GE Mastr II (Master 2) Radio Modifications

GE Mastr II Radio Modifications

The General Electric Mastr II (yes that's correct spelling but most folks call it Master 2) radio is one of the most popular conversion radios available to the amateur community. If you do a search on "amateur repeater" on the web you will find hundreds of GE MastrII repeaters. I give all the credit to Darryl, N4OME, for introducing me to the Mastr II (and creating the monster). I found the Mastr II to be easy to convert and easy to work on. I have done several things with mine that are not documented in any of the conversions I have found on line and I will document them here as time permits.

UHF exciter multiplier padding
The UHF exciter tunes up quite easily in the range it is designed to operate in. Unfortunately I found myself with a coordinated frequency which was very low in the amateur band and the exciter first doubler output and second doubler input would not tune to 442 MHz. My exciter was designed for the 450-512 MHz range. I removed the bottom cover from the radio and the bottom shield from the exciter. Using the schematic and pictorial diagrams I located the proper spots to do my modification. On the bottom of the exciter board I added 4.7 pf NPO discaps in parallel with C145 (15 pf), C194 (8 pf), and with C195 (6 pf). This brought each of these three values closer to the recommended values of 22, 12 and 9 pf for the 406-450 MHz range. After performing this modification, the exciter tuned exactly like it should and yielded good power output.

Receiver audio level adjustment
The receiver audio output was distorted. Using the oscilloscope I looked at the VOL/SQ HI lead and observed the squelch noise and audio was being clipped. I found that I could adjust R608, AUDIO LEVEL ADJ, and reduce the levels to a point that they were not be clipped and distorted. This will slightly offset the squelch adjustment but not a great deal in my case. It definately improved the quality of the audio coming out of the radio.

Do NOT repeat those CTCSS tones
Most of the conversions recommend taking repeat audio directly from the VOL/SQ HI and feeding that as repeat audio to the controller. There are two problems with doing this. One of them is the need for de-emphasis and that is discussed below. The other is that all the incoming CTCSS tones are fed directly to the controller and repeated. These tones should NEVER be repeated. All CTCSS tones from your repeater should be generated cleanly locally. Any CTCSS tones you repeat will only distort the ability of the mobile receivers to decode your own tones. The Mastr II has a good high pass filter built in to keep these tones from reaching your speaker but unfortunately it is on the variable side of the volume control and not really suitable for your repeater application. The best way to handle this problem is to use the high pass filter built into a CTCSS decoder/encoder like the Communications Specialists TS-64. You will already be feeding the VOL/SQ HI signal to the decoder input to decode tones so it is a snap to take the repeat audio from the output of the high pass filter they provide for that purpose. Using this high pass filter, there will be absolutely no trace of the incoming CTCSS tones in your repeat audio.

Proper receive audio de-emphasis - untwisting the DTMF
The problem encountered by everyone building a repeater out of a Mastr II is one of the audio sounding "tinny" (too much treble, not enough bass). Even if you are willing to listen to the audio this way it is causing another problem that may manifest itself through control problems. That problem is one called "tone twist" of the DTMF signals. This is where the levels of the high frequency tones are much different than those of the low frequency tones. Ideally they should be the same. Nothing is perfect but if you look at the waveform produced by a properly de-emphasized radio, even the Mastr II normal audio output, you will find that the tones are very nearly the same. Using the audio from the VOL/SQ HI for repeat audio provides you with an audio signal that has a lot of pre-emphasis. That is to say that the high frequencies are boosted before transmission and need to be attenuated when received.

Many have written that this can be achieved rather easily using a simple resistor capacitor network. This has been addressed in several places and you will find different ideas about what the values should be. There is a good discussion of this at http://www.batlabs.com/predemp.html and at http://www.repeater-builder.com/rbtip/pdemph-post.html which cover all the techie stuff and then there are Mastr II specific references at http://www.n8zcc.com/ge/mastr2/getsignals.htm and at http://www.nhrc.net/mastr2/duplexing.shtml and at http://www.repeater-builder.com/ge/mastrIIsignals.html. All of these recommend a 6 db per octive de-emphasis network and starting with a 15k resistor and .22 uf capacitor is just that, a good place to start. The actual values you use are determined by where you start, that is where you get the audio, whether it is from the output of a high pass filter or directly from the VOL/SQ HI, and the input capacitance and resistance of the circuit you are feeding, typically your controller. Do NOT skimp here. Placing the de-emphasis someplace else will not correct the tone twist and may cause other problems.

NOW... HERE is what I do:

There are many old Mastr II radios out there in pieces in the junk pile begging for a use. Scavenge an IF/Audio/Squelch (IFAS) board from one of them and carefully remove the long 14 pin special audio module from it. It's known as U604 on the GE schematics and parts layout. This may take some effort. The easiest way I have found is to gently add a tiny bit of solder to each of the 14 pins and then use solder wick to remove all the solder from the holes surrounding each pin. Take this module and mount it on a piece of proto board (I like the Velleman ECS1) and wire it and an LM317 regulator as shown in this diagram:
http://w4zt.com/repeater/circuits/geaudio.html
This will provide you with properly de-emphasized audio for your controller that sounds natural and with no guesswork.

CTCSS transmit audio path via shielded cable from the controller to the radio
If you put your CTCSS encoder in the radio, or use an original GE encoder, you wont find this to be a problem. If, like me, you'd rather have the combined unit in the controller where you can CONTROL it then you have to be concerned with how to get the signal back into the radio. I found folks recommending hooking the output of the CTCSS encoder to the microphone lead. This didn't make any sense to me because you already have the problem of mixing the transmit audio with the local microphone (you do have a local microphone don't you?). The Mastr II has an input for CTCSS on the exciter which normally the GE Channel Guard board feeds directly. This input is properly mixed and has its own deviation adjustment on the exciter. The only problem is, how do you get the audio there? Well this problem is not really a problem at all! I found the best and easiest solution after just a little time staring at the schematic of the exciter, front panel system board wiring, and control cable. The Mastr II radio has an audio OUTPUT from the receiver which is designed to be feed to an external PA amplifier (you've heard them on cop cars and on trains if you've been around them). This signal is fed from the receiver through the control board to pin 22 of the control cable. As it turns out, pin 22 is the center of a shielded cable! The shield is pin 21 and is labled CONTROL A-. The simple modification is on the front panel system board. You'll need the pictorial diagram for this (maybe I'll dupe it here later). Open the jumper between H100 and H101. Jumper H101 (pin 22 of the control cable) to P908 pin 7 which is the original Channel Guard board output. Now you can use the shielded lead with the YELLOW/GREEN center conductor to carry your CTCSS signal from your controller to the radio. Set the CG level pot on the exciter and on your CTCSS encoder to give your desired level. EASY!

Cutting the ears off uncovering the connectors
The Mastr II radio has these horrible metal ears over the RF connector. Once you've done the duplex mod you'll have ears over BOTH the connectors unless you do something about them. In doing my duplex mod I chose to completely remove the front panel of the radio. This is the prescribed way but you CAN do it without removing it. Having the front panel off in your hand makes it an easy task to clip those ears. I've seen a few radios that'd had its ears broken off at odd angles and generally making a mess. you can use a hacksaw, bandsaw or jigsaw to make short work of those ears if the cover is free of the radio. Look at these pictures (click the small ones to see larger ones):

Repeater and Station Combination Numbers

UHF Mobile Combination Numbers

VHF Mobile Combination Numbers

(charts were checked for correctness)