VX4204 - 50mW output but 15A current?

[g8tzl2004]

I have two 25W VHF VX4204's.

TX output is only around 50mW (maybe less) but current consumption is 15A....significantly more than expected for 25W.

I presume the PA's are bad.

Has anybody come across this before? Any views? Is this a common issue with VX4204's.

Both radio serial numbers are sequential ie xxx466 , xxx467...but maybe that is a coincidence. Both had been used by different companies with different frequencies programmed into the radios.

Thanks

 

[cmdrwill]

At that current, and no power output could be bad antenna switch circuit. The antenna switch could have been damaged by high SWR or another transmitter/antenna too close.

 

[g8tzl2004]

15A is 2x what you would expect for 25W TX output.

If the antenna switch is bad, would SWR protection circuit not back off TX output will a resulting drop in current rather than an increase in current?

 

[g8tzl2004]

I can actually access my local repeater using the faulty Vertex radio - its Txing about 25-50mW but as I say, drawing 15A in TX mode. RX works great. Does this suggest that antenna switch circuit is OK?

 

[902]

Use a Fluke RF probe or RF millivoltmeter and look at the RF voltage on both sides of the PIN diodes that switch the antenna circuit. If there is a severe disparity, look at the switching logic from the microprocessor and the bias on the diode. Assuming that's okay, the PIN diode may be defective. If all that is okay, start looking further down the line for cracked chip caps (you should use a magnifying glass and a dental probe (do not key the radio while you are holding the probe). Follow the path of the RF. At some point, you've got a lot of signal and it's more than likely not getting past a certain point. Find that, and you probably fix your radio.

 

[902]

I can actually access my local repeater using the faulty Vertex radio - its Txing about 25-50mW but as I say, drawing 15A in TX mode. RX works great. Does this suggest that antenna switch circuit is OK?

Actually, no, because only 1/2 of the circuit is working.

 

[g8tzl2004]

The required PA drive level is around 50mW to give 25W output.

The radio is TXing 50mW rather than 25W. Does the fact that the radio is drawing 15A (rather than 7A for 25W TX output) suggest that the PA block is OK. If the PA block was dead, would the radio be drawing only around,say, <1A? Or can a PA block short so that it draws 15A.

In my simplistic way of thinking, the Vertex radios TX side seems to be OK up to the 50mW drive. 50mW is going into the PA block (as required) but only 50mW is coming out of the PA block? If 25W was coming out of the PA block but a cap was cracked so that 25W is not getting out to the antenna, why would the current draw be 15A rather than the expected 7A for 25W TX output? Sorry for what are probably basic questions.

 

[g8tzl2004]

I did some searching for similar Vertex/Yaesu faults. I came across the following:

"Radio drawing over 10A on TX with just a few mW coming out. RX current draw and operation normal. Radio trying to max out gain of final and driver stages but no signal is passing. That is why current draw is high. The fact that current draw is low in RX mode bodes well for final PA transistor. Solution: bad solder joint on surface mount capacitor which couples RF from driver to final".