Maxtrac Pin Shift and VCO's Explained

Maxtrac Pin Shift and VCO's Explained

The following is a contribution from one of the Maxtrac 900 conversion guru's:

A few things come to mind when I first got into the Maxtrac and was trying to get things clear in my mind, one is what Motorola calls PIN SHIFT. Pin shift is a signal that controls which part of the VCO that is being used (or active). In general, the left VCO is for RX and the right for TX. However there are exceptions to this. In a Maxtrac with trunking firmware (a B6 or B7 unit) the left VCO is used for RX and TX on trunking repeaters, in talk around or conventional - the left VCO is used for RX and the right VCO is used for TX.

Now when amateurs modify the Maxtrac and start to program the 10 conventional channels to amateur frequencies is when it gets confusing - as long as your TX frequencies are above 902.500 MHz no problem. The hooker is, Motorola in the firmware chip picked 902.500 as the frequency that determines the state of the pin shift signal (don't know what the number is in the conventional firmware, probably the same?). If you program a TX frequency above 902.5, no problem the firmware tells the micro-p the state of the pin shift signal to select the right VCO for TX. However, if you program a TX frequency between 902 and 902.500, the firmware will select the left VCO. That is why some have cut and jumpered the PTT signal to control the state of the pin shift signal to assure the right VCO is selected regardless of the programmed TX frequency.

When the pin shift signal "at pin 5 of the VCO" is a '0' (zero volts) the left/RX VCO is the active section (as I recall there is one transistor inversion - so the Pin Shift signal as identified on the schematic would be a '1' or 5 volts). If pin 5 of the VCO is a '1' or 9.6 volts the right side of the VCO is active. You can ground pin 5 of the VCO or J8 pin 14 (J8-14) with a clip lead to change the state of the pin shift to verify this function, or control the VCO active section for trouble shooting - there is a series 10K resistor between J8-14 and the logic IC so you don't have to be concerned about stressing anything when grounding J8-14.

When actually tuning or moving the center frequencies of the VCO's there are several ways to tell if the VCO/PLL is locked in the RX mode. First, the steering line voltage. Second, connect a signal generator/service monitor and use a 400Hz or 1KHz tone or look for a steady RED BUSY light on the front panel. Third, my favorite, is a Radio Shack frequency counter (Catalog number 22-305) lay it on top of the radio with antenna across the VCO with the lid off (do not extend the antenna of the freq counter), if the displayed freq is steady the VCO is locked. For TX, just key the radio and if the TXBsy red light illuminates, the VCO is locked (of course make sure you have a load or antenna connected to the radio), the RS freq counter works for TX as well.

With some practice and experience you will find the stock VCO has a lock range of just at 20 MHz. I have been working on an alternate (simpler) method to extend the VCO range versus the 15 volt DC-DC converter. I have the RX VCO tracking in excess of 55MHz, but have only had limited success on the TX VCO - much higher Q circuit, however I should have it solved soon.