no audio or beep on Radius Mobile

Mikey · Post by **Mikey** » Fri Sep 22, 2006 2:22 pm

I have a Radius mobile radio that when powered on i cannot here the power up beep; also cannot here any audio out of the internal speaker or an external speaker hooked into the back. This has a 5 pin logic board so a jumper is not the problem, i've reseated the logic board and checked the internal connnector for the internal speaker. any ideas as to what the problem is. This just started doing this, it was working fine.

Mike

kcbooboo · Post by **kcbooboo** » Fri Sep 22, 2006 3:21 pm

The volume control could be bad. If you hit the knob hard enough, it will push the control into the radio and make it non-functional. The original pots are no longer available, but a close replacement can be bought, and with a few minutes work drilling some new holes, it'll be as good as new.

A broken wire going to the MIC/VOL board in the control head, or on the connector that plugs into the logic board, can also make the radio quiet. The speaker could have blown out too. A couple of quick tests with an oscilloscope will tell where the problem is.

Bob M.

Mikey · Post by **Mikey** » Fri Sep 22, 2006 4:42 pm

I thought of the speaker going bad, but wouldn't an external speaker plugged into the 5 pin acc. connector on the back remedy that problem?

kcbooboo · Post by **kcbooboo** » Fri Sep 22, 2006 4:53 pm

Yeah, you're right. I missed that little point.

Obviously "something" is bad. Main speaker output capacitor, transistors, volume control, logic board, radio. Time to get the schematic, a scope, and trace the audio through the system.

Bob M.

kf4sqb · Post by **kf4sqb** » Fri Sep 22, 2006 8:46 pm

Mikey, what's the possibility that one of the speaker leads was accidently shorted to chassis ground? If that has happened, the audio amplifier IC is fried. Bob can correct me on this if I'm wrong, but it's a SIP IC, bolted to an aluminum heatsink, on the RF/Audio side of the radio. If it's blown, you will likely be able to see physical damage to the IC, like a crack in it's case, or I've even seem chunks blown out of some.

kcbooboo · Post by **kcbooboo** » Sat Sep 23, 2006 2:56 am

Radius and MaxTrac radios have a 1000uF capacitor coupling the speaker to the audio amp, which is made from discrete transistors screwed to the heatsink on the logic board. So shorting the speaker normally would not cause catastrophic destruction.

However, applying +14VDC to the speaker output (negative end of that capacitor) will cause the cap to get very hot and explode. Other damage is likely once the cap says bye-bye. Happened to someone I know.

The GTX, GM300, and Spectra have direct-connected ICs that drive both sides of the speaker. Grounding either side can/may/probably will let the lifetime supply of smoke out of that IC.

A schematic and a few quick probes with a scope is the easiest way to isolate the problem, short of replacing the logic board or radio itself.

Bob M.

kf4sqb · Post by **kf4sqb** » Sat Sep 23, 2006 6:40 am

Bob, that's strange, I've repaired a few GM300s that had their audio chips blown due to a shorted speaker lead. But, stranger things have happened.....

jackhackett · Post by **jackhackett** » Sat Sep 23, 2006 1:10 pm

The logic boards with the 5 pin acc. connector use the discrete transistor amp. That's how they can get away with using ground for ext spkr -

Locate the capacitor on the logic board, it's the long axial lead one near the acc. conn.

It's usually pretty obvious when it's blown, if it looks normal carefully measure the DC voltage on the end towards the front of the radio, watch that your probe doesn't short to the chassis. I believe there should be around 6-7 volts there. If that looks okay I'd look at the volume control. You can check that with an ohmmeter with power disconnected from the radio. Between vol cont top and vol cont wiper you should show a low resistance with the control turned all the way up.

If the pot is bad you can get a replacement one from Moto, but as Bob said the ones they have now won't fit the older boards, you have to drill new holes, and the on/off switch is wired differently so you have to jumper two of the pins together. If you get to that point let us know and one of us can give you detailed instructions.