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Postby Monty » Tue Apr 16, 2002 6:06 pm


I have a Motorola GM300 which has a strange
problem. ( at least to me )

The Transmitt & Receive Freqs Shift from 1-3 Khz, sometimes
as much as 5Khz off freq. Temp does not seem to have a impact.

Tried the Cool vs Hot, same symptoms ! Weird...

Also tried Securing all the Hardware, Cleaning all the interface
pins between the RF-Logic Board, and even the VC ON/OFF
Contacts to insure proper DC Voltage and Current. Regulator
appears OK.

Anyone else ever encounter a Similar Problem with a UHF
GM300 and a possible solution?


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Postby EKS » Tue Apr 16, 2002 6:23 pm

Does the shift happen to rx and tx at the same time?. It sounds like a reference osc to me. Look at the components on the RF board and see if any of them looked cracked. I don't know where you are, but here in Chicago, it recently got humid. We went from winter to "construction" season. Cracked components and humidity usually don't mix.

I had an Icom radio do that very same thing. It turned out to be a voltage regualtor. It sounds like you've got some work ahead of you.

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Postby perthcom » Tue Apr 16, 2002 7:25 pm

I see that problem alot now that the radios are getting older.

Its most always the connector pins between the two boards, the Serial connection from the Micro to the DAC that sets the reference Xtal on frequency.

Get yourself a bottle of Stabilant (the technicians Secret Weapon)..and clean the pins, apply the stabilant and you're good to go!

Also, how many out there have found loose screws on the RF board of GM300s.. complaints from customers of "howling" during Rx..????


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Postby Al » Tue Apr 16, 2002 8:15 pm

I've never run into this problem myself, but I think that there are 4 possibilities:

1. Low VCO output level at the prescaler input causing intermittent countdown.

2. Flaky divider beyond the prescaler in the PLL divider chain.

3. Flaky reference oscillator divider between the ref oscillator and the phase detector in the PLL

4. Intermittent bypass cap from Vcc to gnd somewhere in the PLL divider chain allowing sneak pulses to louse up the count sequence.


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Postby Nand » Tue Apr 16, 2002 8:31 pm

Last edited by Nand on Sun Jan 18, 2004 10:58 am, edited 1 time in total.

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Postby MicorRT » Wed Apr 17, 2002 6:39 am

Have seen many similar problems... If you hit very hard on the top of the unit while it is transmitting and the freq jumps on or farther off.... it has the classic feedthrough problem. They are using stainless steel feethroughs between the Logic deck and RF deck. I replace both female board connectors and the male feed throughs and then they will work like a champ. Apparently the stainless discolors causeing enough of a voltage diff between the Logic and RF decks to make it wander off Freq. Hitting the unit will jar and change it causing the connection to become better or worse a cheap and dirty way of telling quickly if it has the problem. Also found putting a little bit of clear silicone around the coils that already have "glue" around them in the VCO will greatly reduce microphonics make sure not to let the silicone come in contact with the shields of the VCO compartment.

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re: 'microphonic' rx

Postby Ray D. O'Mann #2 » Wed Apr 17, 2002 5:56 pm

We have seen hundreds of GM300's in all bands with a wide variety of faults, including freq drift, int/no rx, howling feedback rx, rumbling unintelligible tx, etc. most as a result of J6, the header that you said you've cleaned. For a while, our bench tech had been replacing J6 altogether, including the female ends on both the logic and rf board. However, a couple of yrs. ago we just started cleaning the header with a fiberglass burnishing tool, and coating it lightly with non-drying electrical joint compound (same stuff the power guys use on transformer posts etc.) and spraying out the female connectors. This has worked on 99% of cases, and I have never had a repeat offender after this treatment. However as the above posts indicate, there could be other issues.

My 2c -RDO2

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Postby April » Thu Apr 18, 2002 8:12 pm

One idea:
Motorola finally admitted (to me) to this problem. The VCO coils were bad and will be microphonic too. The newer coils solve most of that (in GP and P110 too) but it is hard to get the GOOD coils because they look like the old ones. "Gluing" the coils does not allways work.

Also some radios have had leaking capacitors that make the transmit frequency drift when keyed, after it stays keyed for a while it does not drift anymore.

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What radios do you own?: More than I can count

Postby RADIOMAN2002 » Fri Apr 19, 2002 7:05 am

While it might sound low tech, for whowling rx audio. I found just putting a piece of double stick styrofoam tape across the vco coils clears them up. Have had this problem with new radios right out of the box. The tape clears up the problem, if that didn't fix it it was back to Mother M for factory repair, they created the headach, let them fix it.

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Postby Monty » Fri Apr 19, 2002 9:54 am


Well ???

This one is for the books.

The fellow ( Cab Driver ) Failed to mention he was in a
major collision.

After removing the entire Logic Board / RF Board, the problem
was located to a Broken Crystal Pin in the VCO Housing.

After removing the Crystal From the Board, I found a small
Crack around the Xtal Pin and I suspect it lost the hermetic
Seal. However, since the Xtal was glued to the board, it still
had a fair seal

Of course Motorola says one can not replace the
Crystal on the board, but after replacing it, and
a complete realignment of the radio, it has returned
to its original state.

Many thanks to those who took the time & effort to


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Postby perthcom » Fri Apr 19, 2002 7:15 pm

Further to the howling problem, we started to check the screws on every GM300 radio before it went out the door since we saw the problem caused by loose screw so often. Nearly every screw in the RF board side could be turned another 1/2 turn or more before it it was snug. The logic board screws were always tight, just the RF side was not.

As for stabliant, this stuff is great. It comes in a small bottle, the compound is mixed in alcohol, which is a carrier to wash it into the connector and then evaporate. The stuff turns conductive in the presence of an electric field. Check it out at

cheers :D

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