148 final gets hot?

Flash · Post by **Flash** » Thursday 18th Dec 2008, 19:00

Radio Make and Model: Cobra 148 (old style)
PCB Number:
PLL Number: MB8719
Test gear available: pretty much everything
Symptoms: final gets very hot when radio is turned on and not TXing (mic not keyed)

What is working:

What has been tried: The radio had low TX power about 1 watt and very little forward audio swing. Tuning did not help. I noticed someone had replaced the 2sc1969 with a 2sc1964 so I removed it and installed a IRF 520 and related componant mod. (Per Cb Tricks)
The power only came up to about 2 watts still with very little forward audio swing.
While checking the TX chain for trouble and holding the key down for several seconds I could smell something getting hot and saw slight amount of smoke from around the final area. I unkeyed and touched the IRF520. It was very hot and getting hotter even with the mic unkeyed. I turn the radio off and let it cool. Turned it back on with my finger on the transistor and it started heating up without keying the mic.
Removing the drive from the 2166 did not help.
The MB3756 switch is working OK. If I remove the purple and the green 12 supply wires (the ones for bias adjusting) it does not get hot.
I did check the voltage on the 520 and the gate (base) has 4.5 volts in RX mode? I think this is where the problem is but ???

Did it suddenly happen ?: yes
Anything else that may be of help?: While checking the TX chain earlier I noticed the voltage on pin 7 of the AN612 drops from 7 volts to 4.3 when the mic is keyed. All other pins appear normal.

What do you guys think?
Thanks
Dave

timrim · Post by **timrim** » Thursday 18th Dec 2008, 23:03

I think the final is in self oscillation, look at the output on a scope in receive mode.

RedEye · Post by **RedEye** » Saturday 20th Dec 2008, 17:16

Sounds like a problem exsisted prior to the mosfet install. If someone replaced the final before, it most likely was frying them out prior to anything you did. Check for any other "hack" mods that might be causing the problem. jumped diodes, solder bypasses or even a bad solder joint or solder blob crossing circuits.

what are the readings at the final? the driver?