GM 300 Monitor light on all the time

[Daniel Watson]

Anyone have this problem and if so, what was the fix? Thank you

[EKS]

The radio thinks the mic is off-hook.

[perthcom]

Hi
Generally its the microphone that is faulty, try swapping mics with a known good mic (ie MON light goes out when the mic is hung up)

There is also the obvious problem that just pressing the MON button while the mic is hung up, will fix it.

Also check the Programming for the accessory connector. Do you have a remote control or anything hooked to the accessory connector that could be forcing an Off-Hook condition? If you do not require Hook-Switch through the accessory connector, then be sure this function is turned off in the accessory connector configuration.

I'm not sure how much troubleshooting you have done so far, but those are a start.

[Jim202]

Have you measured to see if the hangup clip is grounded?

Jim

[jackhackett]

gm300s don't need a grounded hang up clip, they use the clip to complete a connection between the hang up button on the mic, and the contacts behind the button. These go bad a lot, you can often see a spring sticking out from underneath the contacts.

[Daniel Watson]

Mic and ground, front housing board, all of this checked and re-checked and still monitor light on all the time. Not encoding/decoding. My other GM300's & M120's monitor light goes out when mic returned to mic hook or base mic in use. This GM300 has monitor light on all the time. Is there another item to check besides all the obvious that I have??? Thanks

[Nand]

http://batboard.batlabs.com/viewtopic.php?t=13566

See if this will help.

Nand.

[kcbooboo]

Is the radio really in monitor mode, or is it working fine (i.e. requiring PL/DPL) and just the light is on when it shouldn't be?

If you hold the monitor button down for a few seconds, does the squelch open up, and does the monitor light flash on and off? I'm trying to figure out whether it's the input to, or the output from, the logic board that has the problem.

It might be time to open the radio, and armed with a schematic, check the monitor signal going from the mike jack to the logic board. It should be pulled up to +5 (or higher) volts. This signal might go through a transistor or two before being applied to the microprocessor, but you can trace it with a voltmeter almost all the way.

The logic board sends a signal back to the front panel to illuminate the various LEDs. It's possible that the front panel driver is bad, or the line feeding it open or shorted. Lots of possibilities here.

Bob M.

[Daniel Watson]

Bob

The monitor button works normal in regard to holding it down and opening up the squelch. Depress it and it silences. The radio transmitts the code programmed but the squelch will open with or without PL code transmitted just like it should do with the monitor light on. I have tried a different front housing from another working GM300, different Mics, switched jumper positions and removed jumper, jumped pin 14 from 16 pin connector and nothing stops monitor light. ???? thank you

[Nand]

Nand
The model number of radio is M34GMC20D3AA (25 Watt 8 channel). The number of the board is HLN 8074A, also 84D80489B03. Not sure what you mean collector but where the Q811 transistor was (now removed) 2 of the three solder points measure 4.99 volts and the 3rd one measures 0 with Mic on or off hook. Thank you for trying to figure out this mystery.
Dan

First of all, let me make sure that I understand your problem correctly.
1. The monitor light stays on all the time, no matter what you do.
2. The radio receives transmissions from stations that have no PL or a PL different from the one you programmed in the radio, no matter if the mic is on hook and the radio is not in monitor mode.

This only happens if the microcomputer is told to do so with a low on pin 9 (not high enough) from the collector of Q811. The transistor Q811 inverts the signal from the mic connector hook switch.

Q811 is the lowest in a column of three items in the left bottom corner of the radio. The pin facing towards the front is the collector. It feeds directly into the microcomputer. The left one of the two remaining pins is ground and the right pin is a transistor base connection that is pulled high through a resistor and connected to the mic connector hook switch pin.

Normally when the mic is on hook, the base is at ground and the collector at +5 volts, indicating an on hook condition. In your case, with the transistor removed, it also indicates an on hook condition to the microcomputer because the line is at +5 volt level as you measured, unless the trace is open. This rules out a problem with transistor Q811.

The collector of Q811 is directly connected to pin 9 of the microcomputer. That is the second pin from the left in the row of IC contacts facing the front of the radio. This pin should also measure 5 volts at this time. It is routed to the collector of Q811 through two plated through holes in the circuit board. You may have to check these.

I hope this gets you in the right direction.

Nand.

[kcbooboo]

You didn't say if the MON light goes on and off as you press the MON button. There are three states that the MON circuit should go through:

1. MON light off, radio squelched, PL/DPL required to open it.
Press it once to go to state 2.
2. MON light on, radio squelched, just a carrier is required to open it.
Press it once to go back to state 1, HOLD it down to go to state 3.
3. MON light on or blinking, radio unsquelched.
Press it once to go back to state 2.

If you just press and hold it to open the squelch, then press it once more, you should be in carrier squelch mode. It would take one more press to get you into PL/DPL mode.

With a ground fed into the hang-up pin on the mike jack, Q811 should be turned off and the signal going to the microprocessor would be high. With Q811 removed, it seems like the radio should think the hang-up switch is permanently grounded, so PL/DPL should always be required. The voltages you are reading seem correct with Q811 removed since the emitter is grounded, and the other two signals are pulled high. However, with the hang-up switch grounded, you should see TWO signals at 0v: the emitter and the base of Q811. The collector would then be at +5 volts.

Almost sounds like the previous owner might have had a problem in this area as well and was trying to figure out what was wrong.

I have a GM300 manual with that board's schematic in it. If you would like, I can scan some of the important areas and send you .JPG files via e-mail. Let me know via PM if you would like this, and also give me an e-mail address to send them to.

Bob M.

[Nand]

Bob, the title of the original post indicates that the monitor light is on all the time.

Nand.

[kcbooboo]

Sorry, that little fact slipped out of focus. This will require a re-think.

[kcbooboo]

After thinking about it for a while, the first place I'd start is at the connector on the logic board that feeds the Monitor LED circuitry. See if it's changing state at all. If it is, then the front panel has a problem. If not, then I'd go to the microprocessor and check the signal there. If it's changing state, then some driver component on the logic board is shot. If not, then there's either a microprocessor problem (nearly impossible), a code plug problem (likely), or a programming problem (also likely).

There's not much one can do except replace the logic board if the uP is shot. Someone could have shorted one of its lines to +5v or higher and blown that output pin away. That could account for everything else working fine except the monitor light. But if the squelch still won't go into PL/DPL mode, this probably is unlikely. Code plug could have some bit set that RSS isn't capable of changing, unless the board is blanked and re-initialized. Maybe some option setting is affecting the squelch.

That's as far as my mind will take me at this hour. The offer still stands if you need parts of the schematic sent to you.

Bob M.