If it weren't for these troubleshooting threads, I would probably be a lazier tech.
I'm spoiled, accustomed to having a service monitor, signal generators, counters, 'scopes and a spectrum analyzer at arm's reach.
Sure, knowing how to track down a failure with all that stuff on hand is useful. More efficient, for sure. Getting more stuff fixed with less labor is just a business judgement. Spend more to make more. Joe Base Station isn't fixing a few dozen radios or more a week. He only cares about his radio, and won't have the luxury of all the tech toys. Could never justify the expense.
But coming up with a fault-tracing strategy with only simple tools like a voltmeter and dummy load?
Gives you less information to work with, and forces you to give the process more thought. Takes a bit more creative thought than blindly (!) picking up the probe to your scope and poking it into the radio. It's what I usually do first.
But I ran out of ideas on this one. Troubleshooting is a process. You check parts of the radio to see if that circuit is working. If it is, you move on to the next section on the block diagram that you suspect and make whatever measurement tells you if that part of the radio is working. Eventually you run out of parts that work. You'll stumble across the root of your problem eventually.
Just one problem. Not all symptoms are caused by a fault in a DC circuit. That is, a circuit function that can be checked by measuring a DC voltage alone. A failed resistor, a capacitor that has leakage current or is shorted, a bad diode will disrupt at least one DC voltage from what it's supposed to be. This kind of failure also disrupts the signals inside the radio. Nice part is, a DC voltmeter, ohm meter can lead you to the root of the failure.
But not all failures are in a DC circuit. The radio is full of AC-only circuits. Your transmit and receive signals are AC signals. And if a part fails that only disrupts the signal at that spot in the radio, there won't be a "wrong" DC voltage to measure and point you to it.
A bad crystal is a good example of an AC-circuit fault.
A shorted RF choke is another example. If the part is good, it will serve to isolate an AC circuit from the DC circuit in that part of the radio. But if it shorts, the DC voltages are almost never disrupted. A shorted choke in the wrong spot can shut the radio down, and a DC meter won't point you to it easily.
A 'scope and/or signal monitor would show you this just fine. But only if you have one on hand.
I thought if maybe I spent a few minutes talking about the process of troubleshooting maybe I would come up with another idea, and suggest what to check next.
Can't be THAT many things in there that would disable only USB mode.
But I'm still stumped. ;-(
73
