OK, here’s another fix/modification… I received an MCS2000 with the FL 01/90 error code, which was extremely common in early production runs of this radio. This error disappeared when I removed the radio PCB from the actual body of the radio deck. So, I went to reinstall the PCB in the radio, installing a single screw at a time and powering up after each screw was added. When I added one of the metal clips that pressed some voltage regulators and the audio amplifier IC against the heatsink, the radio gave the fail code. After some research, it turns out that one of the regulators (the 5v regulator) can come unsoldered from the board due to the pressure of the metal clip pressing it down onto the heatsink. Motorola’s fix, rumor has it, is to shorten the metal tang of the heatsink clip to keep the clip from pressing down so hard on the regulator. I simply reflowed the regulator and added more solder, crossed my fingers, and hoped it would work for another while. Buttoned back up, the radio works like a charm!
Then, I decided to construct a programming cable for the radio using the schematics from BatLabs, and in the process I shorted the SW B+ line on the mic jack (pin 1) to ground, causing the display to go out on the control head, and all that was left on were the LEDs backlighting the keys. I removed power from the radio, let it sit a minute, and tried again, but to no avail. The radio would not power up!
Turns out, I burned some traces from the SW B+ pin on the 18-pin connector that goes from the radio deck to the control head, so the control head was not getting SW B+. This can be verified by checking both sides of a blue 10ohm resistor close to the mic jack, on the back (component) side of the control head board. It should show somewhere around 13v, or SW B+. In my case, it was showing only 2-3v, for some reason… So, I jumpered from pin 17 on the control head connector to the bottom side of that resistor, as such:
I also jumpered from the bottom side of the one capacitor shown to the transistor which is a part of the OPT SEL/SW B+ sensing circuit. Depending on the current drawn through the SW B+ pin on the mic connector, the radio decides whether SB9600 serial lines get sent to the mic jack, or if standard microphone/audio signals get sent to the mic jack. If current is zilch, it assumes that it is a microphone. If current is > 2 mA (I believe that’s the figure…) it decides that the attached device is a “SB9600 SMART” device, and sends the SB9600 to the mic connector. This is why a programming cable for the MCS has a 1k resistor from SW B+ (pin 1) to GND (causes 13mA of current to flow from SW B+, switching MCS into SB9600 Accessory mode). I found that I had no continuity between pin 1 of the mic jack and the 470ohm pullup resistor (possibly R0729) which supplies the SW B+ to the mic jack.
Like I said, the board layout is actually from a GM900 service manual (all I could find) but is *very* similar to the MCS2000’s layout. Hope this helps somebody down the road! The radio is now working like a charm once again.