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Kachina MP-25
Kachina MP-25
Scientific Radio Systems SR-MP-25
HF SSB Manpack Transceiver
2 MHz to 15 MHz

Information Specifications
Operating Manual Service Manual - Mods

Scientific Radio Systems SR-MP-25
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14342.5 kHz USB
14346.0 kHz USB
18157.5 kHz USB
18117.5 kHz USB
NVIS / Evening
5371.5kHz USB
5403.5kHz USB
5332.0kHz CW
VHF FM Simplex
144.300 MHz
PL tone=151.4
51.000 MHz
PL tone=151.4

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This article provides educational and reference information on the
Kachina MP-25 and Scientific Radio Systems SR-MP-25 HF SSB Manpack Transceiver
[Note: Kachina Communications, Inc. made the MP-25. Kachina Communications, Inc. closed its HF equipment business in May 2001.]
[Under private label, Kachina Communications supplied SR-MP-25 to Scientific Radio Systems, Inc. which went out of business in Sept 1997.]

[A typical MP-25 set sold new for approximately US$5000 to US$8000, depending upon which accessories and options were in the package.]

The Kachina MP-25 is a manpack type transceiver
which may be used for portable, pedestrian mobile, vehicle mobile, or fixed installations. The transceiver is designed to receive and transmit SSB single sideband suppressed carrier (2R7J3E), semi-break-in CW telegraphy (350A1A), and compatible amplitude modulation (2R7A3E) signals. AM is received as USB.
Frequency coverage is 2000.0 kHz to 14999.9 kHz in 0.1 kHz digital steps using front panel rotary switches. The MP-25 is designed to operate over the temperature range -30C to +55C and is fully waterproof to 1 meter. The transceiver features an internal simplified manual antenna tuner with a locking rotary whip antenna mount and standard 3/8"-24 threaded fittings. Antenna connections include push-on posts for random wire attachment and BNC connector for mobile or dipole antennas. The front panel has a waterproof speaker with on/off switch for monitoring. A companion 12V power supply / battery charger for base station operation also provides an external speaker for clear crisp reception of voice.

Operation and Maintenance Manual (Service Manual)
Kachina MP-25
Warning: 20 MegaByte
pdf file!

Kachina MP-25
Scientific Radio Systems SR-MP-25
HF SSB Manpack Transceiver
Operating Manual and Maintenance Manual (Service Manual)
828012 revision 8/95 (1995)
with diagrams, charts, alignment, troubleshooting, calibration, schematics, parts lists, specifications, voltage charts, images
Super Antenna MP1 - World #1 HF Portable Vertical

MP-25 Standard Features
Digital frequency selection with tactile rotary switch knobs
Built-in antenna tuner for whip, random wire, or dipole
Integral rechargeable battery pack (NiCd or SLA)
Long term operation on battery due to low receive power consumption <200mA
Multi-mode operation - USB, LSB, compatible AM, semi break-in CW, compatible with external modems 
High/low power selector (25 Watt / 6 Watt transmitter output)
Internal waterproof front panel speaker
Uses standard military manpack accessories and audio U-229 connectors
Integral swivel antenna mount for whip antenna
Audible low battery indicator when pack needs recharging
Dependable hardware logic ICs - no vulnerable software or microprocessors
Uses standard 12 VDC for vehicle power and internal battery
Battery voltage, S-meter, and SWR meter

Promotional Sales Brochure (for reference only)
Kachina MP-25
Kachina MP-25 Brochure

(2MB pdf file)

Promotional pamphlet with accessories, configurations, options, pricing, maintenance packages, and other factory sales information.

[Kachina closed its HF business in 2001]



Super Antenna MP1 - World #1 HF Portable Vertical


The following accessories were available from the factory:
MP-25-HS2 Handset [a]- H-189/GR miniature military handset. Fully Waterproof. Dynamic, 150 ohm
impedance.Weight .51 kg.

MP-25-HG Hand Generator [h]- Designed to operate the MP-25 Transceiver and simultaneously charge the batteries. Batteries must be in place when operating the generator. Weight 2.0 kg.

MP-25-CB Carrying Bag [e]- Manufactured of nylon, the bag not only protects the transceiver, but, also provides pockets for the various accessories such as the microphone, folded whip antenna and other antenna accessories. Fitted with adjustable padded straps. Available in olive drab or green camouflage pattern. Weight 0.63 kg.

MP-25-AD Dipole Antenna [c]- This center fed dipole is wound on high impact plastic spools. A calibration chart is provided to show turns to be removed from the spools. Final adjustment is made using the MP-25 built-in SWR bridge. Antenna is provided with 10 meters of RG58-U coaxial cable. The dipole antenna may also be used as a longwire antenna. One section of the antenna is used as a ground plane. Weight 1.3 kg.

KC-SP-1 Solar Power Unit [i]- Allows transceiver batteries to be charged from the sun. Fitted with meter so that the collector may be positioned for maximum charge Limited availability. Weight 1.94 kg.

MP-25-PSC Power Supply/Charger [g]- Powers the radio in base station operation or charges one battery pack, either sealed lead acid or nickel cadmium. A front panel switch converts the base station power supply to a battery charger allowing the battery to be charged while attached to the transceiver. The supply allows operation from 110/220 volts, 50 or 60 Hz. A fuse and current limiting cut-out protect the power supply against short circuits. The supply is electronically regulated and current limited, and the output is over voltage protected. Contains built-in loud speaker for improved audio during base station operation of MP-25 Transceiver. Weight 3.65 kg.

MP-25-AW Folding Whip Antenna [b]- Made of fiberglass. Collapsible and may be stored in carrying bag. Connects automatically when fitted to transceiver. Antenna may be rotated to any angle with respect to the transceiver and locked in place. Weight 0.39 kg. 

MP-25-MB Mobile Mounting Bracket [f]- Heavy duty type, clamps around body of transceiver, allowing a strong attachment to the vehicle. Weight 1.12 kg.

MP-25-MC Hand Microphone [j]- Military type M-80/U, rugged, dynamic type microphone. Used when transceiver is operated with the MP-25-PSC power supply speaker in base station operation. Weight 0.39 kg.

Kachina MP-25
MP-25-BP(NCE) Nickel Cadmium Battery Pack [d]- Clips on the rear of transceiver. If nickel cadmium batteries are not fully discharged they will develop a memory for the lower charge. Unlike sealed lead acid batteries, they should not be trickle charged. Their regulation is superior to that of the lead acid types.

MP-25-BP(LAE) Sealed Lead Acid Battery Pack [d]- Consists of two 6 amp hour, 6 volt, rechargeable, sealed, lead acid batteries in protective box. Clips on to rear of transceiver. Unlike nickel cadmium batteries, sealed lead acid batteries need not be deep discharged. Regulation inferior to nickel cadmium type.

MP-25-CW Telegraph Key - May be positioned on senders knee or operated on a table or other surface.

MP-25-HP1 Headphones - Military type H140/BU. Weight .69 kg.

PN980014 Mobile D.C. Power Cord - Allows the transceiver to be operated from a vehicle battery.

MP-25-MW1 Mobile Whip Antenna - Stainless steel antenna approximately 2.5 meters long (9 Ft.). Weight 2.2 kg

MP-25-BL Ball Mount - For attaching the MP-25-A Wl Mobile Whip Antenna to the vehicle.

MP-25-S2 Heavy Duty Spring - designed to be used with the MP-25-BL Ball Mount and the MP-25-A Wl
Mobile Whip antenna. Helps prevent antenna damage.

MP-25-CH Battery Charger [g]- Similar to MP-25-PSC(E) above but allows the simultaneous charging of 4 battery packs, either sealed lead acid or nickel cadmium. Will operate from 110/220 volts, 50 or 60 Hz.

PN980013 Extender Board Kit- A set of extender boards that allow printed circuit modules to be extended above the transceiver for maintenance and service.

Power Source: External 12 volt supply, negative ground only.
Frequency Range: 2000.0 kHz - 14999.9 kHz in 100 Hz steps USB LSB CW AM 
Modes: 2R7A1A (USB & LSB), 350A1A (CW), 2R7A3E (compatible AM).
Operating Temperature Range: -30 C to +55 C.
Waterproofing: Immersion to 1 meter (20 C).
Size: (W X DX H) 32 X 34 X 11.5 cm.
Weight, without batteries: 4.3 kg (9.6 lbs.).
Weight, with sealed lead acid battery pack: 7.25 kg (16.1 lbs).
Weight, with nickel cadmium battery pack: 7.29 kg (16.2 lbs.).
Metering: Relative received signal strength. Relative transmitter power output. Reflected power (TUNE mode). Battery voltage.
Antenna Tuner: Built-in. Capable of matching to whip or longwire antennas from 2 - 150 meters in length. Special provision to match mobile antenna.
Lighting: Spring loaded switch activates readout display for approximately 10 seconds. Meter illuminated while switch held on. Meter illuminated when mode switch in TUNE position.
Memory: Frequency change may be entered only when readout display is illuminated. Last four digits of second frequency may be switch programmed in readiness for a change. Will not activate until READ switch is activated.
Out-of-Lock Indicator: Audio tone warns if synthesizer out-of-lock, such as when large changes in frequency are made or battery needs charging.
The MP-25(E) battery pack clips on the transceiver without having to withdraw the transceiver from the case.
The MP-25 uses either rechargeable sealed lead acid or nickel cadmium battery packs.
Optional connecting leads allow the transceiver to operate from an external 12 volt DC source.
Signal to noise: Better than 10dB for 0.5uV input signal.
Sensitivity: More than 0.125 watts output for 0.5uV input signal.
AGC: Less than 10dB variation in audio output from 5uV to 0.1 V input signal.
IF Rejection: 2 - 9.9 MHz Better than -90dB ;  9.9 - 15 MHz Better than -60dB
Image Rejection: Better than -90dB.
Internal Spurious Signals: Not greater than the equivalent of .5uV except at 5 MHz= 3uV.
Audio Output: I watt minimum with not more than I 0% distortion.
Clarifier: +/-50 Hz minimum.
Current drain (readout display off, volume low): Typically 170mA.
Current drain (readout display on, volume low): Typically 200mA.
Current drain (readout display off, volume normal listening level): Typically 185mA.
Note: All measurements based upon 12.6 volt D. C. input voltage. 
Power Output: SSB - 25W P-P average. 20W minimum. (low power 6W P-P average)
CW - 10W (internally adjustable).
AM - 6W (carrier).
Intermodulation distortion: Not less than -26dB at 20 watts. Typically -30dB at 25 watts.
Spurious Output: -50dB with reference to 25W level.
Harmonic Suppression (50 ohm output): -40dB.
Carrier Suppression: -40dB minimum.
Frequency Stability: +/-100 Hz from -10C to +50C ; +/-150 Hz from -30C to +55C
ALC: Limits output power to within 1dB of setting.
Current Drain:  Average voice: 1 ~ 2 amps. /  CW (Key down): 4 amps. / CW (Average): 1 ~ 2 amps. / AM: 1.5 amps.
Microphone: H189/GR or equivalent.
Note: All measurements based upon 12.6 volt D. C. input voltage. 

[Editor's note: the above information was provided by the Kachina MP-25 Operation & Maintenance Manual]

Scientific Radio Systems SR-MP-25 transceiver, SR-MP-25-PSC power supply, J-38-style key, M-80 microphone, Super Antenna MP1 SuperWhip ruggedized manpack antenna.

After these types of HF radios are removed from commercial and military service, some find their way into the surplus market, to be eventually acquired by ham radio operators, shortwave listeners, or radio collectors.

The MP-25 transceiver is designed for short, medium and long path communications work. The transceiver will operate from a whip, longwire or dipole antenna. A built-in antenna coupler allows proper coupling to the whip and longwire antennas. Typical system application is given below. [source: Kachina]
Manpack MP-25-BP Battery Pack Whip Infantry Patrol
Portable MP-25-BP Battery Pack Whip, Longwire Field Stations
Mobile Vehicle Battery Whip Moving or Stationary Comms.
Base MP-25-PSC(E) Power Supply Dipole, Longwire Normal Base Station Service

Kachina MP-25 Images

KACHINA_MP_25_HF_SSB_MANPACK with power supply
Kachina MP-25 second version transceiver with second version MP-25-CH power supply charger (photo courtesy of teotwaki)

Photo: Kachina MP-25 top view showing second version battery pack and enclosure  (photo courtesy of teotwaki)

Photo: Kachina MP-25 whip antenna  (photo courtesy of teotwaki)

KACHINA_MP_25_HF_SSB_MANPACK 2nd Version Battery Pack
Photo: Kachina MP-25 second version battery pack  (photo courtesy of teotwaki)

KACHINA_MP_25_HF_SSB_MANPACK 1st Version Battery Connection KACHINA_MP_25_HF_SSB_MANPACK 1st Version Battery Pack
Photo: Kachina MP-25 first version battery connections   -   Kachina MP-25 first version battery pack

KACHINA_MP_25_HF_SSB_MANPACK Tuning Instructions Card
Photo: Kachina MP-25 Tuning Instructions card   (photo courtesy of teotwaki)

KACHINA_MP_25_HF_SSB_MANPACK Tuning Instructions Card
Photo: Kachina MP-25 Antenna Instructions card   (photo courtesy of teotwaki)

Antenna Tuner Operation
The internal antenna tuner is capable of tuning and matching any HF antenna of any type encountered in the field or base station operations. In the Tune mode, the transmitter transmits a low power level CW carrier for tuning purposes. The Antenna Tuner is adjusted using the Tune mode by tuning ANT.TUNE lower knob (capacitor) and ANT.TUNE top rotary switch (inductor) for lowest reading on the meter, indicated by the green color. If SWR is in the red, check antenna or tuning.
Kachina MP-25 SWR Tune Meter Color Code

1.0 to 2.0  
2.0 to 3.0
 3.0 or higher  
Note: SWR levels are approximate

Procedure for Tuning a Whip or Random Wire Antenna
1. Set the ANT.TUNE lower knob to the 2 position.
2. Set the ANT.TUNE top rotary switch to the 22 position.
3. Set the PORT/MOB switch to the PORT position.
4. Switch to TUNE mode.
5. Rotate the ANT.TUNE top rotary switch counter clockwise, stopping at the lowest reading on the SWR meter.
6. Rotate the ANT.TUNE lower knob, stopping at the lowest reading on the SWR meter.
7. Repeat steps 4 and 5 above sequentially for lowest reading on the SWR meter.
8. Return to USB mode.

Bypass the Antenna Tuner
For use with 50 ohm BNC external antenna such as resonant dipole or broadband dipole.
1. Set the ANT.TUNE lower knob fully counter-clockwise to the 0 position.
2. Set the ANT.TUNE top rotary switch to the 23 bottom vertical line position.
3. Set the PORT/MOB switch to the PORT position.
(see photo at right)

PORT./MOB. Switch
The PORT/MOB switch should normally start in the PORT position for most antennas. This includes 50 ohm antennas, dipoles, random wires, and the manpack whip antenna. It may be put in the MOB position for tuning some mobile antennas or fixed station antennas which are connected via the DIPOLE connector (BNC) or it may be needed to achieve an acceptable green SWR with some other types of antennas at lower frequencies (2MHz to 8MHz). In the MOB position, an additional shunt capacitor of 750 pF is applied to the PI-network antenna tuner on the transmitter side, which is especially helpful for tuning the lowest frequencies, but MOB position is usually detrimental to tuning at higher frequencies (10MHz~14MHz).
For more information see Antenna Tuner Instructions Cards

Whip Antenna Mount
The rotating whip antenna mount on the right side of the faceplate is suitable for lightweight whip antennas in field portable operation. When using a mobile or fixed antenna connected to the DIPOLE (BNC) connector or the antenna push terminals, the whip antenna and its hardware should be completely removed from the mount, to avoid RFI, noise problems, and detuning of the intended external antenna system.

Using Whip Antennas
The Kachina model MP-25-AW Folding Whip Antenna is a lightweight collapsible fiberglass whip, originally supplied with the MP-25. The standard military manpack metal AT-271 collapsible whip was supplied with later versions, along with 3/8"-24 fittings which interlock with the quick-disconnect fitting of the mount. Care should be taken to avoid extreme angular shock or leverage on the whip mount while operating /Pedestrian Mobile with heavy or long whips such as the AT-271. After-market antennas with integral loading coils, such as the Super Antennas MP-1 provide greatly increased efficiency (+5dB or more) over the stock whip, and are highly recommended for use with the MP-25. When operating portable or /Pedestrian Mobile with any whip antenna, a single insulated wire counterpoise should be attached to the push terminal ground connection of the MP-25. The counterpoise wire may be a random length or may be cut somewhat shorter than a quarter wavelength at the desired optimum frequency. In practice, 4 lengths of counterpoise wires have been found to be suitably optimum for the entire operating range of the MP-25, when most of the wire is deployed on the surface of earth ground soil.

Recommended MP-25 field counterpoise wire length
2MHz to 3.4MHz
80ft (~25m)
3.5MHz to 5.9MHz
45ft (~14m)
6MHz to 8.9MHz 30ft (~9m)
9MHz to 14.9MHz
10ft (~3m) 

Super Antennas provides after-market pre-tuned teflon wire portable counterpoise radial sets which are suitable for MP-25 and other manpacks in the field. See SuperWire Stealth Radial Sets MR4020, MR8060, MR4010, MR8075.

Kachina MP-25 Antenna Tuner
Settings for 50 ohm
external antenna

Audio Connector Pinout
Microphone, Speaker, Handset, Headset, CW Key
The audio and accessories connector is a U-283 (the 6 pin version of the common 5 pin U183 military audio panel connector) and it is compatible with most 5 pin standard U-229 microphones, handsets, headsets, speakers, etc. The MP-25 does not output microphone sidetone to the handset speaker audio.

Modems, Devices, Repeaters, and +12V Accessories
A variety of interconnect devices may be used to expand or interface with the transceiver. When setting up equipment, observe proper compatibility procedures, especially pin signals and voltage limits for gear using center pin features. The +12V pin is capable of delivering several amperes, limited by the internal circuit breaker, dependent upon how much total current is used for transmit.
If attaching an accessory that has a U-329 plug (6 pin military connector)
beware that there is +12VDC on center pin F.
The center pin is direct +DC supply from the radio battery and is controlled by the radio on switch.
DO NOT CONNECT a 6 pin MUX or FILL device to the MP-25!

KACHINA_MP_25_HF_SSB_MANPACK U283 U-229 Audio Connector Pinout CW 12V
Pin out of Kachina MP-25 front panel
audio connectors type U-283
(compatible with U-229 or U-329 accessories)

Compatible Microphones
Standard military H-189 and H-250 handsets, and M-80 microphones are compatible with the MP-25. The M-80 seems to have the most punch for excellent transmit audio. Many other types of standard military audio U-229  accessories, such as intercoms, handsets, and headsets work well with the MP-25. The microphone input circuit has an impedance of approximately 1k ohm, and is suitable for low impedance dynamic microphones and audio equipment in the range of 50 ohms to 1 k ohms (50, 150, 600, 1k, etc). Both front panel audio connectors are identically connected in parallel and may be used simultaneously.

Compatible U229 Microphones Handsets H-250 H-189 M-80 U-229 Connectors
Photo: Typical MP-25 compatible handsets H-250, H189, and M-80 microphone using standard military U-229 connector

CW Operation
A manual CW key may be attached, connected from pin E to pin A of the U-229 (or U-329) connector. The CW frequency is offset +800Hz (0.8kHz) above the display frequency. In CW mode, the radio operates Upper Sideband, and is suitable for crossmode operation with stations using Upper Sideband (USB) voice. To zero beat with another CW station, align the display frequency to match the received tone of the CW sidetone pitch. In other words, when the other station's CW signal sounds like it is at the same pitch as the MP-25 CW sidetone, it is the correct zero beat frequency. There is no special narrow CW filter, it uses the standard USB 2.7 kHz filter. The logic level HI of the CW pin is +12V, and ground to send. If using a bug key or an external electronic keyer, be advised that the CW circuitry is rated at 35 WPM morse code maximum, and the transmitted dits may be clipped if that speed is exceeded. The default keying is semi-break-in, which has a fairly fast fixed time delay. It is normal to encounter some chirp on the transmitted CW when using semi-break-in at each initial key-down pulse interval. To reduce the chirp, hold down the microphone PTT while sending a CW transmission. Microphone audio is muted from the transmitter when mode switch is in the CW position.

CW key wiring of U-229 female cable connector pin out

CW transmit frequency is offset 0.8kHz above display frequency.
Examples of Kachina MP-25 display frequency to CW zero beat conversion for common ham radio and HFpack CW calling frequencies

5331.2 5332.0
14058.2 14059.0

Photo: Typical CW key with U-229 connector

External Modem
A computer sound device or external modem with audio and PTT interface may be attached to the front panel connectors. When transmitting with a modem, the BAT.READ/LOW PWR switch should be placed in the LOW PWR position, to avoid overheating due to continuous duty power output. The following diagram shows connection of a computer sound device modem to computer earphone output, microphone input, and a simple PTT interface to a serial port made with a transistor and 2 resistors. Most standard computer-radio interfaces may be wired similarly. The normal logic level of the PTT pin is +12V, and ground 0V to transmit. It is suitable to be driven by either open collector or relay closure. It is not suitable for direct 5V, 3V, or 1.8V logic connection (use an open collector buffer instead).

Image: Connection schematic diagram of U-229 female cable connector for computer sound device modem

External Speaker and Power Supply / Charger
The power supply and charger MP-25-PSC (first version) or MP-25-CH (second version) has a good sounding external speaker in it. It is a linear supply providing OPERATION for the transceiver and CHARGE for the battery. The power supply plugs into the front panel J903 connector, and provides +12VDC nominal from selectable 115 VAC or 220 VAC mains.

It is recommended that the battery be either removed from the radio while charging, or else all the panel clamps be loosened, to prevent volatile gas (hydrogen / oxygen) build-up inside the sealed radio enclosure. The power supply OPERATE/CHARGE switch must never be in the OPERATE position while the battery is connected in the radio. The battery may be charged while installed in the radio. Charging Battery Reminder: always loosen the 6 clamps of the manpack case cover while charging, and slightly let the cover separate from the front panel gasket, to release the hydrogen and oxygen gas. The OPERATE/CHARGE switch must only be set to CHARGE while the battery is installed and attached in the radio. There is a separate charge cable which may be plugged into the battery for charging it externally while the radio is operated. For the NiCad battery, the charger power should be turned off when the battery is fully charged (about 13.8 to 14.0 VDC). Keep in mind that the front panel volt meter is intended only for a rough check of battery voltage. The front panel meter is not accurately calibrated enough to determine full charge voltage. So, a separate calibrated voltmeter should be attached to Pin F of the audio connector to test the voltage, and determine meter error so that it can better be utilized as a charge indicator.

The SPEAKER/ON switch on the front panel of the transceiver controls both the internal speaker and the power supply speaker, it does not mute the receive audio ported through the front panel audio connectors. Other types of military loudspeakers with standard U-229 connectors may be additionally attached to either front panel audio connector.

SCIENTIFIC_RADIO_SYSTEMS_SR_MP_25_PSC Power Supply Charger LS-454 Speaker
Photo: Kachina MP-25-PSC Power Supply / Charger Speaker first version (Scientific Radio Systems SR-MP-25-PSC) and compatible LS-454 speaker

KACHINA MP25 Front Panel Power Supply Charger Connector J903
Photo: Front panel power connector J903 pinout (A=Ground | B=+12VDC | C=Speaker)
Power Connector Plug Family Type = MIL-DTL-26482 (shell size 8, standard insert, 3 pin, cylindrical connector)
Radio Panel Power Female Connector = Type MS3114 (Jam Nut). Amphenol Part # SP07A-8-3S or PT07A-8-3S

KACHINA MP25 Front Panel Power Supply PT06A_8_3P_Connector.png
Supply Cable Power Male Connector = Amphenol Part # SP06A-8-3P or PT06A-8-3P or SP06A-8-3P(SR)
Power Connector Plug Family Type = MIL-DTL-26482 (shell size 8, standard insert, 3 pin, circular connector)

Kachina MP-25 Timer Mod
The frequency display timer on the stock MP-25 is about 15 seconds, and it is a little too quick for most users to tune in a station. This modification increases the display illumination and setting time to about 35 seconds.

Kachina MP-25 Display Timer Mod Instructions and Diagrams
This mod requires one part: a 4.7uF capacitor.
The capacitor may be electrolytic, tantalum, or non-polarized rated at +9VDC.
These instructions show a surface mount capacitor.

1. Remove enclosure, battery, and internal top cover. Remove internal shield plate which covers module PC501.
2. Extract PC501 module board. Pry it out evenly of the socket by levering a small screwdriver into the holes on each edge.
3. Locate C514.
4. Solder the 4.7uF capacitor across the exposed leads of C514.
5. Replace the module, plate, cover, and enclosure.
6. Mod is complete. Test it by turning on the radio and observing the display illumination time.

Image: Kachina MP-25 Display Timer Mod Instructions and Photos

KACHINA_MP_25_HF_SSB_MANPACK Timer Mod Schematic
Image: Schematic of Kachina MP-25 Display Timer Mod

Kachina MP-25 Output Power Mod
The MP-25 amplifier board final transistors are capable of much more than 25 Watts output power (each of the two transistors is rated at 85 watts). By design, the power output of the transceiver is ALC-limited to 25 Watts, in an effort to make it impervious to SWR problems encountered in rugged field conditions, and to limit the current for longer battery life. However, for amateur radio use, if the operator is very careful to observe SWR conditions, it is possible to turn up the output power somewhat, especially for SSB voice use. Exactly how much power, is a balance between the desire for increased transmit signal, and the possible damage which may occur to the amplifier's broadband transformer, manual antenna tuner, output filters, and other output components, etc. In amateur radio practice, it has been found experimentally that it is possible to limit the SSB voice output to about 40 Watts, without damage (if the operator is careful). There are a few caveats however: (1) the CW injection, which determines the CW output power, should be set to 6 Watts, (2) AM carrier level should be set to 5 Watts, and (3) any digital external modem level must be carefully limited to below 6 Watts continuous power by careful adjustment of the audio drive (the BAT.READ/LOW PWR. switch should be in the LOW PWR position for modem operation).
Warning: Do this mod at your own risk!
This modification should not be done if there is ANY chance that the the radio may be deployed by operator(s) who are not familiar with the nature of these cautions.

Image: Power output ALC setting: C24
Kachina MP25 Power Output Mod Procedure
This mod assumes that all other alignments of the transceiver are at recommended levels according to specifications detailed in the Maintenance Manual, prior to beginning the modification procedure. The mod increases the SSB power output to approximately 40 Watts Peak, while keeping the CW level at 6 Watts and AM level at 5 Watts.
1. Disconnect the battery pack. Remove the outer enclosure, top cover, and whip antenna.
2. Connect an RF peak power meter and 50 ohm dummy load to the BNC antenna connector
3. Connect base station power supply or external supply voltage set to +13.8VDC
4. Set the antenna tune lower knob all the way to the left 0 position
5. Set the antenna tune top rotary switch to the 23 bottom vertical line position, which bypasses the antenna tuner
6. Attach a suitable microphone and a CW key
7. Turn on the transceiver and set frequency to 7296.0 kHz USB. Set the BAT.READ/LOW PWR switch to center position
8. Locate C24 the ALC adjustment, and make ready a suitable insulated tool for adjustment of it
9. Press the PTT and whistle into the microphone, observing the power output
10. Adjust C24 for the desired power output (Do not to exceed 40 Watts Peak)
11. Locate R240 the CW Level adjustment, and make ready a suitable insulated tool for adjustment of it
12. Switch the transceiver mode switch to CW, and hold CW key down continuously while observing the power output
13. Adjust R240 for the desired power output (Do not to exceed 6 Watts average)
14. Locate R259 the AM Level adjustment, and make ready a suitable insulated tool for adjustment of it
15. Switch transceiver mode switch to AM, press and hold PTT while silently observing the carrier level power output
16. Adjust R240 for the desired carrier level power output (Do not to exceed 5 Watts carrier level average)
17. Re-assemble the transceiver modules, covers, and enclosures.
18. Modification is complete.

Image: Kachina MP-25 power output mod, PC201 module, adjustment of CW Level and AM carrier level.

KACHINA_MP_25_HF_SSB_MANPACK Final Amplifier Board
Photo: Kachina MP-25 final amplifier board   (photo courtesy of teotwaki)

Replacing or Rebuilding the Kachina MP-25 Battery Pack
The Kachina MP-25 battery pack is out of production, so there is a need to find a replacement alternative. The recommended solution is to find a source for the new cells for the internal battery pack and rebuild it. A Gel Cell or Sealed Lead Acid (SLA) type battery can be easily replaced as an internal unit. But, for the NiCd battery, it takes 10 cells of size D, with solder tabs, combined into a pack of a specific form factor necessary to fit in the enclosure. The new NiCd cells can be purchased separately and then soldered, taped, and/or bound by shrink-tubing into the battery enclosure. The form factor is 7 cells in a row, with 3 cells end to end along the top. There are 2 different MP-25 versions of battery pack holders. The First Version is an internal battery with an aluminum frame and vacuum-formed plastic clam-shell enclosure. The following photos and information are provided as an example of rebuilding the MP-25 first version Kachina MP-25-BP (NC) battery also known as SR-MP-25-BP (NC), with a commercially available battery pack core of the proper form factor and configuration.

Disassembly of the battery pack aluminum frame. Remove the connector screws.

Slide battery enclosure out of aluminum frame.

Battery pack aluminum frame, clam-shell plastic enclosure, connector, new battery pack, old battery pack. 10 NiCad D cells soldered in series.

Connector soldered on to the wires of the new battery pack. Battery pack (TRW) connector pinout: Pin 1 is negative (ground) wide pin. Pin 2 is 12V+ positive (hot) thin pin. Re-assemble the clam-shell with Gorilla Tape, but do not completely seal the seam between the clam-shell halves. Keep a gap at the 4 corners of the clam-shell to release hydrogen and oxygen gas that is emitted while charging. Re-assemble the enclosure into the aluminum frame and re-attach the connector screws to clamp the battery pack into the frame.

BatterySpace Battery for Kachina MP-25 Manpack 12V 5Ah NiCd Product ID 6535 ProductNumber PR-CU-R252
The battery pack core utilized in this SR-MP-25-BP (NC) NiCad rebuild was sourced from batteryspace.com (AA Portable Power Corp) Product ID 6535 Product Number PR-CU-R252 , a custom 12.0V 5000mAh battery pack made of 10 x D size NiCd cells with 10" long 18 AWG wire with special configuration. The cells provide about 25% more ampere-hour capacity than the original Kachina NiCd battery pack did. This also means that the pack may need to be charged about 25% longer than the original. A fully charged battery should read approximately 13.5 to 14.0 V on the manpack front panel meter when switched to the battery test position and while charging. Do not over-charge.
Charging Battery Reminder: always loosen the 6 clamps of the manpack case cover while charging, and slightly let the cover separate from the front panel gasket, to release the hydrogen and oxygen gas.

Troubleshooting the Kachina MP-25
Common problem issues that may be encountered, and some possible solutions.

Whip antenna mount breaks at thin brass fitting screw near rotary knob plate. Rebuild with larger fitting (or braze and re-drill it)
Lower Sideband reception and transmit is very weak, tinny or muffled sounding
Replace FL102 BLE (16 MHz IF LSB filter)
...or use Upper Sideband on the HFpack calling frequencies!   :)
Continuous beep sound of unlocked frequency, combined with false digits on some ranges Repair or clean frequency selection rotary knob switches. Sometimes just rotating each knob from 0 to 9 several times will wipe them clean.
CW chirp or keying interruptions due to semi-break-in keying Press microphone button and hold it down during CW transmissions.
Transceiver front panel explodes out in a cloud of fire and ice Always loosen the outer enclosure panel clamps when charging, to prevent gas pressure buildup inside the sealed enclosure case. Only set the power supply charger to CHARGE if the battery is inside the radio. Remove the battery pack when used as a base station then set power supply to OPERATE.
Frequency is out of calibration Calibrate the 2 internal crystals using the alignment procedure in service manual section 5-36 [PC201 Alignment. 16MHz BFO crystal trimmer C202] and section 5-31 [PC601 Alignment. 5MHz crystal PLL reference oscillator trimmer C641]
Display illuminates for too short of time for comfortable tuning Do the Display Timer Mod.
Continuous tone emitted by radio speaker after some time or whenever transmitting
The tone alarm will sound if battery voltage drops too low. Check battery voltage. Charge battery. A tone will also be heard if the CW key is shorted (key down sidetone)
Battery completely dead, won't take a charge for long
Rebuild or replace the rechargeable batteries in the battery pack.
Intermittent problems appear out of the blue after radio has been sitting unused for some time Open radio up, loosen and wiggle all the module boards so they are seated properly in their connector sockets. Sometimes these will corrode after years of disuse, and the module boards will need to simply be extracted and re-inserted.

Kachina MP-25 in the News
The Kachina MP-25 manpack radio seemed to find its way into many remote places of the world, including armed conflicts in Africa and Central Asia. Here are some links to a few old newspaper articles, with some details on rather "infamous users" of the Kachina MP-25 :)
Kachina MP-25 2001 newspaper article   Kachina MP-25 1998 newspaper article
Kachina MP-25 in Afghanistan (2001 article)   Kachina MP-25 in Kashmir (1998 article)

Kachina MP-25
Note: This article is for educational and reference purposes only. No warranty or guarantee of the information is offered.
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