Question on 148F GTL output

[Hawkeye351]

Got a friend that's constantly having an issue with his 148F GTL dropping all output power in AM.

I've gone through this radio twice already.

The bias caps were dried out, so I replaced those.
No mods whatsoever.
Changed final with known working one (2312).
Changed driver with known working one (2166).
Changed TR41 passthrough regulator (1135) to a 6487.
Replaced Bias VR's.
Replaced all 10v blues caps.
Checked antenna input.
Checked for cold or loose solder joints.
Checked for loose connections.
Set driver and final bias.
Set carrier to 3w.
Set modulation to 100% mod.
Realigned radio completely.
Left it turned on for about a week.
Talked on it every once and a while.
Noticed after talking on it for 45 minutes to an hour, the output power just drops completely out. The transmit light still lights, but no output. Flipping CB/PA has no affect. But, flipping the mode control from AM to either sideband and keying up with modulation for a brief second brings the output back, then I can switch back to AM and talk for a while until it does it again. Sideband I think does drop out also when this happens on AM, but flipping from AM to SSB and keying up with modulation brings the output power back. Mode control doesn't seem scratchy, harsh or course when turning, and makes no noises when turning. So, I'm guessing the mode control is fine. Nothing in the radio seems to be getting hot, maybe warm, but not hot.

Any suggestions guys?

 

[Shadetree Mechanic]

Could be that the board has a crack with a broken trace. My Madison had intermittent transmit, and that's what I found.

 

[loosecannon]

make a map of the TX voltages that go through that mode switch in each mode.

note the source, the specific switch contacts and the components the voltages are sent to from the switch.
now put the radio through its paces until the TX falls out, and use your map to see which voltage isn't making it back to the PC board.

I have seen these mode switches crack before and cause issues like this.
LC

 

[sp5it]

Take RF probe and locate where power disappear.
Mike

 

[nomadradio]

If you can 'sniff' the 10.695 MHz internal carrier, I'll guess that maybe it is cutting out?

Maybe.

When the transmit drops out, is it still receiving on the right channel? If the PLL is dropping out of lock, that will kill your transmit power. A "lazy" crystal in the PLL may sometimes kick back in and start running in response to clicking the selector between mode. If you can eavesdrop on the PLL's mixer crystal while the radio is getting warmed up, this would reveal if it's dropping out. The fact that changing modes brings it back suggests a crystal issue of some sort.

73

 

[Hawkeye351]

Sorry for delay, had a few others to catch up on.
Getting back to this one in the morning.