Issues with 900MHz MaxTrac

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d119
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Issues with 900MHz MaxTrac

Post by d119 »

So I've got a 35W 900MHz MaxTrac here. It was originally a trunking unit, but had the firmware replaced with FVN-4019A. The board has been blanked, and the radio reinitialized as follows:

MaxTrac High Sig, 900MHz 35W 16F MaxTrac 300. The radio has a 16 Pin Logic Board.

I read the how-to on the BatLabs main site about the VCO conversions.

So far, I can get the radio to receive on 927.975MHz, but it will not receive on 927.0125.

It will transmit on 902.975, but not on 927.0125.

The radio appears as though its transmitting, but I can not copy it at all on a portable (won't break squelch).

I modified both the VCO's using the solder blob method. The VCO voltages are within the 2.5 to 5.5v range TX/RX.

What gives with this thing? Any suggestions?

If I can't figure it out, are there any MaxTrac gurus that would get this thing straightened out for me?

Thanks!
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kcbooboo
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Post by kcbooboo »

I just did the same thing to a 12w radio: new firmware, blank/init, and play with the VCO.

It sounds like the VCO is the culprit. On receive, the SL test point has to be at about 2.0 to 2.5VDC, and on transmit (902 MHz) about 6.5-7.5VDC. This is for the "left" VCO. The talk/around ("right") VCO is far less critical, but still try for 3-5VDC.

If the SL voltage is under about 1.5VDC, the radio will have problems receiving, even though the VCO is operating somewhat properly.

There are a bunch of "how-to" articles on www-repeater-builder.com, in the Motorola area, MaxTrac section, including modifying the VCO and a full step-by-step trunking-to-conventional blank/initialize procedure. Perhaps there's something written up that will help you get yours working.

Bob M.
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d119
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Post by d119 »

Well apparently I completely F'ed the tuning values in the radio doing F2/RF Board Replacement.

So now even with correct VCO tuning and steering line voltages, the radio just shuts down when I key up on 902 or 927.

Thoughts?
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kcbooboo
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Post by kcbooboo »

I've blanked and initialized MaxTracs before, and on each screen, just pressed F8 to save the values that were defaulted by the software. The radio works, but it's not "optimal" as the book says. The default values won't give you smooth operation across the entire band(s) of operation; that's why you need to adjust the output power and deviation. But it should still work. In fact, the software assumes it will work as you're makig those fine adjustments. So you likely have another problem on your hands.

Perhaps the conventional firwmare is no good or has an error. That would make the radio go nuts. Swapping it with another working radio would test that theory.

Dirty pins between the RF and logic boards could prevent the synthesizer from being programmed properly.

Problems inside the VCO (too much solder, solder in the wrong place, metal shavings from opening it up, etc) would prevent it from working properly.

I'd suggest you try the following to rule out the VCO. Attach a small wire to the VCO's output pin, and feed that to a frequency counter. Attach another small wire to the VCO's steering line. You'd have to remove the RF board to get to these signals. Then turn the radio on and read the frequency. Remember that the VCO on receive will be off by the IF frequency of the radio, about 39 MHz. Program the radio for 900 MHz transmit and 940 MHz receive; the VCO should be around 900 on receive and transmit. Try this with 940 MHz transmit and yhou should see that frequency with the PTT line grounded. Remember to attach a dummy load to the antenna connector.

Then attach the wire from the VCO's steering line through a 1k resistor to a variable (0-10V) DC power supply. Adjust it from 2 to 8 volts and watch the VCO frequency in receive. It should go from around 896-902, corresponding to receive frequencies of 935-941. This would verify the VCO frequency and range. I did this on an 800 MHz radio's VCO once, and I'd presume that it would work on 900 as well. This will verify the left side of the VCO. For the right side, which is only used for talk-around transmitting, you'd need to keep the radio transmitting while doing this procedure, and that's not easy if the VCO/PLL won't lock up. I think you can ground the LOCK line coming from the RF board, but I don't recall if the grounded state means LOCKED or UNLOCKED. If it needs to be ungrounded, you'd have to try disabling the circuit at the transistor that drives that line, possibly by grounding the base. Look at the schematic and figure out what you'd need to do to keep the LOCK line in the proper state so you can keep the radio transmitting while testing the right half of the VCO.

Or you could send the radio to me and I can try some/all of these same things, including re-blanking/initializing and new working firmware. I happen to have a fully converted working D27 in my posession right now.

Bob M.
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d119
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Post by d119 »

Bob,

What would it cost for you to work on it? Drop me a PM... Thanks!
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kcbooboo
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Post by kcbooboo »

PM sent.

Bob M.
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