MaxTrac Low-Band RX issue

I have a MaxTrac with an NMO40 type roof antenna, mounted dead center on the top of the vehicle. On the local highway patrol frequency, I get a very annoying RX cutout of audio, as the volume of the voice punches across. As the syllables get intense, the radio RX/PL cuts out as the reception gets staticky. A softer speaker on the dispatch will come across without this issue. Would this radio need to be aligned/tuned...does this sound like something simple to fix? I can receive the CHP office located about 150 miles away loud and clear at times, as well as units on car-car simplex at extreme ranges, so I don't think I have a poor antenna installation. Disabling the PL with the MON button keeps the radio audio from cutting out, but not from getting scratchy as the volume of the speaker (person talking) intensifies.

Paul

is it a SIMPLEX FREQUENCY? maybe their to far from you and your getting Static from them not enough to break the PL TONE?

Anderegg said:

I have a MaxTrac with an NMO40 type roof antenna, mounted dead center on the top of the vehicle. On the local highway patrol frequency, I get a very annoying RX cutout of audio, as the volume of the voice punches across. As the syllables get intense, the radio RX/PL cuts out as the reception gets staticky. A softer speaker on the dispatch will come across without this issue. Would this radio need to be aligned/tuned...does this sound like something simple to fix? I can receive the CHP office located about 150 miles away loud and clear at times, as well as units on car-car simplex at extreme ranges, so I don't think I have a poor antenna installation. Disabling the PL with the MON button keeps the radio audio from cutting out, but not from getting scratchy as the volume of the speaker (person talking) intensifies.

Paul

Click to expand...

That sounds like a modulation issue, or a deviation mismatch. (Assuming the freq is correctly programmed.)

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davidzimmerman said:

is it a SIMPLEX FREQUENCY? maybe their to far from you and your getting Static from them not enough to break the PL TONE?

Click to expand...

I didn't think of an intermod/interference issue. Which also makes me wonder how away the transmitter site is, and if you are overdriving the IF and/or the detector.

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This is a repeater...the audio cut outs less when the PL is disabled. The frequency, 39.4MHz, always seems to get a buzzing type interference noise when PL is disabled about 50% of the time. Same issue with an HT750 on this frequency. Also, this interference exists with Uniden scanners on this frequency, but when I last experienced the problem on the MaxTrac, there was no white noise or buzzing when the PL was disabled and the repeater was not transmitting.

I was wondering what exact alignment problem would cause the almost what I would call clipping or the transmission as the volume of the received audio spikes at less than perfect signal strength. For all I know, low band has it's own weird RX anomalies, which is why I asked prior to seeking an alignment on the unit.

Also, noise blanker on or off makes no difference, and the engine is not the source of any interference.

Paul

krokus said:

That sounds like a modulation issue, or a deviation mismatch. (Assuming the freq is correctly programmed.)

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Click to expand...

The radio is a 36-42MHz unit, with the RX being 39.4, so I am right in the middle of the split. I've got a few 42+ freq's in the unit too, using the hold the shift key out of band trick.

Paul

Anderegg said:

The radio is a 36-42MHz unit, with the RX being 39.4, so I am right in the middle of the split. I've got a few 42+ freq's in the unit too, using the hold the shift key out of band trick.

Paul

Click to expand...

My first guess is that the receiver is off frequency and your not centered on the channel your trying to listen to. You might want to bench check the radio with a service monitor.

In trying to get the radio on frequency, the simple way is to put in a transmit channel and set the radio on frequency by measuring the transmit frequency and using the radio programming software to move the radio on channel.

But I will state here that you need some sort of standard that you know is correct and on frequency. Being a low band radio, it is not as critical as a UHF radio. But knowing your frequency measuring device is calibrated is a big help. Using a service monitor is even better. Just remember that it too needs to be calibrated.

Jim

Maxtrac LowBand RX Issue

"Would this radio need to be aligned/tuned...does this sound like something simple to fix? I can receive the CHP office located about 150 miles away loud and clear at times, as well as units on car-car simplex at extreme ranges, so I don't think I have a poor antenna installation. Disabling the PL with the MON button keeps the radio audio from cutting out, but not from getting scratchy as the volume of the speaker (person talking) intensifies."

Did the RX get distorted/fussy all of a sudden? Assuming the Maxtrac is mobile, I've had times when I've replaced the receive section ceramic IF filters due to shock and vibration. The cracked IF filter failure shows up as a almost as sensitive RX but the receive is now off-frequency in relation to the center operating frequencies. It happens with Maxtracs/Radius radios on VHF-Lo, High, and UHF on occasion.

Repairing the Maxtrac will take the same time to replace one of the 450 KHz ceramic IF filters (there are two located side-by-side) at the same time as these are cheap to switch both out. Debug on which filter will cost you about 1 hour more of extra shop time while somebody else scratches their pointy head on which filters are to be changed out - Best to swap them both at 5 bux apiece. Alignment is usually not necessary as Maxtrac receiver is pretty simple - just check the Quadrature detector alignment for peak audio and move on.

Another possibility is Propagation. You say that the CHP transmitters are sometimes up to 150 miles away when the distortions appear. Propagation (low-band skip) occurs when the local weather becomes suddenly Hot in the summertime or sometimes cold like winter. A condition of 'phase distortion' will appear at the receiver input and the result will be audible distortion and drop outs.

Good luck!

"This is a repeater...the audio cut outs less when the PL is disabled. The frequency, 39.4MHz, always seems to get a buzzing type interference noise when PL is disabled about 50% of the time. Same issue with an HT750 on this frequency. Also, this interference exists with Uniden scanners on this frequency, but when I last experienced the problem on the MaxTrac, there was no white noise or buzzing when the PL was disabled and the repeater was not transmitting."

Since the anomaly is occurring on more radio receivers then the Maxtrac, its likely 'propagation" as in phase distortion from the transmitter is occurring. Not too much you can do other than move in closer to the transmitter. Local power line interference or fluorescent lighting may also be a cause a "buzzing" interference..

Older Low Band Motorolas (like the Micor, Mocoms and Motracs) had a "Extender". - Basically it was a second receiver connected to the same antenna that was tuned to low bad that would receive the noise impulses on that second receiver then those noise impulses were applied to the main receiver detector then cancelled out by the time it got to the receiver audio section. Yeah, there was a "extender" switch on the older control-heads! A modern day noise-blanker in fact. Yes, these radios were notably bigger too.

Scott

I typically run the "extender" off, as the car this is in doesn't have an alternator...I guessed that was what it was intended for.

If I could better describe the audio problem, I would say it would sound like an FM transmission being received on a NFM receiver setting, like it's clipping or something. The low band transmitters I am listening to are FM, and the radio is obviously FM only, no NFM, but thats the clipping type reception I am getting a lot of the time. I've got a spare Maxtrac that can take this one's place once I can find someone to send it to to tweak it out for best performance possible.

Paul

You should be better off leaving the "extender" (noise blanker) on. Should be!
It is intended to help with that pulse noise (buzzing) you hear.
If the pulses are too long, it can't help.
In the early stages of the receiver RF signal is "split" to the receiver and to the blanker circuitry.
There is a short delay line in the receiver path that allows enough time for the pulse detect circuit to recognize the pulses, then function the "blanking" part of the audio path to blank the RX gain and thereby sort of shut off the receiver in millisecond periods. But its meant for very short pulses.
I would guess (and its just an educated guess) that the RX has a bad ceramic filter or crystal filter as suggested above.
The clipping you describe sounds like PL "talk off" to me.
Where the received signal is off freq, the "loud" audio becomes greater, and the noise + that audio makes the PL tone too hard for the radio to detect. Being off freq (or faulty receiver) causes exactly the trouble you describe.
I would try programming the RX off freq one step high, and one step low. 39.395 and 39.405 and see if either is better. If so, then your VCO may be off freq, or more likely the ceramic or crystal filter might be bad. The TX carrier and the RX carrier freqs can then be different when they are not supposed to be.
Won't hurt to try taking the radio apart, remove the logic board, and clean up the interconnect pins between the logic board and the RF board. Sometimes this fixes it.
I have worked on thousands of the Maxtracs, and the are durable little radios.
But once in awhile the ceramic or crystal filter gets broken.
I have seen that much more often in low band than any other.
CHP has a very robust TX system, so unless you are listening to a zone VERY far from you, the Maxtrac should work as well as their mobiles.