EX500 power sense problems

[AM909]

A guy with a fleet of UHF-1 and -2 EX500 of various ages is starting to see multiple radios mis-behaving in a similar way, and I wonder if anyone else has come across this problem with this model. I realize they're old and cheap, but if it's an easy fix to several of them, it would be worthwhile compared to acquiring either someone else's "working" problem radios or different models with the associated cost of different accessories.

Test 1: Remove battery from radio. Turn power switch off. Attach battery to radio. Radio double beeps, and green light blinks once, which is what a "good" radio does when you turn it on (but not before). Press PTT, nothing happens, as expected because power switch is off. Turn power switch on. Nothing happens. Press PTT. Nothing happens. Radio is not responding to power switch.

Test 2: Remove battery from radio. Turn power switch on. Attach battery to radio. Radio double beeps, and green light blinks once. Press PTT and radio transmits and works normally. Turn power switch off. Radio is off. Turn power switch on again. Radio is still off, and will not respond to anything until you reset it by removing the battery.

In Test 1, If you can manage to turn the power switch on in the brief time between attaching the battery and the green light blinking, the radio will be on and working, just as would happen with a good radio.

The problem is not the power switch or the battery. If the controller board is swapped from a good radio into the bad radio, it fixes it. When the radio works, TX and RX are within spec. I've spent a little time with the manual and understand somewhat the nature of the failure and where it might be, and have taken some measurements, but I wonder if someone has run across this before.

Thanks for reading!

 

[tropiradio]

I had the same problem some time ago with a HT750 which uses basically the same power on/off circuit of the EX, and where the action of the power switch was basically reversed. I cannot remember what was the solution that I found but seem to remember a part of the power switch control circuit was replaced. I think one of the resistors. I was checking my notes, but unfortunately could not find exactly what was done to solve the issue. Sorry can't be more specific, but in your case of multiple radios there might be something else happening. Also make sure the battery contacts on the radios are in good shape, as they tend to create random problems and restarts when slightly dirty or worn. I never liked that power up sequence used in these radios.

In any event this was a one time issue, customer has still quite a few HT750s in fleet, including a few EX500 as well. If I can find any other info will post back.