MASTR II VHF Oscillator/Multiplier Modification

GE MASTR II VHF oscillator/multiplier modification
By Jeff DePolo WN3A and Kevin Custer W3KKC

Concept:
Modify VHF oscillator/multiplier found in a 150.8 to 174 MHz GE MASTR II receiver for better performance when used in the 144 to 148 MHz 2 meter ham band.

NOTE: These modifications can be avoided by using high-side local oscillator injection. To do this just order your crystal, or specify the ICOM gets made, for high side injection.

Background information:
The sensitivity of the stock GE MASTR II commercial receiver may be limited by low local oscillator injection when tuned to the amateur 2 meter band. The problem is more apparent as frequency goes down. The following modifications will improve the injection resulting in up to 4 dB extra sensitivity, or more. Usually the commercial band receiver provides fairly good sensitivity in the ham band. We like to end up with about -116 dBm or .35 uV for 12 dB SINAD. When it doesn't meet this specification, 9 times out of 10, it's due to insufficient local oscillator injection when using low side injection.

The modification:
On the later model Osc/Mult boards, (the ones with the trimmer capacitors), there are a few fixed value capacitors that you need to install or increase in value. There are several different versions of this board, and photos of two popular versions are included below.

If you have a 19D423242G board, use the images below.

Change C407 from a 12 to a 18 pF
Change C412 from a 5 to a 8 pF
Change C417 from (unknown value/maybe not installed) to a 3 pF
Change C421 from (unknown value/maybe not installed) to a 18 pF

Here are images to help identify the locations of these capacitors which are outlined in red.

If you have a 19D423241G4 board, use the photo below for component locations and value changes. Clicking on the image will make it larger.