The EF Johnson 9755 Summit DM 800 MHz 35 Watt mobiles are designed for public safety Multinet II systems, yet suffer from reliability issues vs. their more reliable 9753 Summit DM 15 Watt mobile version.
High power 800 MHz amplifier repair
The Q501 45 Watt MRF847 NPN transistor output (collector) pin gets burned up, about a $50 part if it needs changing. This manifests itself as a crackling noise heard on the transmit audio most often. I have never seen a failed Q501, which is a testament to the durability. By the way, the commercial market EF Johnson 9883 800 MHz trunked radio is rated at 30 Watts with the same transistor denoted Q651. The 9883 also has a stabilization board with extra components not in the Summit, I have not discussed with the factory if these might help the Summit as well.
The repair process for the high power 800 MHz transistor output is as follows–this should be generalizable to any high-power VHF/UHF high power transistor.
- Inspect extent of damage–is transistor output pin still mostly there? Is PC burned black or just a little solder bubbling? In the case too little of the collector is left, you will need a new Q501 part # 576-0004-817.
- Remove all solder from the collector wing, gently lifting it up in the process.
- Using copper foil, rebuild trace under the collector wing. The width is impedance-critical and keep edges meeting flush to avoid a major hotspot and reburning.
- Recommend using high-temperature, high-conductivity silver solder when reworking.
- You can’t use a piece of wire at these frequencies and current levels. It will quickly burn up again otherwise.
- In repeated cases, setting the radio to 30 Watts output may help, or send to factory for a new board.
Interconnect block breakage
J501 or J401 can break due to high levels of vehicle vibration over time. The usual intermittent solder joint checks work here.
Microphone jack breakage
Sometimes officers don’t realize that the Hirose microphone jack is a slide back to release, NOT a twist off. Repairing the flex PCB after it’s torn up from being rotated is not super pleasant but unfortunately not a rare problem.
800 MHz EFJ Summit DM Programming
There have been and will be more firmware upgrades to the radio. You need the factory programmer with the high voltage switch to put the Summit DM into flash mode.
Don’t let the System Key get out–it’s poor security unfortunately as anyone with the System Key can program radios on the system. OK, System Key is far better than PL/DPL/LTR but still, EFJ should be using a hardware key for Multinet II security.
Keep in mind the system policy for priority–or you’ll have a mobile talking over dispatchers!
You should use an N-N male barrel directly to the service monitor input, to allow proper tuning with the special Johanson ceramic tuning tools for the final amplifier output at proper power levels. Excessive power will burn up the PCB. The N-connector on the antenna cable in the car should always be looked at for improper assembly. The low-loss Antenna Specialists grey cable for 800 MHz is something installers may not be familiar with, so don’t overlook problems there.