RfConcepts 2-317 output power low

[WA7KGX]

I suspect one of the RF transistors is not working.
The amp appears to top out at 60-70 watts output with about 170 watts DC input.
The amp should be doing about twice that.
Increasing RF input beyond about 10 watts does not increase power output much.

I am not familiar with this unit, nor with high power solid state VHF circuits.

Suggestions as to what to look for are welcome.

[K8AXW]

Strange way of phrasing your question.  "Input power 170watts?"

Ask your question by the numbers:

Drive Power?
Output Power?
Power Supply Voltage...no load and under load?

From your figures, the amp is drawing about half of it's rated current.
It is operating at about 40% efficiency. (Assuming the power supply voltage isn't sagging under load)

I suspect your observations are correct.  You have lost an output transistor.  However, more info needed.

[WA7KGX]

I opened up the amp, tweeked some caps, and rechecked my measurements.
Both of the SRF3897 RF transistors got quite warm to the touch.

Measurements indicate the Icom 706mkiig was only putting out 24 watts
at 147 mHz, not the rated 50 watts.  The amp was putting out 110 watts.
The DC input to the amp is about 250 watts.   The output voltage meter
on the Astron RM-35M does not even twitch.   According to a field strength
meter, the amp's power gain is 10-12 db.

So it appears the amp is fine but the Icom is a bit anemic.

[K8AXW]

Looks like you've answered your own question!  Good job!  Have you checked the exciter directly into a dummy load?  SWR between the exciter and amplifier?

[KE3WD]

By all means you should check the output of the icom directly into a dummy load. 

It could be that the input stage of the amp or a bad jumper or connection is causing the icom to see something other than a 50 ohm match and thus it is dutifully folding back powe to protect the output from destruction.


73

[WA7KGX]

Connecting the 706mkiig directly to the Bendix 263 and hence the antenna lead
gives 28 watts forward and 5 reverse according to the Bendix "MicroMatch 263".
That is after applying the 1.23 fudge factor for 2 meters.  Of course, these numbers
might be a bit  beyond what my modest metrology can support.

The Astron RM-35M shows negligible output voltage drop in going from an amp or so to 24.
The lights do dim a bit but the power supply does not drop noticeably.

[KE3WD]

Does that mean you do not have an appropriate 50 ohm Dummy Load? 

Teh sentence with "Bendix 263" in it is rather cryptic...


73

[KA5IPF]

You're forgetting a major thing in the power calculations. The voltage doesn't drop at the power supply as indicated on the meter. That doesn't tell you the voltage at the radio or amp. Voltage drop needs to me measured at the device itself, not the supply.

Clif

[WA7KGX]

Does that mean you do not have an appropriate 50 ohm Dummy Load? 

Teh sentence with "Bendix 263" in it is rather cryptic...


73

Around here a flat antenna on a random simplex frequency is indistinguishable from a dummy load ;-)

The Bendix 263 MicroMatch is a 60s era forward/reflected power meter with 10, 100, and 1000 watt scales.
After "calibration", it agrees with the power out on an Icom 756 Pro and MFJ 993B tuner.  Its manual
gives corrections for VHF frequencies.  Its main use here is monitoring output power in the .1 to 10 watt
range usually used for WSPR.  Handy little device.

[KE3WD]

The device is NOT the crptic part of the sentence...


73