I recently created a kit that lets you convert the D700 to a GPS-in-head design like the later TM-D710GA model. When I went to apply the kit to my oldest D700 sample I encountered a problem: the display would just strobe when powering on. The radio was clearly in a boot loop. If I unplugged the GPS device inside the head, it would power on normally. If I set the backlight dimmer to off (or almost-off) it would power up properly. None of the other D700s I had did this.
I measured the voltage of the power supply (signal PB on the schematic) to the head at just over 6V when powered on, even without the GPS. I knew this was wrong because I had investigated the power supply for the head to make sure my GPS conversion would have headroom to power the device. It should have been about 9.6V. With the GPS plugged in, it was dropping to just above 3V. Here’s the AVR circuit for the display head from the schematic:
Q911 does the heavy lifting here: a 1A-rated PNP transistor as a voltage follower, controlled by the UDZ10 10V zener and the small NPN transistor Q910. When I checked my Q911 I found it shorted in every direction except collector to emitter. Thus, I think the very small transistor Q910 was all that was powering the head, and it’s only supposed to be a reference for the bigger Q911 which explains why the 50mA GPS load dragged it down too low. After coming to this realization, I also realized that this D700 had a dimmer backlight than all my others.
The 2SB1132 part in the schematic is long since obsolete, but I found a 2SAR293P5T100 pin-compatible replacement that was close enough. Q911 is on the bottom side of the board, so you have to unsolder the RF connector and remove all the screws to get it out and flip it over. Here’s the AVR section in the service manual view:
Here is the actual picture of the board, with the device replaced:
After this, the PB line of the head cable shows 9.6V on the dot, and has no problem powering the internal GPS now. I’m not sure when my transistor died, but I might not have noticed until the little reference transistor finally burned out. It would be a much more challenging repair, as tiny as it is. I found other reports of people with burned out display power supplies due to crushed/shorted control cables. Hopefully this helps someone in case they experience a sporadic or traumatic failure.