Astro Spectra to Maxtrac interface

bellersley · Post by **bellersley** » Sun Sep 24, 2006 4:31 pm

Awhile ago I posted looking for some info about interfacing an Astro Spectra to a Maxtrac. Well, my plans have changed somewhat.

The Astro Spectra I have, the 15 pin connector is configured for RS-232, not PTT/Mic and such. It's not my radio so I don't want to start monkeying with SMT devices. Since I only need the "repeater" to be one way (RX on the Maxtrac, TX on the Astro Spectra), I was thinking. I have a spectra mic that doesn't work. So - Could I just take handset audio, COR and Ground from the Maxtrac and apply it to the PTT, Mic and Ground pins on the mic plug of the Spectra?

I've checked out the specs as to input levels and such and it looks like it should work alright, but since it's not my radio I just wanted to double check to make sure I don't let the smoke out of this thing!

Thanks again!

kcbooboo · Post by **kcbooboo** » Sun Sep 24, 2006 4:56 pm

That should work just fine. There's a digital ground (for the PTT) and a MIC LO for the mike signal. You will have to tie them together to feed them from the single ground on the MaxTrac. I see no reason why it should not work that way.

The MIC audio sensitivity of the Spectra is about the same as a MaxTrac, so you'll need about 80 mV per 1 kHz of deviation. There will be pre-emphasis on the Spectra MIC input - that's normal. You will want a de-emphasized signal from the MaxTrac to feed it. The headset audio output available in the MaxTrac's MIC jack will work just fine. Use a 10uF 16V electrolytic capacitor - positive terminal towards the Spectra - to deal with the DC that's present on the MIC input terminal. The COR output from the MaxTrac will easily drive the Spectra's PTT line to ground.

There's some Spectra info on http://www.repeater-builder.com, Motorola area, Spectra section, that could be useful.

Bob M.

bellersley · Post by **bellersley** » Sun Sep 24, 2006 7:37 pm

Excellent information as per usual, thanks Bob! I'll give it a try when I get hope. Actually, I was hoping to grab audio from Pin 11 on the Maxtrac, and change the jumper setting so it would provide filtered audio instead of flat, but I don't imagine that audio being any different than the handset audio on the RJ-45.

bellersley · Post by **bellersley** » Wed Oct 04, 2006 7:22 am

Okay. I finally had a chance to put this together and it works! The audio isn't exactly the best but it's good enough for what I need. Thanks again for the input!

I built it with the 10uF cap, but I'm just wondering for future reference if it's necessary - from what I understand, there is voltage at the mic pin on the Spectra, but since pin 11 on the maxtrac is an output pin, is there any chance of damage without the cap?

kcbooboo · Post by **kcbooboo** » Wed Oct 04, 2006 8:00 am

Pin 11 (RX Audio) goes to JU551 which selects flat or de-emphasized audio. The logic board has capacitors to DC-isolate these signals, so you won't harm the logic board. However, the de-emphasized signal also goes to the top of the volume control in the front panel, and that is also where the headset audio (which feeds the MIC jack) is obtained. So without the external 10uF capacitor, you would have DC across the volume control and present on the headset audio line. Who knows what problems this would cause.

For the little bit of guaranteed safety, always use a coupling capacitor when dealing with the MIC audio input on a Motorola radio. It won't hurt anything and it's good engineering practice.

There could be too much audio coming from the receiver into the Spectra's MIC input. You could put a 2.2k, 2.7k, 3.3k, 3.9k, or 4.7k resistor in series with the MIC input to reduce the audio level, or experiment with a potentiometer. Ideally, if you had a way to inject a signal of known deviation into the receiver, and a way to measure the deviation of the transmitter, you could put in a 2-3 kHz deviated signal of 400 Hz into the receiver and play with the resistor so you have a 2-3 kHz deviated signal coming out of the transmitter.

Bob M.

bellersley · Post by **bellersley** » Wed Oct 04, 2006 8:20 am

Interesting. Well, I'm just glad it worked

The audio is far from perfect, it's a little over deviated, but for what I need it for, audio quality isn't a concern and it's certainly understandable. While its certainly not accurate, I'd hazard a guess at saying the transmitted audio is 3db higher than what's being received on the Maxtrac speaker.

I'll have to whip out the service manual and see just what would happen if DC found its way into the Maxtrac.

From experience, it's probably not a bad idea to use coupling caps on anything external. I've poped my fair share of maxtrac audio caps

kcbooboo · Post by **kcbooboo** » Wed Oct 04, 2006 9:14 am

Try adding a series resistor in the MIC audio line to the Spectra. Try the 3.3k first and see if that gets you in the ballpark. If you need more, go with the 4.7k.

I don't think the DC would blow anything up in the MaxTrac right away, but it could affect something else, so for $0.50 worth of prevention, you get a lot of peace-of-mind. Those caps are tiny.

Bob M.

bellersley · Post by **bellersley** » Wed Oct 04, 2006 9:50 am

Well damnit now I'm going to have to rip the whole cable apart! And soldering to that HORRIBLE wire inside the spectra mic isn't exactly easy. It's that stupid copper foil/cotton mix. I'm thinking it'll be easier just to do the mod inside the maxtrac. I suppose I could just put a resistor in parallel with ground on the input pin and it should do the trick