Batboard

OK, here is one for the highly knowledgable or experienced tech. I have a Motorola CDM750 4 channel UHF mobile model number aam25rhc9aa1an. The problem is that after the unit has been powered up and turned on for any amount of time it will lock up. When it does this it still shows a red led on channel 1 but the radio does not respond to any button pushes or ptt from the mic. Also, the unit will not recieve either. It acts like the unit is powered down but still has a light on it until you remove the power cable. Now, after power has been removed the unit will have to set without power for an hour or so and then it can be powered up again for an undetermined amount of time. I have had the latest firmware flashed into it and a new archive cloned to it but nothing seems to help. I wanted to know if anyone has a solution short of sending it to the depot for repair.

Thanks in advance.

I have a CDM750 VHF, and recently had mine lock up in transmit also.

I suspect RF was getting into the radio via the power cabling, and possibly the coax, so I checked the power setting, and I had placed the radio into high power operation, instead of low power I normally use.

There may be a bad shield on your coaxial cable to the antenna, or faulty ground, and the radio is using the shield as the DC return path.
Or RF is getting in via the DC cabling, as in my circumstance, which caused the radio to remain in transmit until power was removed.

One other cause could be due to lack of available current, and in transmit, the radio draws more than is available, causing the radio to lock in transmit.

Since the power switch is controlled by a logic circuit, the high RF in the radio can effectively 'disable' the shutdown routine of the control, as well as current drain forces a 'brain dead' scenario taking the power switch out of the loop.

Whatever the actual cause, in my case, it was current draw and RF getting into the control circuits of the head.

Thank you for your reply but that describes a different problem. I can understand the theory behind what happened to you but my unit will lock up just being turned on in a standby state. I can power up the unit on a power supply and randomly key it up, talk back to it with a handheld and change channels but after no preset time all of those functions stop working. It acts like it just shuts down and is very frustrating.

Usually when we see this in the shop it is caused when the flash rom (AT49 series) stops responding to calls from the processor. Can be verified by looking at the data and address lines for a repeating pattern square wave type signal. When we find that we replace the flash chip and that usually clears it up.

Regards,

Allen

I've had half a dozen do this, some have been returned to factory and "RUR"ed w/ "unable to reproduce customer complaint". It's frustrating as all hell, thats why they are taxicab radios, and not public safety.

Does the radio have an external accessory plug attached?

If you short any of the pins on the 20 pin rear accessory you will experience the problem you are describing.

Radio will turn on and stop functioning.

Radio may work for a short period of time if the pins which have been shorted, open.

Ran into this on forklifts where the radio mounted under the OSHA headcage would be hit by a rack as the driver sideswiped the shelving units quite often. This would jarr loose the plug by pulling it out sideways bending the pins and the qualified individual would then try and reinsert it back into where they thought it went often shorting the pins and causing the radio to stop working although it remained lit. It normally was pin 16 and 17 getting smashed together.

CoM

...oh, and i forgot the marvelous sensitivity of the Pro lines to B+ voltage. Nothing makes you think like hearing a low batt beep from a mobile! They also have this inane ability to turn ON with low B+ voltage to the primary power connector, i've actually had the radios tip me off to failing vehicle batteries...as more than once i've walked thru the garage and seen a chassis shut off, and the CDM radio powered up and frozen. Great feature.

thebigphish wrote:...oh, and i forgot the marvelous sensitivity of the Pro lines to B+ voltage. Nothing makes you think like hearing a low batt beep from a mobile! They also have this inane ability to turn ON with low B+ voltage to the primary power connector, i've actually had the radios tip me off to failing vehicle batteries...as more than once i've walked thru the garage and seen a chassis shut off, and the CDM radio powered up and frozen. Great feature.

That's an expensive load tester.

Good to know. thx!