GM 300 'howl'

tiredfireman · Post by **tiredfireman** » Sun Oct 02, 2005 9:05 am

Good day--
Ran a few searches and came up with nothing on this - help???

Running a GM300 uhf mobile and experiencing a howl that drowns out the reception, unless the volume is turned down to an unusable level.

Using the internal speaker, no external. Tried swapping the 4 ohm for the 22 ohm variety, but no change.

Had this problem years ago in fire apparatus, and it was chalked up to low voltage (rig not running).
With this in mind, I ran the power cable direct to battery +, with no change.
Likely not an alternator/other interference problem, as it is only on one channel, and the VHF works FINE.

Freq. is 470 and the PL is 82.5 -- any chance it doesn't like the PL?? I doubt I can get the county fire to change it

Any help advice would be truly appreciated. You folks have always come thru in the past!!

jim · Post by **jim** » Sun Oct 02, 2005 9:26 am

Reseat the feeder pins that interconnect the two PC boards. Also reseat all of the RF shields inside of the radio while you're in there.

jackhackett · Post by **jackhackett** » Sun Oct 02, 2005 12:52 pm

You need to look at the feedthrough connector, pull out the logic board to get to it, if the connector is marked 64-80535B01, it is bad and reseating it will not fix the problem. You need to replace it with a 2804637J01.

Will · Post by **Will** » Sun Oct 02, 2005 6:05 pm

Also most likley caused by a bad VCO coil, microphonic. A common problem on Maxtrac/Radius and GM series radios.

Also do the re seating of the inter boards connector as Jim stated.
Also check the part number, as Jack says, the feed thrus do go bad.

tiredfireman · Post by **tiredfireman** » Mon Oct 03, 2005 3:46 am

THANK YOU for the info. It will give me a place to start over.
I am leery of pulling apart the 'guts' to these things, but will give it a shot.
Just to demonstrate my ignorance a bit further, what is a VCO coil, microphonic (mike, I get), and where might it be hiding?

Thanks again, Steve

jackhackett · Post by **jackhackett** » Mon Oct 03, 2005 6:24 am

The VCO coil is one of those little square metal cans on the RF board, IIRC there are two of them, one for RX, on for TX, located under the shield in the section towards the rear of the radio.
You've got about a 99 and 44/100th percent chance that it's the feedthru that's bad so I wouldn't worry about the VCO coil too much at the moment.

tiredfireman · Post by **tiredfireman** » Mon Oct 03, 2005 6:43 pm

Thank you. I had planned to "operate" on the beast today, but too many Squad calls.
I am both pleased and sad that it could be something as simple as the connections. guess the designers (in their sterile environments?) don't get out to the 'real world' much.
Steve

jackhackett · Post by **jackhackett** » Tue Oct 04, 2005 5:38 am

tiredfireman wrote:Thank you. I had planned to "operate" on the beast today, but too many Squad calls.
I am both pleased and sad that it could be something as simple as the connections. guess the designers (in their sterile environments?) don't get out to the 'real world' much.
Steve

Here's the deal on those things...
The way those connectors are made, the pins go through a ceramic cylinder that's metal plated on the inside and outside, this forms a capacitor to filter RF off the pins. On the original connectors they used, vibration over several years of use causes the pin and plating to break away from the ceramic, ruining the capacitors and causing all sorts of havoc. If you take the old connector and push the pins you'll see they come right out with the plating on them.

It probably took about 5 years or so before they started seeing this problem, and then they changed the connector they used to one that's hopefully a little tougher (still waiting to see how long they hold up.. so far so good).

tiredfireman · Post by **tiredfireman** » Wed Oct 05, 2005 2:47 am

Aha, This will likely explain it, as the unit came out of a fire apparatus when we upgraded. Perhaps I should just go buy the part before I do the surgery! Thanks again. Steve

tiredfireman · Post by **tiredfireman** » Sun Oct 09, 2005 9:13 am

Good day all--
Well, I managed to take the critter apart, and (this is key) reassemble it without making anything worse.
The howl is noticeably reduced, but by no means gone (especially at full volume).
The connector jackhackett mentions is the 'new' type -2804637J01- and appears clean & shiny (whole unit is very clean).
I have a couple of more questions before I'm tossed off the board !

-- One of the RF shield covers has bent contacts that (I leap to conclude here) seem to be intended to make contact with the heavy aluminum casting near it. Yet, there is a residue (like what would remain from a sticker) preventing it from good contact. Again, leaping logically (?) I removed this gunk so it would make better contact. Good, bad, or indifferent move?? Does the radio case (i.e. trunnion mount) need to have a good physical ground to enhance the shielding, or will the DC wire suffice. It is currently mounted on plastic. (Antenna coax is good ground at both ends).

-- Two, in an earlier attempt to stop the howling, I swapped out the original 22ohm speaker for a 4 ohm/5watt from another GM300 front. Is there any significant difference between the two?? Or, more to the point, did I screw up when I did this?

Thank you all for your patience. Steve

Will · Post by **Will** » Sun Oct 09, 2005 12:47 pm

. "leaping logically (?) I removed this gunk so it would make better contact. Good, bad, or indifferent move??"

Yes, that helps the RF shielding of the Synth and VCO sections.

The radio should handle the speakers OK, but looks like the VCO IS microphonic!

jackhackett · Post by **jackhackett** » Sun Oct 09, 2005 3:35 pm

Just your luck.. you got one of those 56/100 percent ones... ;)

I've only come across a couple of them where it was the VCO (compared to dozens with the bad connectors)

So, how's your soldering ability?

tiredfireman · Post by **tiredfireman** » Mon Oct 10, 2005 4:10 am

Yes... Murphy and I are real close. Soldering is certainly passable. I managed to install the IGN fuse (thanks, again) without setting anything on fire

. I've worked on pc boards before, tho its not my favorite pastime.
I WILL need a lot of direction and part number(s) to do the job however.
If you don't mind investing the typing time, I'll give it a shot.
Thanks in advance. Steve

Max-trac · Post by **Max-trac** » Mon Oct 10, 2005 9:14 am

I had a GM300 that the outer can of the RX VCO had come unsoldered from what looked like vibration......
So pull the RF board and have a visual inspection...

jackhackett · Post by **jackhackett** » Mon Oct 10, 2005 9:39 am

Did you take that connector all the way out when you checked it? If not you could try reseating it.

The VCO coils.. in the shielded part of the RF board there should be 6 square metal cans, the 2 in the middle are the main VCO coils. Try resoldering them, use a bit of solder wick to take off some of the old solder first. Don't heat the pins up too long, don't want to overheat it and break the connection inside the coil... Do the ground tabs on the cans too. I always use silver solder, but on the coils it doesn't matter, you can use regular electronics solder. (you should use silver on surface mount parts though)

Anyone else have any other recommendations?

tiredfireman · Post by **tiredfireman** » Mon Oct 10, 2005 8:20 pm

Thanks for the description. I'll pull it apart, and check the cans out.

The really interesting thing here it that not only is the problem positional (bench tested horizontally, installed "vertically" -face up- ) but if one taps on the case during the repeater hang time, the 'howl' gets worse. Something has to be loose in there I guess.

Will · Post by **Will** » Tue Oct 11, 2005 1:48 am

Motorola employees knew the VCO coils were bad, but no one is left at Motorola that knows this.

You will find that even tapping on the VCO coil's can will produce the 'howl'. Even the fan vibration in the GR300/500 repeaters cause the 'howl'.

Chances that both TX AND RX VCO coils are bad.

tiredfireman · Post by **tiredfireman** » Tue Oct 11, 2005 8:02 am

Hmmmm. I'm becoming a bit dissillusioned about these rigs here. (Maybe I should dust off the Midland???)

Again, thanks. Does anyone have a part number, or what tips would you have if I bring the "body" to the area Moto dealer for parts??

Will · Post by **Will** » Tue Oct 11, 2005 1:46 pm

" or what tips would you have if I bring the "body" to the area Moto dealer for parts??"

Good luck, most "Moto dealers' have not a clue.

I have the info on the correct parts, but not at this location.

Nand · Post by **Nand** » Tue Oct 11, 2005 6:40 pm

A simple fix (not really a fix that fixes it) is to use an external speaker. This would prevent the speaker vibration from affecting the VCO coil.

Nand.

Will · Post by **Will** » Wed Oct 12, 2005 12:59 am

"A simple fix (not really a fix that fixes it) is to use an external speaker. This would prevent the speaker vibration from affecting the VCO coil.

Nand."

This is more a bandaid, what of the vibration in the car or truck as you go down those roads that need lots of work?????

The problem is also on transmit as we discovered in the early 90's

Better yet, get the correct replacement part and no more 'howling'.

Nand · Post by **Nand** » Wed Oct 12, 2005 2:54 am

I agree that its just a band-aid since it doesn’t really fix the problem, it just gets around it. In some cases, that may be all that is needed and it provides the addition of louder audio out of an external speaker.

Also, going from a 22-ohm internal speaker to a 4-ohm internal speaker is going to compound the problem since is also increases the audio level and the vibration.

As for the bad roads, my guess is that they should be fixed as well.

Nand.

tiredfireman · Post by **tiredfireman** » Wed Oct 12, 2005 12:52 pm

Aha--
I suspected that my local dealer would be without clue (from past dealings -- tho the parts guy DOES try), but I had hoped it was not all that widespread. O well, sign of the times I guess.

Thanks for the input on the speaker, I will switch back to the original, while I am poking around. Heck, since the problem has reduced (but did spread to all channels/tones) maybe that will reduce it further.

I had wondered about the external speaker fix, but the original idea was to be able to move the unit from one truck to the other fairly easily. Of course, its still not out of the question, just a bit more connections.

I don't seem to have the problem on tx (yet?).

I understand that replacing these coils requires a re-tune (which makes sense since they are adjustable - duh) but finances are a bit tight at the moment (which is why I'm using someone elses junque). If my Dept. really wants me to have 'state of the art' they'll foot the bill. I always have their portable to fall back on, just the mobile receives so much better in fringe areas.

Thank you all for your help and patience. Steve

perthcom · Post by **perthcom** » Sat Oct 22, 2005 8:19 am

Hi
We've had lots of GM300's howling as they get older.. Just the ciricuit board mounting screws come loose on the RF board.

Just get your torx out and tighten every mounting screw on the RF board ( don't forget the ones under the the shield and there is one hidden under an RF conenctor too i believe. I bet you can turn them a good one or two turns.

fixed all our problems!

check the logic board ones too for good luck!

GM 300 'howl'

GM 300 'howl'

Hads lots doing this!!