PRESIDENT MADISON HOME BASE, SSB OK, NO FM-AM TX

[arnoldlee]

Initialy the TX on FM was ok (for a count of 10) then it would die, but SSB still worked. I thought the output DRV & Final were dodgy so I changed them, the same happened for a while, until after constant checking, into a dummy load, it wouldn't key up at all, still OK on SSB though. A friend has the same problem on his Cobra 148.
Pick your brains time!!!

[timrim]

Check solder connections and check the regulator.

[Rick]

What is the voltage on the Base, Collector, and Emitter of TR41 in AM mode TX.

[arnoldlee]

The transistor in question would be the 2sc1419?
The tx voltages are Base:-7.33...Collector:-11.8..Emmiter:-6.3The rx voltages are Base:-7.63...Collector:-13.00 Emmiter:-12.4

Question--Should the collector be bolted directly to the heat sink or isolated?

Bob

[Rick]

TR41 it should be isolated from the heat sink.
The voltages you have given are about right, but the Collector is a bit low it should be at supply voltage of 13.8 in RX and should drop to about 13.10 in TX.

When the fault occurs, check the voltage at Pin 5 of IC5 TX Mixer chip and the voltage at the Base of TR36, 38, and 39.

[arnoldlee]

Thanks for the rapid reply, The collector wasn't isolated when this started, so I rectified that, it made no difference. I don't have any info on the rigs voltages in general so I can't make any decisions when it comes to checking the other ones. I believe the Transistor 2sc1419 can be replaced by TIP31A, and they are both PNP!

[arnoldlee]

Even though the transistor was showing correct voltages it failed under load. We are talking the 2sc1419, so I borrowed one from the rigs mains power supply and hooked the rig up to the DC Power supply, EUREKA!
Now I'm hoping the equivalent is TIP31A because the originals are not available as far as I'm aware.
Thanks again for your input.
Bob

[Rick]

2SC1419 is an NPN and not a PNP, and yes you can use a TIP31A which 45 volt at 3 amp.