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

As many know, the G.E. MVP makes a very easy-to-convert, compact, and reliable radio in repeater/remote base service when properly prepared. This document is meant to show the procedure for converting these radios to 420-450MHz operation - full coverage of the U.S. 70cm Amateur band can then be realized. Hence, the need to search for rare (and typically expensive) 406-420MHz 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.) - replacing Q215 in the transmitter PA assembly.  (NOTE: The driver PA current exceeds the OEM Q215 rating of ½-Amp; the famous 'power control transistor' that commonly fails)

To modify the 'exciter:'  (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) and save this capacitor. Install a 56pF capacitor in its place.
• Remove C143 (39pF silver mica) and install a 47pF capacitor in its place.
• Re-install the modified 'exciter board' into the station chassis.

Power Amplifier (PA) modification:

• Install the saved 39pF ceramic capacitor across C17 (18pF) of transistor Q202 (Driver PA).

The transmitter is now modified. TX 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.

Receiver Modification Procedures - 'Front-End'/ LO-Mixer Assembly

• 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 mechanically modifying the RX LO/injection cavity assembly. Note: The 470-494MHz MVP will allow the use of low-side injection crystals - down to 420MHz - but the mixer's tuning screws are nearly 'bottomed-out.'

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

Table of 'Exciter Board' part values
 
 

Part Name (as labeled on schematics)	Value before modification  (450-470MHz) / '88' split	Value(s) with modification (420-450MHz) / '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 schematics)	Value before modification (450-512MHz)	Value(s) with modification (406-450MHz)
C7	8pF	6pF
C8	8pF	6pF
C9	33pF	27pF
C10	27pF	18pF
C11	33pF	24pF
C14	33pF	24pF
C15	47pF	43pF
C16	51pF	43pF
C17	18pF	16pF or add 39pF ceramic in parallel*
C22	33pF	24pF
C24	41pF	35pF
C25	43pF	33pF
C26	47pF	37pF
C27	43pF	35pF
C29	33pF	24pF
C30	6pF	4pF
C33	1pF	OMIT
C52	8pF	4pF
C61	33pF	18pF

We guarantee nothing. ;)
In actuality, this conversion has proven successful many times over. Try it and see!

Any feedback is always welcomed.

Copyright © 2003 by Matt Lechliter - W6KGB
Contributions: Terry Downey - W6TD, Gary Belda - K6ENS (SK), Matt Lechliter - W6KGB
E-mail / contact data good in QRZ.COM database
Last html revision Wednesday, June 25th, 2003.
HTML Publishing © June 25 2003 Kevin K. Custer W3KKC
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