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 radio for better performance.

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 low 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.

C 407 from a 12 to 18 pF
C 412 from a 5 to 8 pF
C 417 from (unknown value/maybe not installed) to a 3 pF
C 421 from (unknown value/maybe not installed) to a 18 pF

Here are images to help identify the locations of these capacitors.