Batboard

I read this thread, but unfortunatly there are few clues:
http://batboard.batlabs.com/viewtopic.p ... x+repeater


Well I found an active low COR tap point. But I'm still having a world of headaches trying to make a bidirectional repeater between a UHF maxtrac and a 900 MHz GTX.. Works just great one way. Where as anything on UHF comes out on 900 MHz.

Basically I'm using a pair of 2N3906's much like:
http://www.batlabs.com/maxrpt.html
http://www.batlabs.com/images/maxrpt.gif
(as a matter of fact the UHF maxtrac has been modded for this)

I hi-jacked the flat audio out from the GTX off the 16 pin accessory jack and connected that to the Mic Audio In on the Maxtrac.

I also tapped the point depicted in gtx900a-COR.jpg, which is active low, same deal thats going on in the Maxtrac and cable with 2N3906's.

Transmit stutters on the maxtrac when a signal is comming in on 900 MHz. Futher more I think that the Audio PA tap point is getting loaded down as it passes no audio when I force transmit.

Has anyone done anything like this?

Here is my loose collection of 900 MHz modification notes:
http://www.qsl.net/kb9mwr/projects/900mhz/plan.html

The CSQ signal that comes out of the radios can either go to ground with a received signal, or go to a source of positive voltage. On your GTX900a picture, which is MINE by the way, that point would need to go through some form of isolation/driver transistor before going to a radio's PTT line.

The PTT line on MaxTracs and GTXs needs to go to ground to allow the radio to transmit. If the COR line goes high with a received signal, a simple NPN transistor will do the job. That would be a 3904 part. If the COR line goes low, then you could use a PNP transistor (3906) by grounding the collector and pulling the PTT line low with the emitter. I believe this is what the circuit elsewhere on BatLabs is doing.

Of course, there's no guarantee that the COR signal won't change when the radio goes into transmit, so if you're trying to make this bi-directional, there could be problems. For a uni-directional repeater, this wouldn't be anything to worry about.

The audio may or may not be enabled on the GTX during transmit. It would need to be if the radio times out so it can sound the timeout beep. I don't recall if I measured that point during transmit - you probably should. Also, make sure it's going all the way to ground when receiving, and if not, you'll need a different isolation circuit.

Bob M.