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Old 06-04-2014, 11:05 PM
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Default Help designing dual radio switch (race car harness) Plus, name this component!

Hi gents. I've recently joined an endurance race team. Aside from acting as driver, mechanic and logistics, I'm also one of the only tech savy guys on the team.

A quick background; They have always been unhappy with their communications between crew, chief and cars (we run 3 cars out of one pit and team). Prior to me coming on they were using a mix of P110's, a CP200 and some old Speedcom branded radio's. Two cars were using single P110's, one car was using a P110 and a CP200 with a battery eliminator with a dual radio switch. The CP200 was the main, but the battery eliminator would tend to lose contact. The driver sometimes wouldn't realize for a while (each driver runs a 2 hour stint). During that time, we would be trying to get him on the radio. He wouldn't come on until he switched over to the backup.

During the last race (14 hours at Daytona), I brought a few banks of my radio's in to use, HT750's and 1250's. I reprogrammed them to match their existing frequencies and away we went. The long and short of it is the team owner was thrilled at the end of the race and actually told me "We wouldn't have won had you not improved the communications". Great, nice little ego boost, but taking 1st place in a 14 hour endurance race with one of our cars was icing on the cake. But, the system still isn't ideal. 2 of the cars still run single radios. Swapping batteries every 4 hours during pit stops isn't ideal. While this is a fairly compact track, other tracks can be a mile or even 2 from pits to a back corner and while 4w is likely enough, stubby antennas on the radios, which are mounted down low behind steel door skins, doesn't fly for some of the other tracks.

So, I've ordered a bunch of PM400's to put in the cars. I'm also going to put a HT750 in each car as the backup radio. I figure we can get 14 hours out of a battery if it's only running idle or Rx instead of the main Tx radio.

I started planning a dual radio switch in my head. In my head, it should be pretty simple. A 4PDT switch. Each radio connects to each side of the switch (Mic+, Mic-, Spkr+, Spkr-). Center poles connect to a 4\5 conductor TJ101 connector (pretty standard in open wheel racing, used for earbuds and helmet mounted mic), with a PTT switch in series with one of the Mic wires, before it heads to the TJ101 connector. Then it dawned on me that I should open up the existing commercial manufactured dual radio box we have and reverse engineer it. So I did.

For the most part, everything was how I thought in my head. All of the shields are tied together, 4 wires in from each radio, 4 wires out to the TJ101 connector with a PTT inline. Except, there are 2 little cans on each of the radio "input" (to the switch) wires. Each can has 7wires on it (red (tied to nothing), black and white (tied together), then red, blue, green and yellow that run in between the radio wires and the switch. I've been trying to ohm out the box and create a schematic, but I'm having a hard time with it. For example, I'm getting continuity between the Spkr+ and Spkr- pins that would plug into the radio (the 3.5mm). There should be no continuity since there is no speakers/earbuds plugged in. The little can's are labled "PICO-11???". I don't remember what the last 3 digits were, I'll update that tomorrow, the harness is at my shop. Either way, I googled it and came up with exactly nothing. I assume it's some sort of audio transformer, but I have no idea.

The end goal (which will bring me to other questions) is to create our own radio box, for a few reasons. Primarily, every communication company out there that specializes in racing only deals primarily with 2 pin Motorola. I suppose I could use CP200's, but why when I have a few dozen HT750's and HT1250's. Further, they charge rather high prices for what they are. Nearly $300 for a 4PDT switch, two 2 pin plugs, a 3 pin miniXLR connector (for PTT switch input) and a TJ101 connector. I can scavenge pretty much everything I need from existing junk, except for the TJ101 connectors and the 3 pin miniXLR.

I've never been a huge fan of the 2 pin connector, especially in something "mission critical" like endurance racing. Plus, the 2 pin adapters that I have for my HT's are pretty low quality and simply don't mate well to the 2 pin plug. So, I want to make it so that Radio 1 goes down to the PM400 and Radio 2 has a HT multipin connector on it. I may even make the inputs with miniXLR so I can use interchangeable cables between the box and the radio, should we decide to change the radio setup or a PM400 dies, etc. Going from the box to the PM400 is simple enough, it's just an RJ45. I was going to sacrifice a few speaker mic's to obtain a HT connector with coil cord for the HT750 acting as the backup radio.

I'll have a PM400 at the hot pit with a antenna on a short mast, plus a PM400 back at the garage.

So, first and foremost, do any of you pro's (that have likely been tinkering with radio's longer than I've been alive) want to take a guess to what the little can is?

Second, is there are reason I shouldn't be using the mic input on the PM400 versus the accessory connector on the back?

Third, is there any reason I wouldn't be able to have a PM400 and a HT-series radio co-exist? I suspect the mic gains could possibly be different between them? I'm planning on both radio's running roof or trunk lid mounted antennas (specifically, these Antenex models that came with the radio's. I still need to grab some HT > BNC antenna adapters.

Does anyone have any hints, tips or tricks for me? I may have a few PCB's made to have all of the wiring terminate to instead of the rat's nest that was stuffed in the box.

Any help would be greatly appreciated. Sorry for the novel of a post.
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Old 06-04-2014, 11:07 PM
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Another view of the box I'm trying to duplicate / modify.
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Old 06-05-2014, 5:50 PM
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That may be a small audio isolation transformer. The audio out on portables is NOT referenced to ground/shield and has DC on it.

The Mic on the PM400 is NOT the same as the ones used on portables. There is a second Mic input on the mobile radios, CDM/PM series. That input is programmable.
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Old 06-05-2014, 10:17 PM
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>>3 pin miniXLR.
That is a Switchcraft TA3FL or ML connector for the PTT lead.
I lost my folder with my schematics from my Comtrack and Racing radio days, can't remember the wiring for the Pico A-1025 audio xfmr. I think it has 2 windings on one side and a center tap on the other.

Sherby Blankenship did Foyt's radios at the 24 hours of Daytona, he had a switch to charge the mobile's battery during yellows. One time AJ forgot to flip the switch and every time he keyed all the crew heard was EMI noise. The guys came to Sherby and asked him what AJ said. He told them "Just because I'm the radio guy doesn't mean I can understand the sound any better than you."

Otto engineering can probably supply the connectors, they make or made the cables for Motorola spkr-mics.

At Comtrack we made adapters from spkr mic cables that had a female nexus, then made car harnesses with nexus plugs to fit in. You loose shielding that way but it worked pretty well.

I remember switching the battery on a portable during a pit stop at the 24 hour race, as I was leaning in the right side window Bill Elliot's foot was coming out as he made way for Ricky Rudd, his shoe looked to be the size of an ironing board, I just ducked in the nick of time.

The mobile radios always needed a preamp to use with the 150 ohm dynamic mics, haven't worked in racing since 2007, electrets were starting to become the norm then.

Some teams had fabricators who made battery eliminators to run off a 12v battery so they could use a portable. A track like Daytona had good coverage with just the rubber duck on the portable, even with the 3-5 dB loss.

Power is often over rated when it comes to coverage. I had a cable fail on a duplexer at the Long Beach race this year, I had less than 20 milliwatts feeding about 120' of LMR 400 at 451 MHz and zero coverage complaints.
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Old 06-05-2014, 10:37 PM
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I think you have the right idea. Install a small mobile radio in the car, even a CM200, and ditch portable radios for in-car use. It's not like the CM200 or PM400 are exactly big and heavy and will slow the car down. Run the power down to just a few watts and for an antenna, you might even make up a tuned VHF foil tape antenna that sticks right on the glass and has essentially zero added drag.

The alternative would be to keep a rack of fully charged spare radios right by the pits and rather than fumble with changing batteries on the portable radio, just swap out the whole radio at each pit stop,
which could end up being faster and easier.
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Old 06-05-2014, 11:17 PM
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Instead of swapping radio batteries during the race I would get a battery eliminator with 12v cord and run from the vehicle power or a small but sufficient size 12v gel cell or better yet an A123 Systems 12V7 pack. These are very light weight and 5AH at 12v or more.

Depending on the mic type used you may not need the transformer and possibly just a DC blocking cap if a dynamic mic is used.

I've made some stealthy in the car antennas from very thin magnet wire with a single vertical element and two opposing ground radials that stick to the inside rear window with clear packing tape. Works much better than a handheld rubber duck in the car.

I think it was already mentioned but improving the antenna or raising it at the pit can usually make up for a lousy antenna in the car.
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Old 06-06-2014, 12:08 PM
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Quote:
Originally Posted by cmdrwill View Post
That may be a small audio isolation transformer. The audio out on portables is NOT referenced to ground/shield and has DC on it.
I called PICO Electronics today. The PICO 11855 in this box is a custom transformer made for Racing Electronics. 80ohm primary (red, brown, blue); 32ohm secondary (yellow, white, black, green. White/black need connected together for center tap). I still need to do some mapping out of the circuit to understand how it works. I'm not sure yet which side goes to primary and secondary. And I don't understand (my lack of knowledge) why a secondary side would have 4 wires. speedway_navigator seems to have some experience with a similar transformer so maybe he can give me some Transformer 101 classes. I fully understand them in an electrical application, but not to much for audio.

To answer some questions about the general setup of the car. As said, PM400's in each car as the primary radio. This is to eliminate battery swapping and battery eliminators. We've not had good luck with battery eliminators, not to mention the cord comes straight out of the bottom, which inevitably gets crimped over as it is sitting in the radio box. With the 37 hour Texas race, that is quite a few battery swaps that we would ultimately like to eliminate.

We really don't need an ultra low-profile antenna. For the two Saab 9-3's we run, I'm going to put it on the trunk lid. While not an ideal ground plane, I feel it is better than a ducky stuck down low on the door buried behind sheet metal. Each car will run two antennas, one for the backup HT, one for the main PM400. The Talon will run a similar setup, except that car has been converted to a single seat, no roof roadster. The passenger seat and rear seat areas are completely decked over. I'll run both antennas in the center of the rear deck, which should provide a pretty great ground plane. That area is ~3'x3'. The PM400's in the cars will all be set to low power Tx (whatever that happens to be on a 25w PM400. Radio info says "1-25w" but I can't imagine it would Tx at 1w on low. I could be wrong)

And yes, at the pit PM400, I'm planning on running a 5/8 wave (supposedly no ground plane needed? We run UHF freq's) on a 15 or 20' fiberglass or PVC mast. Basically just enough to get it above most of the infield buildings and bleachers.


Ultimately my biggest confusion and roadblock right now is how to interface the PM400 to the mic and earbuds on the helmet. I take it from some comments made and other reading that the PM400 mic jack on the front is looking for a different type of mic than say, an external speaker-mic on a HT750. If I understand it correctly, the PM400 uses and electret (powered?) and everything in the race world is dynamic? Samson makes a harness to mobile adapter here. It looks simple enough, 16 pin plug to a TA5M connector that ties into the rest of the harness for the Nexus\TJ101 connector and the PTT switch. BUT, it does have a pigtail on it that I'm not sure what it would connect to.

So, cliff notes, what kind of connection do I need to do to get my mic and ear buds to work with a PM400. Direct connect? Isolation transformer? Something else that I have no clue about?

Once I get that done, I should just be able to switch the second radio input (to the switch box) from the 2 pin to a multipin for the HT750 and that should work without issue.

Help?
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Old 06-06-2014, 12:10 PM
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Also, I found a thread on this very board of someone trying to interface a car setup to a PM400, but it doesn't look like it was resolved, or if it was, it wasn't updated.

PM400 16pin accessory and programming
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Old 06-06-2014, 2:50 PM
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Contact Racing Radios to see if they have an off the shelf solution first. There build quality is second to none. If they do it will save you a bunch of headaches. It nothing off the self they do custom work, have all the connectors and parts available and the documentation to do it right.
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Old 06-06-2014, 3:48 PM
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Quote:
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Contact Racing Radios to see if they have an off the shelf solution first. There build quality is second to none. If they do it will save you a bunch of headaches. It nothing off the self they do custom work, have all the connectors and parts available and the documentation to do it right.
While this is an ideal solution for a team with a huge inflow of cash, our team doesn't. We run 3 cars in the ChumpCar series. ChumpCar doesn't pay out, even for a first place win in 14 hours of Daytona like we snagged 2 weeks ago. There is really only one guy financing the entire team. The rest of the crew and drivers put in sweat equity.

As it sits, RE, RR, etc doesn't have a full of the shelf solution. While I'm sure they would be willing to build one (maybe, I talked to one of their tech's and he didn't even know what a HT1250 handheld was), I'm sure it would be very costly. Even a plain jane 2 radio switch box from RE is $279+ship. Times 3 cars, we're in for nearly $900. Did I mention this is ChumpCar? These are cars valued at $500 (or, at least are supposed to be).

It's bothersome when I know the component cost and actually have many of the components laying around and I'm more than capable of putting it together myself, with a little help on the design side. There is nothing wrong with my build quality. I took have wire strain reliefs, seals, wire, etc all on hand. Nothing that RE does is "magical" that can't be duplicated. But this isn't about who builds a better box. This is about adapting a PM400 to our existing earbuds and dynamic mics.

$100 > $900
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Old 06-06-2014, 4:50 PM
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As an example of building your own; the cable in the attached picture is pretty basic. It's a $7 TA5M plug, a $3 16 pin plug and what, maybe $2 worth of wire? Maybe 15 minutes to assemble it?

That cord sells for $105. Are you really going to buy it for $105 or are you going to build it yourself?

While I do realize that these companies are in business to make a profit and that there are individuals and teams out there that don't have the skills necessary to build such things, I have that ability (with a little help from you guys, of course).

As far as that adapter goes, I spoke with someone at Simpson today. There is a flying lead on that, which needs +12v. He said it could be wired directly in with the power supply wire that feeds the radio. According to him, there is a preamp built into the radio, but it needs an external 12v. He said with that adapter, you can run a dynamic mic directly into the radio without the need for a preamp. Doesn't sound quite correct to me, but I've certainly been wrong before.
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Old 06-06-2014, 6:14 PM
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Ok, contrary to popular belief I am not psychic. Had you said chump car in the first place instead of playing coy I would have gone a different route. Sorry to bother you.
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Old 06-06-2014, 8:53 PM
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Ok, contrary to popular belief I am not psychic. Had you said chump car in the first place instead of playing coy I would have gone a different route. Sorry to bother you.
No bother at all. I should have mentioned it before. I truly didn't mean to come off as a condescending *******.
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Old 06-07-2014, 3:21 AM
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It sounds like using the center tap on the 3 lead side would almost be a 1:1 ratio.
Motorola usually has balanced audio on the rx side, you can't ground it to chassis ground like you do for mic audio and PTT.
I suspect that's why the xfmr is in there.

Not sure why you need a box, when I first started working for Comtrack they had boxes built for the Standard 734 radios we sold (Moto had not started Radius then), this box had some type of filter and a shorted quarterwave stub to reduce EMI.

Vince Granateli's team was having radio problems at Indy, so I told them to just use the radio without the box. Mo Nunn kept saying we had to have the box, I always admired him for his engineering expertise in setting up a race car but was surprised at how stubborn he was about changing the radio install.

Finally he relented and Roberto did a test lap. Before I made the change he could only talk from the exit of 4 to half way down the backstretch. He was able to talk at speed all the way around, Vince kept calling him on the radio for a radio test, finally Roberto told him to quit testing. By the end of the month all the teams had ditched the magic box.

I made a Y cable so the Newman Haas spotter could could key 2 portables with one headset, no iso xfmr, this was done about 8pm on a Saturday before the race the next day, purely seat of the pants engineering after a last minute request by Ed Nathman.

The old Moto headset had a Nexus plug, so I made a dual plug to single jack cable, each radio had a side adapter to Nexus jack.

Racing radios had a PA-201 mic preamp for dynamic mics, David Clark has headsets with mic preamps, bulkier looking than the M-87 or M-101.

You might be able to parallel the mic and speaker leads from both radios to the helmet and just switch the PTT lead from the wheel to the radio, but I suspect you will still need the mic preamp which is powered by the mic hi lead.
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Old 06-10-2014, 10:06 PM
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Quote:
Originally Posted by speedway_navigator View Post
It sounds like using the center tap on the 3 lead side would almost be a 1:1 ratio.
Motorola usually has balanced audio on the rx side, you can't ground it to chassis ground like you do for mic audio and PTT.
I suspect that's why the xfmr is in there.
As I'm not well versed enough in designing audio things, do you think a generic 1:1 audio transformer would be suitable for my project?

I know the PM400 speaker output is for 4/8ohm speakers. Not sure what the accessory connector on a 2pin radio is setup for. I would assume the same (for speaker mic's and such). Typical earbuds as used in racing are going to have an impedance of 16-32ohm, so maybe that is what their transformer is for?

I don't mind breadboarding some things to mess around with earbud levels. I'm more concerned with the MIC side of things. I don't want to inadvertently fry a mic because the radio is sending out voltage for a preamp or similar that I'm not hip to. I also don't fully comprehend. The following is from the PM400 manual;

Quote:
Pin 2 - External Mic Audio Input impedence:500 ohms
80 mV rms at 1 kHz for 60% deviation.
This path is enabled when external mic PTT is keyed.


Pin 3 - External Mic PTT Put this pin low (less than 0.66 Vdc) to key transmitter and enable
external mic audio path. This path is pulled low via a diode when front
panel mic PTT is pulled low to allow sensing of mic PTT by accessory.
This pin pulled high to 3.3 Vdc via 3.3k ohms
At this point, I don't know how to even connect a mic to the accessory connector of the radio. Ultimately, that is where I'm looking for some help.

Quote:
Not sure why you need a box, when I first started working for Comtrack they had boxes built for the Standard 734 radios we sold (Moto had not started Radius then), this box had some type of filter and a shorted quarterwave stub to reduce EMI.
"The box" is just to switch between the vehicle powered PM400 or the backup, battery powered HT750. I'm trying to make the box as least complicated as possible. If I can get away with a 4PDT switch to switch the mic / speaker lines and nothing more, fantastic! But I'm assuming there is a good reason that Racing Electronics had a custom transformer made for their dual radio switch box. I revert to not knowing enough about audio design to make that call.


Quote:
Racing radios had a PA-201 mic preamp for dynamic mics, David Clark has headsets with mic preamps, bulkier looking than the M-87 or M-101.

You might be able to parallel the mic and speaker leads from both radios to the helmet and just switch the PTT lead from the wheel to the radio, but I suspect you will still need the mic preamp which is powered by the mic hi lead.
Again, out of my league.
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Old 06-11-2014, 12:50 AM
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>>do you think a generic 1:1 audio transformer would be suitable for my project?

yes as long as the impedance does not load the audio, I had seen teams try to use aviation headsets, those are usually 600 ohms. I think the reason Jack Babb (head engineer for racing radios a few years back) went with the Pico was the metal shielded case.

The 150 ohm mic can handle the voltage coming from the radio,
I think the mic preamp on a portable will be way too hot. The simpler way would be 2 portables or 2 mobiles.

Hook the mic to pins 2 and 7 on the PM400 accessory jack.
RX audio is pin 16 RX high and pin 1 RX low.
PTT is pin 3 to gnd / pin 7. I suspect your TX audio will be too low to hear, but in reading the service manual I see a 'low cost mic' (no amp) sense, so it might work after all

I don't think the PM400 shuts off the front speaker when audio is pulled from the read like the old mobiles did, Bearcom does hardware mods to the front panel so dispatchers in race central at the Long Beach race can use earpieces and not blast out audio disturbing the other dispatchers in the room.
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