M120 - No Transmit

Taft · Post by **Taft** » Tue May 30, 2006 9:29 pm

Hi. Please help me identify the cause of my m120 problem. There is power to the transceiver but no transmit. It indicates zero power at all thru the power meter but if can receive. I'm thinking it might me the final transistor. Can somebody please tell me where can I find the board schematic diagram for M120. Thank you!!

AEC · Post by **AEC** » Tue May 30, 2006 10:14 pm

What's the model number of the radio, and do you know what the output power is supposed to be?

Many are 25 watts, but 45 watts is also very common, both take different power transistors, as one is a flange type(45 watts) and the other is of the stud variety(25 watts).

If you see a stud with a nut on it poking out from the rear of the heatsink, it's probably a 25 watt radio.

Let me know and I'll get you the numbers of both devices.

AEC · Post by **AEC** » Tue May 30, 2006 10:20 pm

To add to this trouble, did you run the service menu in the RSS after you programmed it, to make certain the levels are set properly?

If no RF is being produced, you may have a VCO out of lock, but since you mentioned you can receive you still may suffer from a VCO out of lock condition, or the ref. osc. is bad.

I'll have to check the manual and see what's what first here..

There are other tests you can perform to check each stage in the radio and many only require a meter to check a.c and d.c voltages in both transmit and receive.

Remove the P.A heatsink and shield and check for B+ in transmit on the collector of the P.A, it should be at or about 13.6 volts d.c

With an RF probe, you can check for RF at the driver stage of the amp as well.

wa2zdy · Post by **wa2zdy** » Wed May 31, 2006 4:47 am

Does the receiver mute when you push the PTT button? Does the transmit LED illuminate? Do you get a warning tone when you push the PTT? Are you sure the channel isn't programmed for receive only?

Rayjk110 · Post by **Rayjk110** » Wed May 31, 2006 10:06 am

I know there is an option to have the GM300/M120/M10 Line of radios in the RSS to have the radio setup in "Repeater Mode". I've got a repeater composed of 2 VHF radios, (M120 + M10) and I know both are setup this way. I will try later today and key the RX radio to see if it has the same diagnostics of yours; possibly indicating it may be wrote as the RX radio in a repeater setup, as those radios are commonly used as such with the GR300/500.

Taft · Post by **Taft** » Wed May 31, 2006 6:53 pm

Thank you all for the response. To answer AEC's question: The model number is M44GMC20A3AA. And yes there is stud poking out from the heatsink with a nut. I haven't done any of the measurement first since I want to make sure it is not a transistor issue. To WA2zdy and RayJK110: The transceiver is being used as a link radio or sometimes to access my local repeater. This had been running for the last 12 months or so with no problem until it stops transmitting 2 days ago without doing anything (either reprogram or modify). Thank you again.

jackhackett · Post by **jackhackett** » Wed May 31, 2006 8:29 pm

The stud mount transistor doesn't tell you it's a 25W unit (as we can see from the model number it's a 40W), they use a stud mount as the driver and a flange mount as the final in the 40W radios.

Any one of a number of things could cause you to suddenly lose transmit power, a few are:

The solder joints on the final getting too hot and melting is a common one, you can usually tell that one just by looking at it, sometimes the joint just looks grainy, sometimes there are blobs of melted solder shorting things out, sometimes it looks blackened and burned and the chip caps have come unsoldered.... and sometimes it looks fine, but resoldering it fixes it anyway.

Another possibility is the PIN diodes used for T/R switching, these seem to like to quit working right after a nice lightning storm (it is getting to be that time of year after all)

And it could be just good old component failure in any of the transmit stages.. how are you set up for test equipment?

Taft · Post by **Taft** » Sat Jun 03, 2006 10:11 pm

I only have a DVM and SWR/Power meter (SX-400).

jackhackett · Post by **jackhackett** » Mon Jun 05, 2006 8:51 am

Can you measure the current the radio draws when you key it?

Taft · Post by **Taft** » Mon Jun 05, 2006 8:53 pm

It's almost nothing. I could see that from my power supply since I have a voltage and current meters.

jackhackett · Post by **jackhackett** » Tue Jun 06, 2006 7:31 am

Ok, so when the PIN diodes go bad it will usually still draw current on TX, it just doesn't pass the RF out to the connector, so we can probably rule that out..

You could check the voltage to the controlled stage, that will be on the orange wire from the radio, probably the easiest place to check it would be on L2722, which is a coil right between the stud mount transistor and the smaller transistor before it, there should be a ferrite bead on on leg of that coil.

Also the yellow wire is 9.6volts on transmit, you can check that on L2714 right next to where the yellow wireconnects to the PA.

Other than that you'd either have to have some way of measuring RF from on the various stages, or shotgun it.. replace parts until it works (not recommended when you're dealing with things the price of PA transistors)

Will · Post by **Will** » Tue Jun 06, 2006 1:19 pm

Jack is correct, you do NEED a RF voltmeter to measure the low levels from the exciter/VCO stages to the controlled amplifier. The controlled RF amp is the first transistor on the PA assembly and they are known to fail.

But as Jack said, the cost of the RF transistors is way too much to be replacing them until it works.