MTR2000 AC/DC power supply

[RFdude]

Anyone know if there are any issues with feeding the MTR2000 AC/DC power supply with DC from a large, central +24V power plant? AC would not be used (ever). Battery charging would be through +24V rectifiers, not the MTR power supply which puts out over 28V.

There is a +24V DC only supply available for the radio, but this is more of a logistical issue for us. It would be more convenient to stock one power supply for both AC and DC applications.

The MTR supply has switchers to provide regulated 14V and 5V, while the PA operates directly from the 24V input. However, I vaguely recall discussions that the AC/DC supply has some sort of peculiarity that can cause it to fail if DC (21 to 27V) is supplied directly to the DC input without first suppling AC, then reverting to DC. Off-the-cuff, I find this hard to explain. Argus makes a special power supply that handles this switch over AND charges a battery, but this solution is valid on single radio basis.

So, one of the techs gave this a try (DC into the AC/DC power supply) on the bench and the sacrificial radio is now lifeless. No pop... no blown fuse... just dead. AC doesn't work anymore either.

Any ideas from the Borg?

RF Dude

[HumHead]

I would have to dig some more, but a couple of thoughts:

1) I'm sure that in the course of working on MTRs I have connected and disconnected the DC revert power without first going onto AC any number of times. I've never lost any magic smoke.

2) When the MTR is running on AC, there is voltage present on the revert connector. There were some considerations attached to that, but I don't recall the details off of the top of my head, and I would rather not guess. I seem to recall it being a bad thing to have the batteries connected when the unit was on AC. The installation that I currently manage has a disconnect relay driven from the cabinet AC to disconnect the battery bank when the station is on AC. The Argus charger that /\/\ offers also performs the same function. None of this is likely to be an issue if you are running on DC only, at least until the day that someone goes and sticks an IEC cable into the back of a DC powered MTR...

3) If your unit is an AC unit, running it off the revert input will mean that it will constantly think that it is in AC fail mode. This means that you will have to set up the AC fail RF power level as your default, and will have to do something about the AC fail alarm beep transmission.

[RFdude]

Thanks HumHead for your thoughts. Your point (3) regarding the AC fail beeps were overlooked... so you have alreay saved me some trouble by bringing this up.

BTW... I did have an interesting issue with the DC ONLY version of the MTR2k power supply... Seems that when using these PS with large +24V power plants, turning on a circuit breaker or closing the LVD would provide a sharp "step response" or near instantaneous ramp up of voltage into the MTR2k PS that it would NOT FUNCTION some of the time. Seems that sometimes the 14V switcher would not start switching under this condition, and thus no 14V and no 5V. Only certain combinations of power plant and circuit breaker would duplicate this condition. Seems that Airpax 30A CB have a lower internal impedance than Heinemann. The problem would only occur when using an Airpax CB, and then, only about 33% of the time. But it was repeatable. Motorola's power supply vendor investigated and has since been adding a bit of resistance in the input circuit to slow down the power up ramp.

RF Dude.

[HumHead]

Interestingly, I had a problem a while back with an MTR2000 that was not sending the AC fail beep over the air. You could always turn the problem into a "feature":

http://batboard.batlabs.com/viewtopic.php?t=27110