Problem:

There are many conversions available on the Internet describing how to convert a VHF GE mastr II reciever for use in the 220 MHz amauteur Band. Some do not work well, some are not well documented and therefore not easily reproduced. This article describes a complete conversion that is reproduceable and works well once completed. Portions of this conversion are from the following folks:

Receiver Oscillator Mulitipler Board

There are three versions of the Oscillator Multiplier Board. Only the two later model oscillator boards will work for this conversion. The version of the board that WILL NOT work, has 3 tuning coils instead of the three capacitors. The components of the middle model board are numbered using three numbers and the later model uses one or two numbers. I have used both numbering systems here for clarity.

1. Replace C12 / 412 with 12pf.
2. Remove C421 and C417. C421 and C417 may already be missing.
3. Install a jumper from the center of L403 to the next turn of L403 closest to R404.
4. Install a jumper between the bottom two turns of L403 closest to the position reserved for C418.
   These jumper locations are shown in the image below:

The new crystal formula is: xtal = (f - 11.2) / 12 example: 17.675 = (223.30 - 11.2) / 12

Some crystal manufacturers don't do well when presented with orders "outside the box". If this is the case, simply calculate the fundamental frequency, put it back into the original formula and order the equivalent high-band frequency. If you are ordering from Bomar Crystal, you can simply tell them the 220 frequency and that it's for a GE MASTR II 19A129393/12. The 9393/12 is the important part.

Mixer / IF Board

1. Short out approximately 1 turn from the outside of L502 as shown in the photo below. This short is from the outer turn of L502 to the ground leg of C502 closest to the board edge.

Receiver RF Assembly

The RF Assembly has 2 enamel coated coils and 5 silver coils. These coils are very difficult to un-solder. An electric kitchen stove at the lowest heat setting can be used to pre-heat the casting. Do not get it hot enough to melt the solder. Once the casting has been preheated, use a solder gun to remove and replace the coils. A propane or butane torch can also be used with care.

After removing the coils, remove turns from the "cold" end of thse coils until they have the number of turns specified in the table below. Once the coils have the correct number of turns, gently pull on the top and bottom of the coils to spread them apart until they are the original length.

Coils L301/311 and L305/315 have taps on the bottom turns. Both taps were originally soldered directly across from the solder ends of the coils. Replace both taps approximately 1/8 turn closer to the soldered end of the coils.

When you replace the coils, the bottom of the ceramic should be just touching the cover. You can use the cover and circuit boards to help position the coils while soldering the coils back into the casting. To do this, insert screws into the ceramic insulators to fasten them to the cover and secure the cover to the casting with a few screws. Once the coils are resoldered into the casting, remove the cover and see that everything looks as it was in the origional VHF casting. Check to see that the coils are straight and aligned with the capacitor hats on the adjustment screws. Be sure the cover can be easily re-installed and that the screw holes lign up properly. Now is the time to correct any mis-alignments. Once you are sure everything looks OK, replace the cover on the helical assembly.

DO NOT OVERTIGHTEN THE SCREWS IN THE INSULATORS!!!

There, you have been warned!! The ceramic is VERY fragile. If you break an insulator, do the best you can with what you have left. I have used longer screws to get down carefully into threads that are not broken. If you can find the pieces and they are big enough, you may be able to repair the broken ceramic. I have had differing success at this method. What seems to work best is super glue on the joint wrapped by a piece of tape to hold it in place until the glue dries.

A modified L301/L311 thru L305/315 coil

Conclusion:

Reassemble the radio. Tune the radio set as outlined in the manual. You should now have good sensitivity (Better than ~.35 uV for 12dB SINAD) on 220.

 
 
Research and compilation done by M. Scott Zimmerman N3XCC
Origional authors credited at top of page.
HTML Copyright©  3/23/07,  M. Scott Zimmerman N3XCC
Page last updated 3/23/07 N3XCC