Repeater Circuit

[Tom Line]

Been experimenting. Anybody familiar with what schematic would be close to what's hidden inside this cheap little Chinese HT repeater box? There's no documentation, and it's glued into plastic. Seems to key mic with zero ohms on the base of the plugs by presence of audio signal. There's a pair of coils, transistors, and some caps for sure. The rest is a mystery. It likes to get stuck easily in xmit mode for sure; not a loop, one HT will just get stuck like the audio/mic cables are picking up RF and thinking it's audio. Not too reliable but kind of nifty. (Please don't bore us bashing it, just trying to understand what's inside here.)

 

[brandon7861]

The transformers are probably impedance matching the 8-30Ω headphone audio to 600ohm for the mic input and the transistors and capacitors probably generate a DC signal from the audio to activate PTT. Maybe the circuit looks something like this???

 

[Tom Line]

The transformers are probably impedance matching the 8-30Ω headphone audio to 600ohm for the mic input and the transistors and capacitors probably generate a DC signal from the audio to activate PTT. Maybe the circuit looks something like this???
View attachment 57559

I bet that's exactly how it works. I found that transmitting cts tone to the system helps it kick into transmit quicker so it won't chop off the first word spoken. Volumes have to be a 100% or the PTT goes to 50 ohm or so not 0 ohms like it should to trigger the mic. It has a tendency for one device to get stuck on transmit. Been experimenting with different antennas and frequencies, slugs on the wires, etc. Being real careful not to cause interference. Tried slugs on the wires last night but didn't help much. Shame they poured glue in it.

 

[brandon7861]

I'd bet the reason it sticks in transmit is because the wire is picking up it's own RF and that is also being rectified by that detector diode which holds the PTT to ground.

Since it is filled with glue, maybe you could try adding a capacitor across the audio inputs at the box (not at the radio, it has to be after the RF is picked up and before the diode) to ground.. The capacitive reactance would have to be high to audio and low to RF, perhaps 330pF. That would present a 3Ω path to ground for RF at 2m while presenting 160kΩ to 3kHz audio. The other option is to try a ferrite on the entire cable right at the radio. If you can melt/dissolve out the glue, maybe you can put in a slower diode or maybe alter the RC constant in the detector section so that it is less responsive to RF.

 

[Rwb]

Been experimenting. Anybody familiar with what schematic would be close to what's hidden inside this cheap little Chinese HT repeater box? There's no documentation, and it's glued into plastic. Seems to key mic with zero ohms on the base of the plugs by presence of audio signal. There's a pair of coils, transistors, and some caps for sure. The rest is a mystery. It likes to get stuck easily in xmit mode for sure; not a loop, one HT will just get stuck like the audio/mic cables are picking up RF and thinking it's audio. Not too reliable but kind of nifty. (Please don't bore us bashing it, just trying to understand what's inside here.)

View attachment 57518

View attachment 57519

i like that you said dont bash it.might be 1 thing but when it works its ok. i agree with you 100%

 

[NZ8N]

That looks like epoxy.

Amazon product ASIN B00THUUVSU

 

[Tom Line]

When it gets stuck in a transmit loop, unplugging the unstuck radio doesn't stop the loop. I tried some filters at the middle and ends of the wire with no effect. However, when they are right next to the box, the thing seems to work pretty well.