GM300 VHF high current draw (out of band)

sb · Post by sb » Sun Oct 16, 2005 10:28 am

Hi all.
I have a VHF gm300 programmed on 145 MHz. Power is set to 15W and currently drawing is 2x biger then on 170 MHz/15W.
I understand if radio is hacked out of band that there is no provisions for
soft pot alighnment .
How to solve this problem ?

RadioSouth · Post by **RadioSouth** » Sun Oct 16, 2005 10:37 am

You need to supply more info. Bandsplit of radio ? 25 or 45w ? Actual current draws (2X what?)

Max-trac · Post by **Max-trac** » Sun Oct 16, 2005 11:00 am

If it is running full power due to being out of band, you will have to put a manual pot to turn the power down.. Like a Maxtrac.
Nobody has figured out how to change the softpot freqs (yet).
What does a wattmeter say?
It should be a 25 or 45 watt radio...

Nand · Post by **Nand** » Sun Oct 16, 2005 11:22 am

If the transmitter draws twice as much current when running on 145 MHz as when it does on 170 MHz, with 15 Watts out in both cases, it only means that the final is out of tune for the out of band frequency. The only way to correct this is by tuning the final and driver. This should increase output power. Once tuned, you should lower the “Controlled B+” voltage going to the PA to a level that is safe. You need a manual for the radio in order to do this correctly. You still wont have the soft pots working correctly when you do this.

Nand.

sb · Post by sb » Mon Oct 23, 2006 2:29 am

Hi!

Well I found GM300 when cleaning my shack and try to solve the problem again

The solution is on this page:
http://www.repeater-builder.com/maxtrac ... ntrol.html

But GM300 logic board is different from the MaxTrac logic board

Is there a foto documented mod for GM300 ??

kcbooboo · Post by **kcbooboo** » Mon Oct 23, 2006 3:51 am

The GM300 is electronically very similar to the MaxTrac. But there are at least two different board layouts. Armed with the service manual and the right board part number, I'm sure the GM300 could be modified to have the manual power control circuitry added to it.

But before doing so, I'd want to measure the controlled B+ going to the power amp. If the excessive current draw is accompanied by excessive output power and excessive controlled B+, then the manual power control will fix the problem. But if the controlled B+ isn't changing yet the current is, then there's a tuning problem in the power amp, and a manual power control will not help.

The parts on the GM300 logic board are smaller than on the MaxTrac. This might mean they're easier to get to. It seems Motorola used a lot more surface-mount components on GM300s because they were made a few years after MaxTracs and the technology had improved.

Bob M.