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Converting the G.E. MASTR II UHF 'High-Split' to UHF 'Mid-Split' for use as a Mobile Relay or Duplex Link radio By Matt Lechliter W6KGB

By popular demand, the MASTR II edition of the band-split conversion is here! Re-written from the original MVP/Exec II version, it rounds out the 'top 3' radios list. The G.E. MASTR II makes a very easy-to-convert, reliable radio in repeater/remote base service when properly prepared. This document is meant to show the procedure for converting these radios to 420-450 MHz operation - full coverage of the U.S. 70cm Amateur band can then be realized. Hence, the need to search for rare (and typically expensive) 406-420 MHz radios (i.e. '77' split) is non-existant. It also provides versatility; the same radio may now be used for either role. Have a link radio that blew up? Swap it out with a ready from your now abundant full-band inventory!

Transmitter Modification Procedures - Exciter Board and Power Amplifier

Parts Required:

• (5) - 3.3pF disk ceramic capacitors
  
  
• (1) - 5pF disk ceramic capacitor
  
  
• (1) - 12pF disk ceramic capacitor
  
  
• (1) - 47pF disk ceramic capacitor
  
  
• (1) - 56pF disk ceramic capacitor
  
  
• (1) - 1-Amp/25-VDC - NPN Power Transistor (Jim-Pak #TIP-120 or eq.) - this replaces Q215 in the transmitter PA assembly, the famous 'power control transistor' that commonly fails. The driver PA current exceeds the original Q215 rating of ½-Amp;

Exciter Modifications   (Reference LBI-30564B - 'Transmitter' section)

• Remove the 'exciter board' from the station chassis.
  Add the following items to the bottom of the PC board:
• Install a 3.3pF capacitor across C118 (pins 1 & 3) of T102.
• Install a 3.3pF capacitor across C121 (pins 1 & 3) of T103.
• Install a 3.3pF capacitor across C122 (pins 1 & 3) of T104.
• Install a 5pF capacitor across C192 (pins 1 & 3) of T105.
• Install a 12pF capacitor across C193 (pins 1 & 3) of T106.
• Install a 3.3pF capacitor across C194 (pins 1 & 3) of T107.
• Install a 3.3pF capacitor across C195 (pins 1 & 3) of T108.
• Remove C138 (39pF ceramic) carefully and save this capacitor. Install a 56pF capacitor in its place.
• Remove C143 (39pF silver mica) and install a 47pF capacitor in its place.
• Optionally remove L114 (.047µH) and replace with a 10 ohm resistor (now R106).
• Re-install the modified 'exciter board' into the station chassis.

Intermediate Power Amplifier (IPA) (driver) modification:

• Install the saved 39pF ceramic capacitor across C217 (16pF) of transistor Q202 (Driver PA).

The transmitter is now modified. Transmitter frequency crystals are ordered as normal. An alternate method is to replace various capacitors on the exciter board, and a table of part values is provided at the end of this document. A high-power (40W+) Power Amplifier deck is rarely necessary in link service. This modification is for the IPA (driver). Modification of the High Power Amplifier is possible, but doesn't necessarily yield 'to-spec' results due to its tuned strip-lines (Wilkonson combiner strips), unlike the amplifier decks in MASTR Exec IIs or MVPs.

If your radio has an HPA deck, remove the jumpers between input and output on the HPA, and replace with a coaxial jumper between the IPA and Harmonic Filer assembly.

Receiver Modification Procedures

All changes are performed on the 'Front-End'/ LO-Mixer Assembly:

Parts Required:

• (2) - 10pF ceramic capacitors
  
  
• (1) - 8pF ceramic capacitor (or you can salvage one, see C426 below)
  
  

Procedure:

• Carefully remove C426 (8pF ceramic) on Oscillator/Multiplier Board and install a 10pF ceramic in its place. Save this capacitor.
• Remove C2302 (3pF ceramic) on Oscillator/Multiplier Board and install a 10pF ceramic in its place.
• Remove C412 (6pF ceramic) on Oscillator/Multiplier Board and install the original C426 (an 8pF ceramic) in its place.
• Remove the four (4) screws (two-each along each side of the chassis) of the receiver RF casting assembly.
• Disconnect the receiver RF cable connector at the 'antenna relay' board.
• Disconnect the three (3) wires interfacing the RF to the IF assembly (two white wires and one orange wire).
• Physically remove this sub-assembly from the station chassis.
• Remove the bottom cover plate of the RF 'pre-selector.'
• Remove the helical coils one by one very carefully. Heat the coil and casting, gently wiggling the coil loose from the casting with a pair of long-nose ("needle nose") pliers holding onto the straight portion of the coil soldered to the casting. Do not grab the coils by their windings! Be sure to have a high-temperature (>1000 degree) iron to solder/de-solder these points.
• Add by 'butt soldering' (be sure to tin well) a ¼-turn segment to the coil ends of L301, L302, L303, L304, and L305 at their 'tops.'
• Gently replace the coils to their original positions. Make sure to center the tops of the coils over the associated tuning plates.
• Reattach the bottom cover plate of the RF 'pre-selector.'
• Re-install this sub-assembly into the station chassis.
• Reconnect the three (3) wires interfacing the RF to the IF assembly (two white wires and one orange wire).
• Reconnect the RF cable connector at the antenna relay board.
• Re-install the four (4) screws (two-each along each side of the chassis) of the receiver RF casting assembly.

It is highly recommended that one should use high-side injection LO crystals to preclude the requirement to mechanically modify the RX LO/injection cavity assembly.

Note: The 470-494 MHz MVP / MASTR II will allow the use of low-side injection crystals - down to 420 MHz - but the mixer's tuning screws are nearly 'bottomed-out'.

High-Side Injection:   Crystal Frequency (Fc) = (Fo + 11.2 MHz) / 27

Low-Side Injection:   Crystal Frequency (Fc) = (Fo - 11.2 MHz) / 27

Table of 'Exciter Board' part values

Part Name (as labeled on the schematics)	Value before modification (450-470 MHz) / '88' split	Value(s) with modification (420-450 MHz) / '78' split
C118	27pF	30pF or add 3.3pF in parallel
C121	27pF	30pF or add 3.3pF in parallel
C122	20pF	24pF or add 3.3pF in parallel
C138	39pF	56pF
C143	39pF	47pF and optionally switch 39pF w/ C193
C192	18pF	24pF or add 5pF in parallel
C193	27pF	39pF or add 12pF in parallel
C194	8pF	12pF or add 3.3pF in parallel
C195	6pF	9pF or add 3.3pF in parallel
L114	.047µH	Change to R106 - 10 ohms

Table of Power Amplifier part values

Part Name (as labeled on the schematics)	Value before modification (450-470 MHz)	Value(s) with modification (420-450 MHz)
C207	8pF	8pF
C208	8pF	8pF
C213	OMIT	5pF
C215	47pF	47pF
C216	43pF	51pF
C217	16pF	18pF   or add 39pF ceramic in parallel*
C224	35pF	41pF
C225	35pF	43pF
C226	43pF	47pF
C227	37pF	43pF
C230	OMIT	7pF
C235	37pF	47pF
C236	35pF	47pF
C238	43pF	47pF
C252	7pF	OMIT
C257	OMIT	5pF

Table of 'Local Oscillator/Multiplier Board' (Receiver) Part Values

Part Name (as labeled on the schematics)	Value before modification (450-470 MHz) / '88' split	Value(s) with modification (420-450 MHz) / '78' split
C407	18pF	18pF (NC)
C412	6pF	8pF   (optionally keep C426 and reuse here)
C417	3pF	3pF (NC)
C426	8pF	10pF
C2302	3pF	10pF

We guarantee nothing.   ;)

In actuality, this conversion has proven successful many times over. Try it and see!

Any feedback is always welcomed.

Contributions: Terry Downey - W6TD, Gary Belda - K6ENS (SK), Matt Lechliter - W6KGB

Copyright © 2003 by Matt Lechliter - W6KGB   All Rights Reserved, including that of paper and web publication elsewhere.
E-mail / contact data good in the QRZ.COM database

Original HTML © 25-Jun-2003 Kevin K. Custer W3KKC   All Rights Reserved.

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