GM300 VHF low output power

[sailor]

Hello,

I am far from being a radio tech, but I can get by with the basics...

I was given a VHF high GM300 16 channel radio. I was told it was working when taken out of service (but who really knows). I wanted to move this into the amateur band so I immediately programmed my channels and removed a bunch of signalling options. When I tried to TX I was only seeing about 1/2 Watt output. I re-programmed all 16 channels from 144 to 162 and tried again, this time monitoring the SL voltage. It was between 2.7V and 4.6V across the entire freq range indicating a VCO lock. Above 150MHZ the output power was almost 1 Watt but never higher.

Checked the TX Power settings in RSS and it was around 86 I believe. Changes made little difference.

Is this likely an output transistor? Or is there something else I should be looking for? If it is, are they fairly easily replaced, and cost effective?

[com501]

Disassemble the PA from the back of the radio and closely inspect it for bad solder joints before you do anything. Also make sure that the radio has GOOD receiver sensitivity on a service monitor. Common failure is someone stuck something into the antenna connector that spread the pins so it isn't making contact. Check THAT, and look at your PA/drivers for migrated solder or burnt boards from overheating. Chances are, if you find some bad connections around the PA, a simple clean and reflow will fix the issue.

Also, NEVER run the GM300 on full power as a PA for a repeater, the heatsink is inadequate for that usage.

[Bill_G]

When diagnosing low power out from any radio, monitor the current if possible. Low power out AND low current draw suggest an active device failure (ie: xter, module). Low power out AND normal current drain suggest a passive device failure (ie: cap, relay, connector).

[sailor]

Well, I pulled the PA module off and the solder points do still look good and clean.

Radio is pulling 5-6 Amps on Tx putting out between .25 and .5 Watts between 145 - 159MHz.

[WB6DGN]

Also, check the PIN diodes in the T/R switch.