Hello all;
I just pulled out of service the above radio. Its replacement worked properly.
The thing receives fine but transmits a 1 to 1.5kHz deviation alternator whine underneath the voice which is at normal deviation. /\/\'s tech department gave me the following fix, and I quote, "loose screws which hold internal die-cast cover to bottom heat sink assembly".
Well, after opening up this UHF rig, I found no separate internal cover, only the external metal cover secured by oh, about 6 or 7 large torx screws, but that's a cover, an EXTERNAL cover underneath the plastic shroud and those screws while not torqued down tight, weren't loose either.
Can anyone help deciphering this "tip"?
Problem is, that I have no way to test the "repair" (occasioned by the complete disassembly and reassembly of the radio... even lifted the circuit board out of the heat sink chassis and eyballed it under a magnifying glass... yea, I did recoat the power transistors and audio amp chip with white heat sink goo when I put it back together.
Now I could drop the radio into my van and hope that my alternator is noisy enough to test my "repair" but what's the odds on that happening. BTW, the customer is about an hour and a half away... can't just run over and do a test in the bus the radio came out of.
Was hoping that some of the denisens here might have already been down this road. Found an open bypass cap in a tx audio amp voltage source, or something of the like.
On another note; has anyone investigated the possibility of feeding an audio signal into the control loop of a linear power supply, to impress a pseudo alternator whine onto the DC output? I've had several radios which would have been much easier to troubleshoot if I could have been able to simulate vehicle electrical noise on the bench.
TIA, Stan
CDM750 Alternator Whine
Moderator: Queue Moderator
/\/\'s tech was right!
Well, I had a chance to test the "repair" and the Motorola tech department was right on!
All it appears that you have to do is take the radio apart and put it back together... Just don't loose the hardware
Stan
Now if I can just figure out how to add noise to my otherwise quite supply...
All it appears that you have to do is take the radio apart and put it back together... Just don't loose the hardware
Stan
Now if I can just figure out how to add noise to my otherwise quite supply...
My two bits worth:
Perhaps the noise is caused by a "Ground Loop". We had problems with Mitrek if the mike low got grounded.
If the noise was in the power source the noise would be an AM signal on the carrier, although you may also hear it on an FM receiver.
If the noise is getting into the microphone audio it will be FM, with no AM component.
I would suspect a resistance between the chassis ground and the power plug ground. That might explain why taking the radio apart and properly re assembling cured the problem. You might have resistance between the solder by the screw holes and the aluminum chassis.
Another thing that can cause endless grief is a broken ground bond between the engine and vehicle chassis. Part of the grounding of the cab is via the radio mount or antenna ground.
Ever see the coax smoke?
I serviced a trucking company once that was having many intermittent problems with MICORs. You could easily spot the trucks at night, the lights were flickering.
After a few hundred thousand miles the ground cable from the engine to cab separated. The ground for the cab was via the Bouden cables as well as the radio wiring.
All it took was a ground jumper to cure all the electrical problems.
The customer was very happy.
Another source of alternator whine is an open alternator diode, or a sulfated battery.
I had one trucking company that insisted their electrical systems were perfect: they could start a Diesel engine..The air starter worked perfectly.
Often the radio man will be called when the vehicle is in the shop for maintenance. The radio passed bench check, works perfectly when re installed. The alternator, or batteries were replaced during maintenance.
Then perhaps the radio has problems after the vehicle was serviced.
The power wires were left un connected, or worse yet improperly connected, such as on a vehicle with 12 system and 24V starting.
(There are two 12V strings of batteries which are switched in series to start the engine, the 12V accessories are connected to the string that is grounded)
Perhaps the noise is caused by a "Ground Loop". We had problems with Mitrek if the mike low got grounded.
If the noise was in the power source the noise would be an AM signal on the carrier, although you may also hear it on an FM receiver.
If the noise is getting into the microphone audio it will be FM, with no AM component.
I would suspect a resistance between the chassis ground and the power plug ground. That might explain why taking the radio apart and properly re assembling cured the problem. You might have resistance between the solder by the screw holes and the aluminum chassis.
Another thing that can cause endless grief is a broken ground bond between the engine and vehicle chassis. Part of the grounding of the cab is via the radio mount or antenna ground.
Ever see the coax smoke?
I serviced a trucking company once that was having many intermittent problems with MICORs. You could easily spot the trucks at night, the lights were flickering.
After a few hundred thousand miles the ground cable from the engine to cab separated. The ground for the cab was via the Bouden cables as well as the radio wiring.
All it took was a ground jumper to cure all the electrical problems.
The customer was very happy.
Another source of alternator whine is an open alternator diode, or a sulfated battery.
I had one trucking company that insisted their electrical systems were perfect: they could start a Diesel engine..The air starter worked perfectly.
Often the radio man will be called when the vehicle is in the shop for maintenance. The radio passed bench check, works perfectly when re installed. The alternator, or batteries were replaced during maintenance.
Then perhaps the radio has problems after the vehicle was serviced.
The power wires were left un connected, or worse yet improperly connected, such as on a vehicle with 12 system and 24V starting.
(There are two 12V strings of batteries which are switched in series to start the engine, the 12V accessories are connected to the string that is grounded)
Aloha, Bernie