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Strange happenings with a 900 MaxTrac

This forum is for the discussions targeted at converting various models of Motorola equipment to operate in the 900MHz Amateur Band.

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n9wys
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What radios do you own?: Too many to list! :-)

Strange happenings with a 900 MaxTrac

Postby n9wys » Thu Mar 12, 2009 6:52 am

Gentle people...

I purchased a 900 MHz DeskTrac from the popular and fun auction site. I went about converting it, and this is where I have run into problems. In order to give you a full report on the issues, my description will be detailed... so make some popcorn and settle in to read the novel.

Well, when I first got the DeskTrac, I merely went in and prpogrammed in my frequency list to see how it worked - starting out. RX was fine, but no TX. I figured that would happen, so I set about modifying the VCO. After wrestling with it for nearly two weeks, I reached out for another learned amateur, who was much more well-versed on the MaxTrac than I, for assistance... In the end, I sent him my RF board so he could perform his magic.

Turns out the left half of the VCO was dead... Don't know exactly why that happened yet, but the good "doctor" is continuing his "post mortem examination" on the unit to learn why it died. In the interim, he modified the VCO on a "new" RF board from a 2-channel Conventional 900 MaxTrac, and then sent that back to me. When it left his hands, the VCO locked in the entire amateur range of 900 MHz freqs.

I got the RF board back and installed it in my DeskTrac. Immediately, the radio reported it had 10 conventional modes plus one trunking mode. I wondered about this, but attributed that to the logic board in my radio... I read the radio with trunked RSS, and the MODE UTILITY menu showed all 10 conventional and one trunked modes. BUT, if I went into the CHANGE/MODIFY screens, I only could "see" the one trunked mode and TWO conventional modes. So I went back into the mode utility, removed all the conventional modes and then added them back. Then I went back into CHANGE/MODIFY and entered the frequency data for each mode, and wrote the codeplug back to the radio.

Now it gets REALLY weird... The VCO will not lock at frequencies above approximately 902.4875, where it would before it was returned to me. Transmit VCO Steering Line voltages were measured as follows:

>> Ch 01 - 902.5250 TX 0.7 - 1.2V (Varied) NO TX
>> Ch 02 - 927.5250 TX 3.19 V (Steady) TX OK
>> Ch 03 - 927.5000 TX 3.18 V (Steady) TX OK
>> Ch 04 - 902.6875 TX 0.7 - 1.6 V (Varied) NO TX
>> Ch 05 - 927.6875 TX 3.23 V (Steady) TX OK
>> Ch 06 - 902.4875 TX 6.64 V (Steady) TX OK
>> Ch 07 - 902.8000 TX 0.7 - 1.6 V (Varied) NO TX
>> Ch 08 - 902.9750 TX 0.9 - 1.7 V (Varied) NO TX
>> Ch 09 - 902.4875 TX 6.65 V (Steady) TX OK
>> Ch 10 - 927.4875 RX 3.12 V (Steady) TX OK

You'll note that the 927 (simplex) freqs are all fine, this anomaly affects the repeater input frequencies in the 902 range... but 902.4875 locks the VCO with 6.65V where 902.5250 remains unlocked with voltages fluctuating between 0.7 and 1.6V. So in a difference of about 0.0375 MHz (37.5 kHz) the VCO goes from 6.65V and locked to below 1.7V and unlocked? What's up with that??

I wonder whether the "change" from using conventional RSS at the "repair facility" to my use of trubnked RSS may have caused this? Can anybody give me any direction with which to proceed? Both myself and my mentor are stumped at this point...

Thanks!
Mark - N9WYS

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Max-trac
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Re: Strange happenings with a 900 MaxTrac

Postby Max-trac » Sat Mar 14, 2009 6:50 pm

The trunking firmware shifts over to the other side at 902.5.
The conventional 903.000
I would blank it, change to conventional firmware and initialize.

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n9wys
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Re: Strange happenings with a 900 MaxTrac

Postby n9wys » Sun Mar 15, 2009 6:28 pm

Max-trac wrote:The trunking firmware shifts over to the other side at 902.5.
The conventional 903.000
I would blank it, change to conventional firmware and initialize.


Is there some way to hexedit the "changeover point" in the RSS? Has anyone attempted this, or does anyone know WHERE this is stored in the RSS? I'd LOVE to be able to change that to 904.0.

Also, if I blank the radio and convert it to conventional, will it still work OK with the Deskrac I have it installed in? Or doesn't the DeskTrac "care" whether the radio is trunked or conventional? (i.e., will it be able to determine the radio type by the type of control panel on the front?)

And finally - I've never done a conversion on a MaxTrac... what is involved? Do I need to change a chip somehwere? Are they readily available or will I need to get it from Motorola?

In this instance, the RF Board I am using now comes from a conventional radio, but the logic board is from a trunked radio.
Mark - N9WYS


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