A GM300 that was in service as a repeater receiver failure has gone pretty close to deaf. The duplexer in the system failed, and the GM300 receiver was being fed about 500 milliwatts of power from the transmitter.
After troubleshooting the system and realizing what the duplexer was doing, we replaced it and of course the receiver. The radio is presently taking about 12.6 microvolts (-85 dbm) to open the squelch. Does anyone have suggestions on where to start with troubleshooting the radio and attempting to restore it to full operational condition?
Thanks
Jay
GM300 with bad receiver
Moderator: Queue Moderator
Re: GM300 with bad receiver
The most common problem with deaf GM300 receivers are these two parts:
dual diode across the RX input CR2, usually shorts out; remove and see if that helps things. If not:
RF amplifier transistor Q1, usually opens up; must be replaced to fix the radio.
Both of these parts are on the solder-side of the RF board. They fail when excessive RF is input to the board, usually when a duplexer is mistuned, is defective, or takes a high burst of energy. It's quicker to just replace both parts at the same time. Order a couple of each (I got mine from Motorola) so you'll have spares for the next radio.
Bob M.
dual diode across the RX input CR2, usually shorts out; remove and see if that helps things. If not:
RF amplifier transistor Q1, usually opens up; must be replaced to fix the radio.
Both of these parts are on the solder-side of the RF board. They fail when excessive RF is input to the board, usually when a duplexer is mistuned, is defective, or takes a high burst of energy. It's quicker to just replace both parts at the same time. Order a couple of each (I got mine from Motorola) so you'll have spares for the next radio.
Bob M.
Re: GM300 with bad receiver
Well, there is a lot of time since last posting to this thread, but anyway, I hope my small addition will be useful for someone (and for me too in case my own notes will be lost somewhere). 
Yesterday, I got pretty deaf UHF GM300 radio. First check revealed that CR2 dual schottky diode is blown completely (both diodes is open), and CR3 is short. Preamp transistor survived.
To speed up things, I "borrowed" CR2 and CR3 from useless (for me, and right now) 490-520 MHz UHF RF board, and then started to dig around to order replacement parts for my "donor" board and to have some spares to be prepared for such disaster, if it will occur again sometime.
CR2, SOT-23 dual diode with M4FU marking, Moto part # 48-80154K03 was solved pretty fast - same Moto p/n is marked as 353L in GTX service manual, and I'm almost completely sure it is MMBD353 (MMBD353LT1), Dual Hot Carrier Mixer Diode. It was out of stock in Digi-Key, but I found MMBD352 there, should work equally well in this place (also dual diode, with same specs, but polarity of each diode is reversed, but it is connected in parallel anyway, so there will be no difference).
CR3, SOT-23 schottky diode with white paint C0 marking (Moto p/n 48-80939T01) taken a bit more time. Now I almost completely sure that it is Avago (formerly Agilent and HP) HSMS-2820.
Concerning cryptic M50X01 low noise bipolar transistor (mine had MH marking, in a style common for Japanese parts) - answer come from one of russian language amateur forums. There was report that MH is marking of Toshiba 2SC3606 transistor, and author of that note already ordered this part and made some repairs with great success.
That's all. I hope this note will save some time for someone. Success reports and corrections (if any) is welcome.
Yesterday, I got pretty deaf UHF GM300 radio. First check revealed that CR2 dual schottky diode is blown completely (both diodes is open), and CR3 is short. Preamp transistor survived.
To speed up things, I "borrowed" CR2 and CR3 from useless (for me, and right now) 490-520 MHz UHF RF board, and then started to dig around to order replacement parts for my "donor" board and to have some spares to be prepared for such disaster, if it will occur again sometime.
CR2, SOT-23 dual diode with M4FU marking, Moto part # 48-80154K03 was solved pretty fast - same Moto p/n is marked as 353L in GTX service manual, and I'm almost completely sure it is MMBD353 (MMBD353LT1), Dual Hot Carrier Mixer Diode. It was out of stock in Digi-Key, but I found MMBD352 there, should work equally well in this place (also dual diode, with same specs, but polarity of each diode is reversed, but it is connected in parallel anyway, so there will be no difference).
CR3, SOT-23 schottky diode with white paint C0 marking (Moto p/n 48-80939T01) taken a bit more time. Now I almost completely sure that it is Avago (formerly Agilent and HP) HSMS-2820.
Concerning cryptic M50X01 low noise bipolar transistor (mine had MH marking, in a style common for Japanese parts) - answer come from one of russian language amateur forums. There was report that MH is marking of Toshiba 2SC3606 transistor, and author of that note already ordered this part and made some repairs with great success.
That's all. I hope this note will save some time for someone.
Success reports and corrections (if any) is welcome.