a .3 to .5 uV sensitivity you should be able to hear lots of signals
on most bands and hear significant background noise between stations in
the unsquelched mode. The state of the antenna coupler does not seem to
have much effect on the sensitivity of the receiver so the antenna coupler
tune mode does not need to be actuated if only receive mode is desired.
This will help conserve power in battery operation. The PRC-70 radio uses
a signal to noise type squelch. A 150 hertz tone is not required to break
squelch on any frequency. (The radio is designed to be downwardly and upwardly
compatible with any HF/Lowband VHF radio the military ever procured.) When
the 10 MHz. frequency selector and many times when the 1 MHz frequency selector
are actuated, the noise of a small tuning motor can be heard. This motor
selects the correct antenna coupler capacitor and harmonic filter element.
Selection should be complete 1-2 seconds after frequency change and the
sound of the tuning motor should disappear. Note: With most PRC-70 radios
I have seen, occasionally when the operating frequency is changed to a new
frequency, the radio might not receive on a desired frequency or the sensitivity
is low. This condition can be detected because the background noise from
the ear piece of the handset is low or the number of signals heard is allot
less than expected. To resolve this problem, change the 10 MHz frequency
switch to a frequency 20 or more MHz away from the intended frequency and
then back to the desired frequency. Many times this will restore normal
operation of the radio by resetting the tuning motor and switched sections.
6. Transmit operation Prior to transmitting, make sure that a correct antenna
is connected to the radio. Set the radio to the desired frequency and mode.
Remember that the radio will not transmit in the FM mode below 30 MHz unless
specially modified Turn the power switch to -TUNE- and release to initiate
the tune sequence. A low power signal (about 2 watts is applied to the coupler
and antenna during the tune cycle. While in the tune cycle a series of clicks
can be heard as the tuner selects the optimum inductor for the antenna.
The cycle should last less than 10 seconds. DO NOT TRY TO TRANSMIT UNLESS
THE TUNE CYCLE HAS BEEN SUCCESSFULLY COMPLETED. Damage will occur to the
predriver or driver transistors if high SWR conditions are encountered.
While internal circuitry will not allow the PRC-70 to operate in the high
power mode when the tune cycle has not been successfully completed, the
radio will produce low power. (The operators manual (TM 11-5820-553-10)
suggests that the radio can be operated successfully when it won+t tune
but doing this will eventually burn out the predriver and/or driver transistors)
If the tune cycle hasn+t completed within 10 seconds, a problem has occurred.
Typical problems and remedies are as follows. 1. After tune sequence is
initiated, Tune sequence stops and a beeping is heard in the handset --
antenna is not connected or antenna is shorted. Antenna connection should
be corrected or short removed. The tune mode should then be reactivated
to achieve proper tuning 2. Initiating tune sequence causes supply current
to rise to approximately 5 amps, no relay clicks are heard and the current
does not fall after 20 seconds. Immediately turn the power switch off. After
10 seconds turn radio back on and change the 10 MHz frequency switch to
a frequency 20 or more MHz away from the intended frequency and then back
to the desired frequency. Many times this will restore normal operation
of the radio by resetting the tuning motor and switched sections. Again
initiate the tune sequence to achieve proper tuning. 3. Initiating tune
sequence causes almost no current rise and no relay clicks are heard. Shut
off radio to end cycle and turn back on. Remove bottom cover and check power
amplifier input and output connections. Push in all mini coax connectors
to make sure they are properly seated. Retry tuning sequence. No change
in symptoms means that a failure has occurred in the power amplifier or
transmit broadband modules. Questions and Service For further information
regarding the operation of this PRC-70 radio please call or write to: Jim
Karlow KA8TUR 4907 Oakwood Court Commerce Twp, MI 48382 (248) 685 - 3077
****************************************************************** RT-70,MORE
THAN JUST A TANK RADIO! The RT-70 though usually referred to as a Tank Radio,was
used in various other roles dating from 1951 to approx 1965,& continued
on in service with U.S. secondary combat units,& allies even longer. It
was originally conceived in response to a need for communications between
tank personnel & their support infantry(armor radio frequencies were in
the range of 20-27mc,while infantry was 38-54mc). Though this need had been
recognized early in WW-II,it was not properly addressed until the Korean
Conflict. Up until the requirements of the RT-70 were added to that of the
new family equipment(RT-66-68 etc). Field telephones were mounted externally
on tanks,thus an infantryman needed to run along side the moving vehicle
to pass information. Though this practice was continued,it was no long the
only means of infantry soldier/tank communications. So the RT-70 now enters
the seen as a liaison radio to provide the service outlined above. In addition
it could be used in a similar role between infantry & artillery when space/weight
requirements were limited. With the addition of some minor components,it
also was found useful in several other configurations. The most overlooked
was it's Back Pack setup as the PRC-16. With the addition of a CY-590 battery
box,& associated antenna accessories. Here the RT-70 is mounted with it's
ancillary equipment to a standard pack board. Used in this manor it could
& did serve in several capacities unsuited to any other system. #1. as a
backpack set for artillery spotting. #2. as a manpack system providing a
very light weight portable ground station. #3. as a stand alone vehicular
radio(with the AM-65). #4. & most notable is it's installation with the
CY-590 battery box in light aircraft for Artillery spotter & observation
duties. In the above listed roles,the RT-70 had the distinct following advantages
over it's cousin set,the PRC-10 which was used to a greater extent in similar
fashions. #1, though it was heavier,& didn't cover as wide a frequency range,the
RT-70 had the advantages of greater frequency stability,longer operational
time by virtue of it's larger batteries,two presetable frequencies,a wider
selection of antenna types with better compatability(with it's integral
antenna trimmer),& a much broader selection of optional power sources. #2.
In the Manpack role,the RT-70 combined those assets noted in #1,with it's
ability to be powered by a hand crank generator. #3.As a vehicular radio,the
RT-70 with its companion power supply(AM-65). It's advantage of smaller
size could be combine with it's ability to be installed in vehicles having
6,12,or 24vdc electrical systems. In comparison the PRC-10 with it's AM-598
power supply/audio amp,was larger & limited to 24vdc operation. In addition
it's AM-65 supply also provided multi channel audio amplification for either
accessary radios,or interphone operation in high noise/crew served vehicles.
The PRC-10/AM-598 had no such provision. #4,when used in an aircraft all
those advantages noted in #1 come into play with an emphases on the dual
present freqs & longer battery life. We might stray here for a moment to
note that it was common practice to strap backpack radios in light aircraft
for the noted uses. This beginning with the introduction of prototype SCR-194's
& 195's in late 1935. Their success was not impressive until the advent
of the BC-620 & 659. Later the SCR-300 replaced the earlier sets. All this
because early aircraft of this type did not have electrical systems of their
own,thus the radio needed be supplied by it's independent source(it's battery).
On the subject of the longer battery life advantage. It must be noted that
front line tactical radio equipment in normal routine operation,is not operated
continually while it's personnel are deployed. Radio's in service with a
patrol for instanse,would not be turned on when they deployed,then turned
off upon their return. Rather the operational communications procedures
required pre-determined check in times. Or the radio might be used to call
for unexpected assistance or coordinate actual combat activities. Also the
absence of one of these pre-determined check in's would alert higher echelon
to trouble. Thus normally operated PRC type radio's did not need be operational
for several hours at a time. Where as,in the application noted above,prolonged
operation was always a necessity. To conclude,some notes for the collector/operator.
The RT-70 is one of the best & simplest radio's to restore,repair,or get
into operation. Unlike many portable set's with numerous voltages & multi
celled bastard batteries,the RT-70 requires only 6 & 90 vdc. It's one of
the most common & affordable surplus radios available(for the present).
It's advantages over the PRC-10 family are numerous. & never under estimate
it's mere 500mw output. Until all the old farts I once talked to in the
area,died off. It was an every day thing to communicated with them over
15-20 mile paths using a stock RT-70 & 1/4 wave ground plane. So get you
one,beginner or expert,put it on the air & use it! Dennis Starks; MILITARY
RADIO COLLECTOR/HISTORIAN STARKS ELECTRONICS,wholesale supplier of used
communications equipment. -------------------------------------------------------------------------------------------
Dennis, What bits do I need apart from the battery box to make the RT-70
into a PRC-16? Thanks, ______________________________________________________
John A. Kidd Collector: military radio. Tullamarine, 3043. Interests: military
aircraft Victoria. AUSTRALIA & vehicles, Chrysler cars. JOHN A COMPLETED
PRC-16 CONSIST OF; RT-70 CY-590,BATTERY BOX G-8,HAND CRANK GENERATOR(OPTIONAL)
H-33,HANDSET CX-1209,POWER CABLE(THIS WILL MOST LIKELY NEED BE MADE) MT-673,
MOUNT(OPTIONAL) MT-652,ANTENNA BASE AB-22,& AB-24,MAST SECTIONS TM11-288,MANUAL
THE MT-652 IS SECURED IN THE HOLES LOCATED ON THE TOP OF THE FRONT PANEL
OF ALL RT-70,66,67,68,ETC. THE AB-22,24(OR ANY OTHER OF THIS FAMILY OF ANTENNAS),ARE
THEN SCREWED INTO THE MT-652. THE ORIGINAL BATTERIES USED WERE,5ea BA-403
1.5vdc,& 1ea BA-419 90vdc. THE MT-673 COULD BE MOUNTED TO A STANDARD PACK
BOARD,TWO WHERE REQUIRED FOR SIDE BY SIDE MOUNTING. ONE FOR VERTICAL MOUNTING.
THE STANDARD DOGBONE CONNECTOR WILL NOT WORK BETWEEN THE RT-70 & CY-590.
IT'S NOT LONG ENOUGH. *****************************************************************
TRC-77,FINE RADIO,WRONG TIME. Purpose; The TRC-77 was specifically designed
for use by Special Forces troops in Vietnam. However the timing was not
good for a very distrusting group of people that had been flooded with all
manor of new fangled contraptions. Most of which did not work, or it least
wouldn't for several years to come. And they were not, understandably, willing
to risk their lives any longer as guinea pigs. Perhaps some explanation
is needed here. Vietnam saw the mass development of thousands of high tech
gizmozes. At the same time three generations of radio equipment & types
of technology were in use,some dating from 1942. It's now 1963,the transistor
is on the seen(though still a little unreliable)with scientist & engineers
dieing to use them in practical applications. U.S. Special Forces teams
have traditionally been our test bed for new simi-experemental equipment
of all conceivable types. By way of radios they had already been supplied
with GRC-9's,GRC-109's.PRC-64's,Hughes HC-161(later to become the PRC-74),&
a host of many other types. Thus they had been plagued with a logistic system
that was trying to support both these & the multitude of other radios mentioned,&
couldn't. Special batteries could not be readily replaced, & for the most
part,these new fangled contraptions could not be powered in any other way.
Also field service was impossible as replacement parts didn't exist. These
problems & others led to immediate refusal to even try the TRC-77. As a
result,it was doomed before it ever saw the light of day. A special note
here; the above problems were in fact so acute, that by 1972 many Special
forces groups were discarding their new solid state equipment in favor of
the older, field serviceable, earlier to maintain, & support, tube type
equipment, which also featured a multitude of power supply options. The
poor TRC-77 was then relegated to large scale storage in a warehouse,further
contaminating the fragile Supply/Logistics system. Then one day the South
Vietnamese government, having a wish to build a special operations "Commando"
group, requested radios suitable for their needs. The Signal Corps responded
with,boy do we have the radio for you. So the poor TRC-77 did get to see
some service,& this is the only known instance to date. Description; The
TRC-77 is a manpack transmitter-receiver,about the size of a PRC-25. It
operates from 3 to 8mc with an output power of about 10-14 watts. While
it is capable of reception of AM,SSB,& CW signals,CW is it's only mode of
transmission. Channel selection is via two six position channel selector
switches,one for transmitter the other for the receiver. Both transmit &
receive frequencies are crystal control,& the set has the ability to use
several different types of crystals. The receiver is all solid state while
the transmitter is a simple two tube type with 2E26 output. Primary power
was supplied by it's 12vdc battery. Features;The radio was provided with
a built in antenna tuner that allowed great leeway in antenna selection.
This consist basically of a multi taped toroid coil with an pilot light
output indicator. Primarily designed to use wire type antennas,the manual
specifically demonstrates what they refer to as a "crows foot". Though we
call it a Backpack Radio,it would better be described as a Manpack type.
This because,the radio was not designed to be operable from the users back.
Rather, is was necessary to remove the radio & set it up for ground operation.
Anybody looking inside a TRC-77 will immediately be impressed with the extreme
quality of construction. Each set appears to have been completely hand built,
with all serial numbers encountered being below 800 this is entirely possible.
Those familiar with experimental radios, will at once see this resemblance.
All radios appear to have been built by ARVIN which is the same company
as Crosely of WW-II fame. They being responsible also for such radios as
the PRC-1,5, BC-654,1306,& GRC-9. Most TRC-77's encountered today will be
in an unused condition, or even new in the box(I've opened several of them
myself). Anyone wishing to know more about the above, radio development
in general, & a host of other very interesting topics, is strongly advised
to read,Military Communications,A TEST FOR TECHNOLOGY,The U.S.Army in Vietnam,by
John D. Bergen,printed by the Center of Military History,U.S Army, & available
from the government printing office(CMH-Pub 91-12). To close, notice that
TRC designation on this radio, this is one we usually associate with extremely
large, multi truck transport types. In fact this radio is one of four known
TRC types that were field portable & intended for use by Military Intelligence.
I ask that anyone having more historical information about this radio &
it's use,please let us all know. I thank you all for interest in preserving
our history as it is represented by this equipment. Dennis Starks Military
Radio Collector /Historian *****************************************************************
ANOTHER CLUE TO THE TRC-77's USE; From Steve Hill BTW, I spoke to the guy
who said the TRC-77 was used by the Aus army. He didn't really have any
more info, but said he saw them in USED condition in disposal stores. Thats
all he knew. Steve Hill VK4CZT |
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