465 - 495mhz uhf M120 lost RX?

[motoguy]

gday all i have this uhf m120 radius that i planned on using for a repeater, somehow during the last 6 months sitting around it has completley lost all recieve? it cannot even recieve a signal from a radio rigth next to it? anyone have any idea what could be causing this and if its a common problem/fix ?. thanks to all who take the time to read this post and to help me out getting this radio working again.

Motoguy

[motoguy]

ok im not sure if this coudl be a problem, but i can hear when i transmit with a handheld rigth next to the unti and that is all, ive gone in to the board replacement menu in the software and the crystal setting wich are in there they are all set to zeros? is this correct? can anyone tell me the correct details wich should be in there. cheers.

[Satelite]

Hello :
Crystal values are entered during a setup and as i recall if you go back into service menu it does not actualy read the radio crystal info entered previously so it would not give the entered crystal info from earlier setup.
So id expect to see the zeros.
Does not mean the crystal is good but id qeuss its not the culprit.
My bets on a defective Y52 crystal = 44.645 mhz rx oscilator crystal.
If you need a scan of it on the bd PM me your email address and ill email it to you.
Satelite

[jackhackett]

You need to determine whether it is receiving off frequency, or if it has very weak receive. If you can, measure the TX freq, if that's on the RX should be too.
If it's on freq but has very poor sensitivity, it's most likely the RF front end or the PIN diodes in the PA. A bad 44.645 crystal usually causes no receive at all, but it's a remote possibility. There's also the feedthru connector between the RF and PA boards, they have a tendency to go bad, usually the symptom of that is an off freq radio.


edit: One other thing... make sure someone didn't unplug the RX cable that goes from the PA to the RF board, hey.. it could happen

[motorola_otaku]

Where are the pin diodes located? I have a VHF GM300 with good TX but absolutely no RX. Tried re-soldering the interconnect connections with no joy.

[RADIOMAN2002]

What I have found especially with the GM series radios, is if there is a high SWR for any reason at any time, it blows out the PIN switching diode in the front end of the receiver board.

[motoguy]

i shall test the TX and adjust that accordingly via the warp in the software and il get back to you.. i planned on using this radio for a repeatrs RX .. but aTM its lookin prety dismal! il get back to you .Cheers

Motoguy

[jackhackett]

Don't adjust the warp if it's more than a few hundred hertz off. The freq being off by around 1khz or more is a sign of a bad feedthru connector (do a forum search, it's been discussed quite a bit).

[motoguy]

ok tested the TX side of things, it transmits fine right on freq.. the RX is there but very very very deaf..... cheers

[jackhackett]

Try injecting signal into J4, the jack where the receive coax from the PA plugs into the RF board, that will tell you if the problem is in the PA or RF board.
If you still get poor RX there, I'd check CR2 on the bottom of the RF board next, that's the diode array that protects the front end, check across it with the diode function on a DMM (it's capacitively coupled so you can do it in circuit, it's 2 back to back diodes so it'll show a diode drop in both directions, if bad it'll probably show a dead short.