Spectra Repeater

thehead7 · Post by **thehead7** » Tue Oct 28, 2003 11:01 am

I'm making preparations to build a repeater out of two spectras... At this point, I can see a few issues:

1) Preselector: I'm trying to figure out how to get around the preselector issue. I want to rec at 902.???. I have heard rumors that the preselector can be retuned with a stripline modification, but I haven't found any actual data on it...

2) VCO: Not too big an issue. Just takes some fiddling...

3) Audio, COR, PTT: According to the info on this site, it is possible to have flat audio passed to the accy connector. It would also seem to me that there is someplace where you could pull a squelch output. Since I will be running DPL, et cetera, I don't want actual COR, but the actual squelch (including tone squelch) output... Make sense? I want to be able to put this output on one of the pins of the accy connector (perhaps one of the VIP pins?) If anyone has any info regarding this, please let me know. Also, the specific modification required to pass flat audio and data PTT to the accy connector...

I'm kind of obsessive about how I do stuff like this. I intend to have everything done as neat and clean as possible, therefore I don't really want to have a cable draping from the front of the TX where it uses the mic connector, et cetera. Also, it makes a lot more sense to me to use flat audio, as the entire project is based on motorola 900 radios operating within commercial spec (except frequency, of course...) Hopefully, when I get done with everything, it'll all be in a nice, neat, 4U rackmount enclosure with pretty little lights and buttons... (We'll see...)

I'm hoping to elicit as many comments as possible, especially from the "devil's advocates". This repeater will be used primarily for emergency communication coordination, so I want it to be as reliable as possible...

Thanks,
Head

batman21 · Post by **batman21** » Wed Oct 29, 2003 4:13 am

Maybe rip out preselector and just use your bandpass cavities for the front end filtering? me and some friends are in the process of trying this with a 2-GTX repeater but it's not on air yet.

thehead7 · Post by **thehead7** » Wed Oct 29, 2003 6:39 am

batman21 wrote:Maybe rip out preselector and just use your bandpass cavities for the front end filtering? me and some friends are in the process of trying this with a 2-GTX repeater but it's not on air yet.

hmmm... That's a thought. The thing is, I'm going to probably end up with a set of "mobile" duplexers. I think I still might be able to get good iosolation. I might give that a try....

Okay, here's a question: What is the peice of coax that goes from the VCO to the preselector? I understand that it facilitates tuning of the receiver, but how? In other words, how do I connect it to the RF board? Would just tying it in with something resembling a "t" work? I don't have the service manual, and I haven't looked at it yet to trace it...

Thanks,
Head

batman21 · Post by **batman21** » Wed Oct 29, 2003 7:06 am

I dont have the manual in front of me but I can try and look tonight unless someone answers before then. I suggest you get the manual though.

The other stuff is fairly easy, like you said. The vco can be pulled without too much trouble and the other stuff is either on the back or can be brought though to there.

thehead7 · Post by **thehead7** » Wed Oct 29, 2003 7:54 am

Yeah, I figured out how to pull the VCO without taking apart the rest of the radio (Cuts a few minutes off my mod time... It's worth it...)

Surgical instruments work great for electronics work... Haha...

I intend to get the service manual, but I think if I order it from M it will a) take too long to get here, or b) be really expensive to get it here fast...

I guess I should give them a call this afternoon...

Thanks,
Head

John G · Post by **John G** » Wed Oct 29, 2003 8:21 am

The coax coming from the VCO board to the frontend module is the local oscillator injection. It pulls out of the VCO board to allow the frontend module to be removed. The preselector can be bypassed without too much difficulty but you would need to have some sort of preselector between it and the duplexer. Especially if it is the mobile type which are usually only notches. I have a Spectra in front of me now, torn apart so I could look at options for the frontend board. Eliminating the built in preselector is the easiest if you can use an external preselector. I am actually considering doing a whole new front end board with Murata filters, a Minicircuits mixer, and low noise preamp. The main problems are mechanical.

thehead7 · Post by **thehead7** » Wed Oct 29, 2003 11:29 am

I would think about the same, but my SMT skills are a bit lacking... I think setting it up externally would be the way to go for me. Perhaps the thing for me to do is to figure out how to defeat the internal bandpass filter, and merely add an external one as well as a preamp...

John G · Post by **John G** » Wed Oct 29, 2003 12:24 pm

Remove the cover over the frontend module. With the module nearest you, there is a shield that covers most of the right side of the ceramic substrate. In the upper left, you will see a printed transmission line that seems to meander from a capacitor left and end in a solder blob. Remove the module and with a good soldering iron, heat that blob. You should be able to pull the wire that goes through the substrate to the preselector out. When it is out, solder the center conductor of some sub miniature coax to the point where the transmission line ends. The shield will solder to the copper looking ground area of the substrate nearby. You can route this outside the case or mount something like an SMA or BNC through the side of the case and terminate the coax there. There is a strong caveat here. The Local oscillator injection is connected to this same place. If this isn't terminated in 50 ohms ot there about, you could kill the L.O. level going to the mixer and degrade the sensitivity. The Spectra mixer is unusual in this respect and for that reason, I don't particularly like it. Good luck. Let us know how this works. Incidentally, there are a good many filters floating around these days that would make a good preselector.

thehead7 · Post by **thehead7** » Wed Oct 29, 2003 12:54 pm

Thanks!!

I'll give this a try, hopefully, tonight. Thanks for the detailed info.

-Head

thehead7 · Post by **thehead7** » Thu Oct 30, 2003 7:36 am

Okay, I decided to experiment a little bit before I completely wrote off the pre-selector and bypassed it. I pulled off the shield on the right of the module and fiddled around with silver paint. I ended up putting a trace accross the middle of all three traces under the sheild. That seemed to do something... I am almost positive that in the process I have a) reduced sensitivity, b) made the filter too wide, c) probably allowed everything that I don't want to get in, in.

At least it gives me something to play with for a while...

My next obstacle is the squelch. I don't want true COR, per se. I want to find an output that goes high when squelch AND tone are received. I don't want to use an external tone board... Given this, I cannot simply get my COR output off of the "busy" led...

I thought I had seen a post about this, but I cannot find it now...

Thanks,
Head

John G · Post by **John G** » Thu Oct 30, 2003 9:05 am

The filter under the shield on the right is the injection filter. I haven't actually swept it yet, but it is probably wide enough as it is. The preselector is underneath and runs the length of the module. If you want to play with silver paint, this is the place to do it. The problem is that you can't get access to the bottom of the filter while it is installed in the radio. I am working on a fixture to allow it to be swept while outside the radio.

thehead7 · Post by **thehead7** » Thu Oct 30, 2003 12:06 pm

Hmm... I knew it was too easy...

I did notice better sensitivity, though. Of course, my means of measuring this was a handheld that I had lowered down to minimum... I know, it's about like trying to measure lumber with a car odometer...

I really need to get my hands on some test equipment... Maybe I should look at the local CC. I wonder if they'll let me just take a lab, without the class... HAHA I don't think they have the gear, anyway...

Thanks,
Head

thehead7 · Post by **thehead7** » Fri Oct 31, 2003 11:25 am

Okay, I feel stupid now...

I found where to get channel busy indication and PL/DPL detect... After I found it, I found the information on the board here...

For those who'd like the info here and now: The channel busy indication can be had from pin 10 of the audio amp chip... This puts out b+, so it will probably have to be converted to TTL for use... PL/DPL detect is on the connector coming from the command board to the RF board... It is on the bottom row, fifth pin from the right, with the faceplate facing you...

I intend to logically AND these together and use this on the COR input of the controller...

It also seems that (in at least all of the several radios that I've checked) they are already set up to pass detector audio and data ptt to the rear connector... I haven't checked for TX DATA IN yet, but I assume it is there...

So, I guess I just have to get my pre-selector finalized and get a controller and cans... After that, I can start work on my 21 element collinear with about 10dBd gain...

Regards,
Head

ALF 935 · Post by **ALF 935** » Thu May 13, 2004 5:37 pm

my secret???????
That pin 10 connects through a 10k resistor to the input of a ULN 2803 driver IC. The output can negatively source up to 300mA. My supply voltage for this driver is 5 volts and all inputs/outputs can be used at one time.
This IC drives my multiband ( 2mtr/70cm/900/lo-band/10 mtr fm) linking controller. Very useful, and quiet, no relays .
Jimmy