Maxtrac ghost transmit w/ acc. plug in use

[eportel6607]

Hi guys,
Have had an intermittent issues on my Maxtrac. It's a 16pin logic board model and is setup for use with a vehicle VHF repeater. Pin 14 is set to detect a carrier with or without PL. The extenders are made from HT1000. I have made several of these and haven't had any issues with them until recently.

On the vehicle I added a quarter wave VHF antenna (matched near perfectly) to the vehicle repeater frequency. The mobile Maxtrac is low band. Because of the RF from the low band radio (60watts) I know form experience that I needed to add some isolation to the receiver of the vehicle repeater radio (HT1000) so as with my base system I added a Comet diplexer (duplexer) with a frequency split of 1.3-90Mhz and 140 to 512Mhz. I only use the high side connector as it's all that is needed to isolate the Rxer of the HT1000 extender system from overload/desense.

As I said, since I added the vhf hi band quarter wave the Maxtrac (low band radio) will sometime stay transmitting after a transmission from it. In other words regardless if the vehicle repeater (HT1000) is on or not if I manually transmit on the Maxtrac (or if the vehicle repeater is on and the Maxtrac is "keyed" remotely) it will sometime stay transmitting. It might stay keyed up for just a few extra seconds or might go until the TOT shuts it off. When it does happen I can sometime just touch different parts of radios (radios are in a communications rack in the vehicle), maybe under the radio, touch some wiring...not move it but just touch it and the Maxtrac would stop transmitting. This is why I think it's a RF coupling issue.

I'm sure it has something to do with the control wiring from the vehicle repeater's transceiver to the Maxtrac...since this issue won't happen if I simply switch out the accessory plug (16 pin connector) for the vehicle repeater with a stock one having only the jumpers for the internal speaker and/or emergency switch bypass.

It appears that maybe the PTT line (pin 3) is getting some stray RF on it and keeps the radio transmitting?? The PTT line is active when low. So the vehicle repeater has circuitry that converts its normally "active high" COR output to active low and then feed into pin 3 on the Maxtrac accessory port.

I was originally thinking that I could place a small capacitor (.001mf) across pin 3 (PTT) and pin 7 (gnd) but don't see this working since the pin3 (PTT) is already active low. If it was active high this may work. Or am I wrong on this and it would force the RF to ground??

Any idea guys how I can resolve this intermittent issue?

Thanks
chris

 

[krokus]

The induced RF could drive the DC level low enough to register as a logic low.

 

[902]

The induced RF could drive the DC level low enough to register as a logic low.

I agree. eportel6607, get several snap-on ferrite cores and put one on every wire going into and coming out of your Maxtrac and HT1000 arrangement. See if that makes the problem go away.

Is this a Motorola factory-built extender or something you made?

What kind of shielding did you put your HT1000 into? Is it in an RF-tight box with feedthrough capacitors or something different? If you can describe it, that would be great.

Where is your Maxtrac antenna mounted? How much cable did you run? Are you running full power, or did you reduce the PA output any (sometimes reducing the output makes the PA break into oscillation)? Doesn't sound like you reduced anything, but just going down a mental checklist.

The other thing is terminate the unused port of that diplexer with a 50 ohm load and put a .01 uF disk ceramic capacitor from your PTT line to ground to see if that helps shunt some of the RF.

 

[cmdrwill]

Do you have pull-up resistors on the input pins?

 

[eportel6607]

I agree. eportel6607, get several snap-on ferrite cores and put one on every wire going into and coming out of your Maxtrac and HT1000 arrangement. See if that makes the problem go away.

Is this a Motorola factory-built extender or something you made?

What kind of shielding did you put your HT1000 into? Is it in an RF-tight box with feedthrough capacitors or something different? If you can describe it, that would be great.

Where is your Maxtrac antenna mounted? How much cable did you run? Are you running full power, or did you reduce the PA output any (sometimes reducing the output makes the PA break into oscillation)? Doesn't sound like you reduced anything, but just going down a mental checklist.

The other thing is terminate the unused port of that diplexer with a 50 ohm load and put a .01 uF disk ceramic capacitor from your PTT line to ground to see if that helps shunt some of the RF.

Thanks guys very much for all of your input.

Sense I've posted this message I've tried some things.
I did try the cap across pin 3 and 7(gnd). The results...well at first the Maxtrac would transmit at all! So I removed this cap. I had the same condition...the Maxtrac wouldn't transmit. Clearly the cap wasn't responsible for not allowing the Maxtrac from transmitting. I later found that the Vehicle Repeater (VH) wasn't grounded due to a bad contact under the battery eliminator unit. I rearrange the power cables and re-routed the control cable to the VH. I tested the system out yesterday and all went well. I used the system in Home Depot with my vehicle at least 15 feet way and was using the system talking to one of our control points about 32 miles away.

I will add the chokes to all line going in and out of the Maxtrac/VH. I will re-attached the cap between pin3 and 7 again as it certainly can't hurt to have this in place.

Ok not to answer some of the other questions.

There VH (HT1000) is not enclose in anything. Well except for the stock HT100 case. The internal speaker and mic were removed from the HT1000 case. In its place is one cap, 2 pull up resistors, 2 transistors taking care of all the switching for the system and there is a resistor to take care of attenuating the audio going to the mic from the Maxtrac. Yes this is a "home brew" unit not a Moto factory unit. I have made many of these a few years back with GP300/P110 with great success and VERY cheap to build. Did I mention that they are cheap to build??

The antennas on the vehicle are NMO roof mounted with the low band antenna about 30 inches away from the 1/4 wave VHF antenna. I can simply move the vhf antenna to one of the other mounts further back on the vehicle but the efficiency of that back mount isn't as good due to the lack of a ground plane. Beside the base station setup is very similar with no problems in almost 2 years of use.

The low band mobile is full 60 watts power but the VH is set to 1-2 watts TX power.

I was also thinking about adding a dummy load to the other unused port (low freq side) on the duplexer. I added a dummy load as the issue was happening. The issue remained. I also have the same exact duplexer system on the base station system with an open port and the issue didn't happen there.

The control cable between the VH and the Maxtrac is about 15".

Also I should mention that the HT1000 are of course as you all know "public safety" quality units unlike the P110. I can certainly tell that the isolation on the HT1000 is FAR great than that of the P110/GP300. As an example, before I placed the duplexer in line with the VHF side (when using the P110s as the VH) I could only get about 250ft away before the RXer in the P110 was overwhelmed from the low band RF. After I added the duplexer I could get over a mile away. I'm sure if I had a duplexer with more than 40db of isolation that I could get nearly the full range of the radios...but a mile is more than enough.

Thanks again guys.
If you guys every want the schematics for this system let me know. I have the P110/GP300 one now as I've built them in the past. I have yet to make the schematic for the HT100 version...but the concepts are very similar. I can't tell you how powerful and useful this system is...expecially when tied into our repeater. I have had countess police officers from different departmenst ask "how are you guys talking on portable in this warehouse forty seven miles away to the other end of the state to another unit on portable? We can't do that!" Up until our state when P25 we had more reliable portable coverage than RISP did.

Thanks guys for all your help and suggestions.
Chris

 

[902]

Hi Chris,

Yes, I used to have a bunch of GP300 link transceivers that I pulled at one point. It was the radio out of its case and just blocked up onto a heatsink and an interface board. For a while, I replaced them with Hamtronics boards, but eventually went to microwave and eliminated the UHF FXO stuff altogether.

I'm fussy about those things, that's why I mentioned putting a termination on the open port. Just OCD.

Glad you're up and running! My next thing would have been doing an RF-tight box for the HT1000 with feedthroughs.

BTW, you have my greatest respect for keeping low band active.

PM me with your contact info. I'd love to see the schematic!

 

[eportel6607]

Hi Chris,

Yes, I used to have a bunch of GP300 link transceivers that I pulled at one point. It was the radio out of its case and just blocked up onto a heatsink and an interface board. For a while, I replaced them with Hamtronics boards, but eventually went to microwave and eliminated the UHF FXO stuff altogether.

I'm fussy about those things, that's why I mentioned putting a termination on the open port. Just OCD.

Glad you're up and running! My next thing would have been doing an RF-tight box for the HT1000 with feedthroughs.

BTW, you have my greatest respect for keeping low band active.

PM me with your contact info. I'd love to see the schematic!

Thanks for all your input.
I'll PM you for the rest of my comments regarding low band and related stuff.

Thanks
Chris

 

[eportel6607]

hey guys,
Just wanted to close this topic. I found the issue to the problem. Judging by the fix that had to be implimented, the problem shouldn't have existing in the first place. After extensivly checking and doing all of the suggestions that you have all mentioned none fixed the problem. It wasn't until I bypassed the duplexer did the problem go away. My focus was now on the duplexer. Despite the fact that the antenna grounds are good and no issues with electrical grounds on any of the equipment adding a ground wire to the aluminum duplexer chassis box and grounded it to the vehicle ground fixed this issue. There must be something wrong with the connector (SO-239) on the unit because I have another Comet duplexer of the same model and this issue has never happened before. Thanks for all your efforts guys. It's much appreciated.

chris