G0MRF 5W 472kHz PA

I made one of these kits https://www.g0mrf.com/5W%20linear.htm for a ham in the Czech Republic, and sent it in June, but it has never arrived. I made two other kits at the same time, so I machined up a new case from a bar end of some 7xxx aluminium mystery alloy. Went a bit mad with the slitting saw on those fins, but had a lot of fun making it. It will soon be on it’s way to its new home, not far from Prague.

G0MRF amplifier board in a custom machined aluminium case
Lid in place

As I was making the case, I managed to snap an M3 tap in one of the holes. After a lot of failed attempts, I managed to mill it out using a new carbide 3.5 mm end mill, only to snap that off deep in the hole because I forgot to allow for the missing bits of the tap, and the mill grabbed when it hit the tapered tip of the tap. Not willing to admit defeat, I drilled a 2 mm hole from below and knocked the remains of the tap and mill out of the hole. The of course, I had a raggy hole too large to tap, so I reamed it to 4.00 mm and made an oversize stainless steel sleeve, drilled and tapped M3, then used the mill and a mandrel to press the sleeve into the hole. Almost an invisible repair…

The repaired M3 tapped hole with 4.02 mm threaded sleeve insert pressed in

I machined up some 2.2 mm spacers to put the board at the correct level above the case, and drilled and tapped the output transistor mounting hole at an angle so as not to spoil the look of the fins.

PCB spacer machined from aluminium rod

I found that the lid was a bit too close to the output filter toroids, so I milled out a pocket to give more clearance.

Milled pocket under the lid to clear the output filter toroids

I was going to counterbore the holes for the lid mounting screws, but they were too close to the edge, so I just milled out the corners.

Finished case showing the milled-out corners of the lid

After finishing the case and fitting the amplifier PCB, I ran some tests to check the gain and linearity of the amp. It starts to sag a bit above 2 watts output, but will go to more than 6 watts when saturated.

Input dBm 
Output dBm
Gain dB      
-10
-23.7
33.7
-6
27.8
33.8
-3
30.8
33.8
0
33.4
33.4
3
35.2
32.3
6
36.2
30.2
9
37.0
28.0

The spectrum of the output has some amplitude noise at -76 dBc or so, which I think is coming from the power supply I was using for tests. I tested for harmonics, but the signal generator (HP E4421B) is putting out a second harmonic at -66dBc, so the -41dBc result is probably not that bad. Third and higher harmonics are well suppressed.

At 9 dBm in, the initial current draw is 1.07 amps at 13.8 volts. I have set the bias at 140 mA. Saturation is at around 7 watts, but continuous carrier at that level is probably not advisable.

I ran it for 10 minutes at 5W and the output transistor only got to about 45 C, ands the heatsink reached 42 C, with no proper airflow.

After 20 minutes at 9dBm in, the gain had dropped to 27.0 dB and the current draw increased to 1.24 amps. Output dropped to around 4 watts after 8 minutes or so.

I checked the temperature of the components using my FLIR One infra-red camera, and found some hotspots

The IR and visible images are offset about 8mm, so the really hot parts are one of the resistors near the PA and the PA RF choke. The output 1:4 transformer and the driver transistor are also quite hot.

Resistor near the PA is at 117C
The PA choke is at 108C
The class A driver runs quite hot, it is at 77C here after 20 minutes at 4-5 W

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