Batboard

I tried something that in theory should have worked but didn't.
Here is what I did and what happened.
I decided that I wanted to be able to turn on my radios (Maxtrac w/W7 head) without needing to turn on the truck.
I had some high current diodes so I stacked two of them cathode to anode.
I fed the ignition feed to the anode of the first, and it's cathode to the second ones anode.
I then connected a switch from a fused constant source to the cathode anode connection between the diodes.
I took the last cathode and connected it to the orange and green wire on the head connector.
The radio would power up, the siren model worked and it would receive. However the radio would NOT transmit.
Tried different cable, still the same. Radio would work on the bench. I removed the diode and wired direct, it would work at that point.
I assume that the either .7 or 1.4 volt drop I was getting across the diodes was being sensed by the radio and not allowing it to transmit.

I am planning to reinstall the diodes and have them switch a relay so that there is not a voltage drop.

Good idea in theory, but not in reality. Once relay is in I will be able to switch full console on and off so it's better anyway.
Another place it occurs to me that this could be a problem is a dual battery setup where the batteries are split with a charge isolator.
It could be a problem if you used the secondary batteries to power the drawer units and the main power for the green / orange wires. (which oddly is what I was working towards doing)

A Maxtrac with a W7 head? That must be an SP build...

You were sort of on the right track with the diodes, but used them in the wrong manner. Instead of putting
them in series, put one to the ignition and the other to your power switch. Put the cathodes together to
feed the radio. This way you have only one diode drop. It would be better if you could find some low
resistance diodes like the ones used in the dual battery isolators. Only problem is that they will be backwards
from how you want to use them. Most of the diodes you find today are silicon. You really want some high
current gemanium ones.

Jim

You shouldn't need diodes. Take a SPDT switch, connect switch common to radio green/orange leads, one contact to ignition switched power, other to constant power.

In position 1 radio is connected to ignition switched power so it goes on with the vehicle, in position 2 it's connected hot so goes on without turning on the vehicle.

Your on the mark Jack. I guess that's what happens when your tired and can't think straight.

Put the common of the switch to the radio. Wire as Jack has instructed.

Jim

jackhackett wrote:You shouldn't need diodes. Take a SPDT switch, connect switch common to radio green/orange leads, one contact to ignition switched power, other to constant power.

In position 1 radio is connected to ignition switched power so it goes on with the vehicle, in position 2 it's connected hot so goes on without turning on the vehicle.

Yep, I should have thought of that myself.

Thanks, I can do that.