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  Converting a GE MASTR II
42-50 MHz to 50-54 MHz

By David Edwards W2GBO


LBI numbers have been updated to access
PDF files available on this web site.
LBI-4989 and LBI-4993 replace LBI-30019.
LBI-4732 and LBI-4896 replace LBI-4728.
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Modification of the General Electric
19D416478G4, G13 Receiver RF Assembly and
19D423078G4, G8 Oscillator/Multiplier Board for
Operation within the 50-54 MHz 6 Meter Amateur Band

The specified frequency range for this receiver assembly is 42-50 MHz. There are several tuned circuits that will not resonate at the proper frequencies for receiver operation above 50 MHz as they are, therefore the L/C ratios must be adjusted slightly.

Before starting this modification, please read the following instructions thoroughly while examining the assemblies to become familiar with the location of the components requiring modification or change. Reference to GE maintenance manual LBI-4989, or LBI-4993, or LBI-30020 (Noise Blanker) is necessary. Some of the components are extremely fragile and easily broken. Extreme care must be exercised to avoid breakage. The use of a large soldering iron, in the 400 to 500 Watt range, and/or a propane torch is required for modification of the RF assembly. Please use extreme care during this operation to prevent personal injury, as the RF assembly metal casting will become quite hot during the process.

Disassembly instructions for the receiver are contained within the LBI-4989, LBI-4993, and LBI-30020 maintenance manuals. Make note of the location for all removed hardware to make reassembly convenient.

  1. Remove the Oscillator/Multiplier and IF Audio Squelch boards from the MASTR II radio chassis.

  2. Remove the RF Assembly metal casting.

  3. Remove the Mixer/IF/Noise Blanker circuit board from the RF Assembly metal casting.

  4. Unsolder the DA jumper from hole H1 on the RF pre-selector circuit board and remove the board from the RF Assembly metal casting. Clean the solder from hole H1.

  5. Unsolder the antenna coupling capacitor, C2 from the center pin of A301-J1 (antenna input connector).

  6. Remove tuned circuit assemblies L301/C301 and L302/C302 from the RF assembly metal casting. This operation requires the use of a large soldering iron and/or a propane torch. The ground ends of the coils are soldered to the casting side, while the tuning capacitors are soldered to the casting top near the tuning adjustment holes. Use care with the torch flame. Do not let it enter any of the tuning adjustment holes, as damage to the internal components will result. Use gloves to prevent severe burns. After the tuned circuit assemblies have been removed, clean the solder from the casting holes for ease of reassembly. Note that assembly L301/C301 includes a "neon bulb" and is the antenna tuning circuit.

  7. One at a time, unsolder the tuning capacitor end of the coils and remove two turns of wire from each of the two assemblies. Cut off the excess wire and re-solder the free end where it was previously connected. Do not use excessive heat.

  8. Replace the two tuned circuit assemblies in their original positions in the RF casting, and solder them into place. Set the RF assembly casting aside and let it cool.

  9. Re-solder C2 to the center pin of A301-J1 (antenna input connector) that was previously disconnected.

  10. Remove capacitors C7, C8, C9, and C10 from the RF pre-selector circuit board assembly (18 pF, -80 ppm). Clean the solder holes and replace each capacitor with 13 pF, -80 ppm.

  11. Remove capacitors C506 and C508 from the Mixer/IF/Noise Blanker circuit board (12 pF, -80 ppm). Clean the solder holes and replace each capacitor with 8 pF, -80 ppm.

  12. Remove capacitor C404 from the Oscillator/Multiplier circuit board (8 pF, 0 ppm). Clean the solder holes and replace with 2 pF, 0 ppm.

  13. Remove capacitor C408 from the Oscillator/Multiplier circuit board (8 pF, 0 ppm). There is no replacement necessary for C408.

  14. Remove capacitor C414 from the Oscillator/Multiplier circuit board (12 pF, -80 ppm). Clean the solder holes and replace with 10 pF, -80 ppm.

  15. Mark each modified assembly or circuit board showing it has been modified for operation between 50 and 54 MHz, as there is little physical evidence to indicate such.

  16. If this receiver is to be used in repeater service, the optional noise blanker (if present) must be disabled. On the Mixer/IF/Noise Blanker circuit board, add a jumper between holes H6 and H7. Connect P551 to J551. Connect the receiver antenna coaxial feed-line directly to the receiver antenna input connector, A301-J1.

  17. Reassemble the component circuit boards into the RF assembly metal casting. Return the RF casting, Oscillator/Multiplier and IF Audio Squelch boards to the receiver assembly. Be sure all connectors and DA jumpers are re-connected; secure all shield cover plates in their proper positions.

  18. This receiver assembly is now ready for tuning and operation within the 50-54 MHz 6 Meter Amateur Band. Obtain the proper ICOM for the desired channel frequency of operation and tune the receiver per instructions contained in maintenance manual LBI-4989, LBI-4993, or LBI-30020 (Noise Blanker).

For successful operation of this MASTR II radio system above 50 MHz, the transmitter exciter, PA assembly and the receiver assembly all require modification.

10-94, W2GBO. Rev. F, 02-97

Modification of the General Electric
19D416659G4, G8 Transmitter Exciter Board for
Operation within the 50-54 MHz 6 Meter Amateur Band

The specified frequency range for this transmitter exciter board is 42-50 MHz. There are several tuned circuits, which will not resonate at the proper frequencies for transmitter operation above 50 MHz as they are, therefore the L/C ratios must be adjusted slightly.

Before starting this modification, please read the following instructions thoroughly while examining the circuit board to become familiar with the location of the components requiring modification or change. Reference to GE maintenance manual LBI-4732, LBI-4896, or LBI-4898 is necessary. Some of the components are extremely fragile and easily broken. Extreme care must be exercised to avoid breakage, especially with step 3.

  1. Remove the exciter board from the MASTR II radio chassis.

  2. Locate slug-tuned coils L101, L102 and L103. Carefully unsolder and remove the aluminum shield around each of the three coils. Clean the solder tabs on the shields and the circuit board holes to make reassembly convenient.

  3. These coils are electrically identical and are wound with insulated copper wire, which is quite fine. One at a time, carefully disconnect the top lead of each winding from its solder post on the coil form. There may be a wrap of clear plastic tape just above the solder posts which must be removed. Remove ten turns of wire from each coil, cut off the excess wire and re-solder the ends to their respective solder posts. This wire is easily tinned without scraping the insulation. Do not use excessive heat on the plastic coil form solder posts. Return the shields to their positions around the coils and solder them into place.

  4. Locate and remove capacitors C123, C125, and C127 (43 pF, -80 ppm). Clean the circuit board holes and replace each capacitor with 39 pF, -80 ppm.

  5. Locate and remove capacitors C129 and C131 (15 pF, -80 ppm). Clean the circuit board holes and replace each capacitor with 10 pF, -80 ppm. Note: C129 may be soldered directly to coil T104, within the aluminum shield on some production runs. There are usually circuit board holes just outside of the shield near J101 in which the replacement capacitor may be installed.

  6. Locate and remove capacitors C134, C136 and C138 (7 pF, -80 ppm). Clean the circuit board holes and replace each capacitor with 4 pF, -80 ppm.

  7. Locate and remove capacitor C146 (7 pF, -80 ppm). Clean the circuit board holes and replace the capacitor with 5 pF, -80 ppm.

  8. Mark the modified exciter board showing it has been modified for operation between 50 and 54 MHz, as there is little physical evidence to indicate such.

  9. Install the modified exciter board into the MASTR II radio system. Be sure to connect the RF output cable (from the exciter output to the PA assembly).

  10. This exciter board is now ready for tuning and operation within the 50-54 MHz 6 Meter Amateur Band. Obtain the proper ICOM for the desired channel frequency of operation and tune the exciter board per instructions contained in maintenance manual LBI-4732, LBI-4896, or LBI-4898.

For successful operation of this MASTR II radio system above 50 MHz, the transmitter exciter, PA assembly and the receiver assembly all require modification.

10-94, W2GBO. Rev. D, 08-98

Modification of the General Electric
19D417927G4 100 Watt RF Power Amplifier Board for
Operation within the 50-54 MHz 6 Meter Amateur Band

The specified frequency range for the 19D417927G4 RF Power Amplifier Board is 42-50 MHz, with a maximum rated RF power output of 100 Watts. This power amplifier circuit board is utilized in the MASTR II 19C321295G8, G16, G24, G32 (20%) intermittent duty, and the 19D417955G8 (100%) continuous duty station power amplifier assemblies. The inductance of several air core coils on this circuit board assembly must be reduced slightly to improve the operating efficiency above 50 MHz.

Before starting this modification, please read the following instructions thoroughly while examining the power amplifier circuit board to become familiar with the location of the inductors requiring modification. Reference to GE maintenance manual LBI-4732, LBI-4896, or LBI-4898 is necessary.

  1. Remove the top cover from the MASTR II radio chassis. Remove the shield cover from the top of the PA assembly, exposing the RF power amplifier circuit board.

  2. Locate the following 23 air core inductors: L201, L202, L205, L206, L207, L210, L214, L215, L216, L217, L218, L219, L220, L237, L238, L239, L240, L241, L242, L243, L244, L249 and L250. Each of these inductors is close wound with Heavy-Formvar copper wire.

  3. One at a time, spread the turns of each listed inductor such that the turns are spaced approximately one-half wire width. The use of a small, insulated screwdriver, or other suitable tool that will not remove any insulation from the coil turns, is necessary. Note: Due to the high RF Voltages that are present on several of these inductors, the removal of any insulation from the coil turns must be avoided to eliminate any possibility of an arc, which could destroy your PA assembly.

  4. While driving this amplifier with a modified exciter operating above 50 MHz, adjust the power output per instructions contained in LBI-4732, LBI-4896, or LBI-4898. Do Not exceed the 100 Watt RF power output rating, as this will considerably shorten the life expectancy of the RF power transistors. These devices are no longer available from the manufacturer.

For successful operation of this MASTR II radio system above 50 MHz, the transmitter exciter, PA assembly and the receiver assembly all require modification.

10-94, W2GBO. Rev. D, 02-97

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This page originally written as a PDF in 1994.



Article text © Copyright 1994, 1998 by David Edwards W2GBO.
Conversion to repeater-builder format © Copyright 2017 by Robert Meister WA1MIK.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.