cdm trouble shooting newbe question on voltage readings

[insight3fl]

Since I am not an engineer / tech and have done little troubleshooting on systems this intense, my question is when the schematic says "=5V" and my digital VOM says "4.86" is that "close enough" or must it be exactly 5V to assume that circuit is ok? This is in regard to no power on and using the manual trouble shooting diagram (for the rotary / on off). I hope this is not a ridiculous question....

Thanks,

Dennis

[Bill_G]

Depends on the voltmeter and on the circuit. If it's a good quality device (like a Fluke), in good shape, high probability it's accurate, then 4.86v seems a tad low, but might be totally normal. 4.96 I could live with in general. I wouldn't question it. But, let's say you're measuring 4.86 on the output of the 5v reg - I'd wonder if there was excessive draw, or low drive, or a bad reg. But, if it was a nominal 5v on one input to an opamp, and I measured 4.86v, I probably wouldn't worry about it.

[insight3fl]

That puts it into perspective. The voltage is what is being used to trigger the power up on the cdm 1550. The non working unit gave me those measurement on the head test points on the potentiometer switch. Without verifying the same values on a working head, I plugged that head into the subject radio and still no power up. I am hesitant though to plug the head from the un-working subject into the working one for fear I mess that unit up with a bad head....but I did not think these values would cause a no-power up issue being only off a few tenths.

Thanks

[Bill_G]

If you have a working unit, then compare the voltages.

I've had a few hundredths of a volt mean the difference between working, and not working. For example, the Motorola UT GPS board drives the active antenna with current limited 5V down the standard coaxial cable. Open circuit (no antenna attached) is usually around 4.92V, and with a load (antenna connected) it drops to 4.86V fairly reliably.

[insight3fl]

Thanks... I am going to run the same voltage test on a working head. The only other long shot is the ignition sense "pico fuse" with the radio being programmed for ignition only. I have of course been feeding that pin 10 all the while but I have read that there is a fuse (underside of the board?) that can interrupt that signal.

[jackhackett]

The voltage on the on/off switch is only regulated with a simple zener diode circuit, so it's not going to be super accurate. Seeing as the radio didn't work with a known good head the on/off circuit probably isn't the problem.

There is no "ignition sense" fuse, but there is one that feeds power to the control head.. location depends on specific model. It's either a very small surface mount fuse or on some models it's just a circuit trace, check the service manual for exact location. It feeds FLT_A+ (12 volts or so) to the control head so you could check for that.

Usually a dead CDM is caused by the reference oscillator going bad. To check it you need a service monitor or a receiver sensitive enough to pick up the 16.8MHz signal from it. It's not a field serviceable part though.

Also sometimes the Flash ROM can go bad and that might cause a dead radio.

[insight3fl]

Thanks Jackhackett.... I think I am pounding my head against the wall on this one and should bail out before I waste anymore time on it. I have learned a great deal the past few days studying the diagrams / schematics and reading everything I can find here. Worth the time so far, but not if there is not end game on this unit other than to re sell it for parts as I bought it. I did verify supply voltage to head and found the fuse is in tact and voltage is to the head.

[insight3fl]

Also confirmed head works on another radio that does power up.

[jackhackett]

Yeah... probably the reference then. If you hit the oscillator module with a little freeze spray they'll usually pop back on and work for a short time, then die again after they warm back up. That's a quick way to confirm the problem, but like I said, it's not supposed to be field replaceable and depot just replaces the whole board.