CDM power output went from 20 to 0. Whats wrong?

[nmfire10]

So I have this one CDM1250 that is arguably the red headed stepchild of all my radios. I recently had the firmware crash during an upgrade but fixed it. That got it working and I tuned it perfectly. However, it was only doing at most 20 watts despite being a 48 watt radio. It worked fine at 20 watts this morning. This afternoon, its got nothing. Power output is so low, the monitor switches to millivolts rather than watts.

I set the PA Bias properly and I double checked it. I don't know what happened. Is there maybe a fuse for the PA inside the radio? I hope its not toast. It still receives perfect.

[Hightower]

I'd venture to say the PA is blown. If it's outputting some milliwatts, then even if there was a fuse to the PA it would still be good due to the few mW measured.

The actual PA RF transistor comprises of a transistor array - usually 2-10 discrete high power transistors. These all work together to deliver the RF output - 48 watts in the case of a HP CDM. Seeing as the power dropped drastically to 20 watts or so, that is usually caused by a few of the transistor's being shorted or blown on the PA RF transistor. If the damaged individual transistors in the RF transistor are shorted, even with the radio turned off, it will get warm.

If you can get a new RF transistor, that more than likely will fix the problem. Once replaced, carefully open the bad transistor and you'll see what I'm talking about with the "transistor array" found in a typical high power RF transistor.

[jackhackett]

Once replaced, carefully open the bad transistor and you'll see what I'm talking about with the "transistor array" found in a typical high power RF transistor.

Carefully is right.. RF power transistors can contain toxic beryllium oxide, you really don't wont to be breaking them open just to see what's inside.

[Karfield]

As I'm sure you've openned the unit already just be sure you verify if it's LDMOS or a BJT board. If by chance it is a LDMOS type also keep in mind that the little preamp on the top side of the board can also go bad so don't dispair too much if you replace the PA and still have issues. Also be careful with the removal of the PA as if you pry too hard you'll pull the traces and end up with a RX only brick. Care also should be taken when prying the board up from it's heatsink as that is where I've managed to damage traces on CDM's.

[chartofmaryland]

Check all of the solder flows around the PA trans. The 1250's PA chip fail the most from overheating and them not being put together with silver solder. The chip may be too far gone but I have been able to save some. Reflow and try again. You can tell very quickly by trying a little pull with a small screwdriver and if it comes off in your hand, by george I think you may have found it.

CoM