CDM1250 / CDM750 loosing filtering of vehicle line noise

[cbroseinak]

I have run into problems with the CDM1250 and CDM750 mobile radios loosing filtering capabilities of vehicle power supply line noise after time. A customer just came in with a CDM750 in vehicle and with engine running sounded horrible, I swapped radio out with a new CDM750 and noise was totally gone. This is not the first time for this happening. I usually have to send the radios back to motorola and they will be fine once I get them back and re-install them into THE SAME vehicle they came out of.

Is there a known series of parts that maybe I can replace with-out sending these in for the horrendous bill I get for motorola repairing these? or maybe I should have motorola repair these under time and materials option instead of flate rate (which is $285)?

[jackhackett]

I've never seen that happen (and I deal with a lot of CDMs).

What I have seen is the chassis screws coming loose, causing all kinds of noise problems. Pop the plastic cover off, snug down all the screws, and see if that fixes it.

Also new CDMs coming from the factory all seem to have loose screws. The screws are numbered with a tightening order and the last few screws will be okay but the lower numbers are loose, as if they didn't go back and recheck them after tightening the others. I've found you have to go back over them all a few times to get them all tight.
If you buy new radios make sure you check that.

[nukedude]

+1 on the loose screws. I have also noticed this on the CDM1550's.

I added very small lock washers under mine to keep them tight. Where I work, the CDM-1250's are mounted in trucks, on small vessels and mobile comm suites. I just thought it was from being in a mobile environment though. So I check some "New Old Stock" radios that have yet to be installed. Some of the screws in them were loose also.

Thank you for posting this, it helped me with a half dozen radios that had loose screws from the factory.

[cbroseinak]

First off, I should have clarified that the noise problem is Transmit noise from the radio. Yes, like bad alternator noise sounds being transmited. Receive is fine. Just seems weird that a new radio or having the radio repaired by Motorola fixes it. And the radios are shipped to Motorola with complaint of "noisey transmit" as the main complaint.

The radios come back and when installed in the same vehicle, the problem is gone. And yes, antenna connectors are always checked to make sure they are tight when inspecting one of these problems.

But I will inspect the chassis top plate on this one I have. That would be a real cheap fix if it works!

Thanks, i'll report back latter.

[KE7JFF]

You know, I remember reading about this same problem elsewhere. If I remember right it was something like a diode failing...I'll see if I can find what I am talking about.

[MassFD]

Almost every CDM we installed in Fire Apparatus had alternator noise in the Xmit audio, we tried all kind of power filtering, ground straps etc. Then we found that turning down the Mic Gain via CPS eliminated most of the noise with almost no reduction in Xmit audio. They must set the gain high to compensate for the low talkers but it also pumps up the noise.

Have never had one that gets worse with age, they were all noisy right out of the box.

[Bill_G]

Almost every CDM we installed in Fire Apparatus had alternator noise in the Xmit audio, we tried all kind of power filtering, ground straps etc. Then we found that turning down the Mic Gain via CPS eliminated most of the noise with almost no reduction in Xmit audio. They must set the gain high to compensate for the low talkers but it also pumps up the noise.

Have never had one that gets worse with age, they were all noisy right out of the box.

Thanks. I have one ambulance that has driven me nuts for years. Tried everything to filter the power, isolate the power, isolate the radio chassis, and isolate the antenna before I finally kinda knocked down the noise to something teeny tiny. It's high pitched noise introduced on the 12V line by a bank of power tool battery chargers. You always know which rescue is rolling before they even say who they are. It never occurred to me to check mic gain because they just use the palm mic, not headsets like the other apparatus. I'll be checking board screws too.

[jackhackett]

First off, I should have clarified that the noise problem is Transmit noise from the radio. Yes, like bad alternator noise sounds being transmited. Receive is fine.

This is pretty much what happens with loose screws, the receive sounds fine but the transmit has loud whining/squealing noises in it.

[cbroseinak]

Wow! This simple fix repaired the problem with this radio and another one another customer brought in and complained about noisey TX. The screws on both these radios were not as loose as some I have seen in the past. in fact I would not realy have considered them "loose" but they obviously were not tight enough. I know there is the 1-6 torque sequence on the top plate, but what is the actual torque spec? By thread size and application I would guess 6ft-lbs max, but is there an actual factory spec?

Although I doubt i'll torque all new radios, every used one I get my paws on will get tightened because like I mentioned, the problem is hard to detect on the bench thru a bench supply/environment. It acts more like a noisey mic PTT switch on the bench and is hard to actually notice when you transmit thru a monitor, but it is there. You put the same radio in a vehicle and it is Whorrible with a capital "W" while the vehicle is running and seems to go away with the vehicle motor off.

Crazy, got to be more to this than I can grasp, but it worked.

[cbroseinak]

Ok, so for grins I just checked a new radio and screw #1 was so loose it probably would havce fallen out if I had inverted the radio, the only reason it hadnt gone further was it was hitting the top wrap-around cover.

Cheesh Motorola...

[Satelite]

Hello:
Yeah and Motorola will only chg you 285 dollars to redo what they didn't do the first time.
Pretty good deal aint it ?
Satelite

[cbroseinak]

I just checked another (4) brand new CDM1250's and on three of them screw#1 was loose by 3turns easily and all other screws were 1/2 to 1turn to get them tight. The fourth one all six screws needed 1/2 to 1 turn to tighten them and I am not cranking these down tight, I really need a small torque wrench with about a 10lb max scale, i'm guessing i'm giving them 4-5ft lbs max. But it would be nice to know what i'm actually giving them. I worry about crushing the power transistor and other components.

Now I am going to have to check everyone of these, especillay any in for programming or serice of any sorts.

[jackhackett]

See?