SBE Trinidad III blows fuse

[Plumber-G]

Hello
I just got a SBE Trinidad III when I plugged it in the transmit was a little iffy, work sometime sometimes it wouldn't, thought I would change the microphone to see if it was the problem, sparks flew and I blew a fuse.
Now when I connect any mike to the radio it blows a fuse (low voltage fuse) Can't find a schematic for the Trinidad III and the Trinidad II uses crystals and the III has a PLL Board. Any suggestions on a similar model? I could get a schematic for or any Ideas what could cause the mic to blow fuses,

Thanks
Plumber-G

 

[Dmans]

Just hazarding a guess-transmit/receive relay shorted??
mike not wired properly??

It seems nomadradio posted a schematic and/or a service manual recently for an SBE but not sure if it was this model or not??

Good luck.

73’s
David

 

[nomadradio]

This is one of SBE's radios that run a hot wire up the mike cord to the PTT switch. The transmit wire goes hot when you press the button, feeding back into the mike socket.

Any mike that connects that wire to ground should blow the fuse. The receive pin of a mike wired for a Cobra radio will short the 12 Volts to ground.

Seems to me we ended up removing the hot wire from pin 4 of the radio's mike socket on those, connecting it to the emitter of a PNP transistor like a 2N2907/PN2907A. The transmit wire comes off the mike socket pin 3 and goes to the collector. A 10k resistor goes to the base leg of the PNP. The other end of the 10k goes to pin 3.

Pin 1 is already ground, and pin 2 is already the mike audio.

Should work with a Cobra mike after that.

Schematic is here. http://www.cbtricks.com/radios/sbe/sbe_29cb_catalina_iii_malibu/index.htm

73

 

[Plumber-G]

NoMadRadio

is the drawing correct?


Thanks
Plumber-G

 

[nomadradio]

Nope.

Two wires have to come off of the inside of the mike socket.

The pin 4 wire goes to the emitter only. Is that the "RW" wire?

The pin 3 wire goes to the collector only. I don't know what color wire gets removed from pin 3.

This leaves both pins 3 and 4 empty on the mike socket for now.

The 10k resistor gets one end to the transistor base, as you show.

The other end of the 10k goes to pin 3 of the mike socket, and nothing else.

Mighta been faster to draw than describe, but there it is.

73

 

[Plumber-G]

I got it !

Think substituting a 2n3906 would work OK?

Thanks for the help

 

[nomadradio]

That one should do the same job.

Lotsa small PNP transistors that will work.

73

 

[Plumber-G]

I completed the mod to allow use of a cobra wired mic today on the Trinidad III as described above. I am having a problem I am not familiar with.

When I key up with No modulation my carrier has ripples in the output ,( seen on O Scope)
The output varies up and down on the watt meter I suspected the power supply so I fired it up using a 12 volt power supply with the same result. It looks like it is modulating or oscillating, in fact the modulation light blinks on and off.

When I apply a 1K audio input great modulation, Power stable

Could this be caused by the newly installed mod??

Appreciate any suggestions

Plumber-G