Mini Circuits ZHL-10W-2G+ sans magic smoke, teardown and repair attempt

[eriklidgren]

Hello
I'm a newbie into RF.
I bought a broken Mini Circuits ZHL-10W-2G+ amplifier that I'm going to attempt to repair. It's labeled "EMC lab" so I think its been used for EMC. I don't know what happened to it, load mismatch? If I can repair it I'm going to use it for immunity testing.
The switching pre-regulators have released their magic smoke. The last two power stages all have a short between source and drain, no visible damage.
After removing the pre-regulators(bypassed them with a blob of solder), and shorted transistors, it will power up and the linear regulators are regulating.
I put rf into the input(HP 8753B) and the first 2 amplifier stages(green) seem ok. The signal is larger after each stage.
There is a component circled in blue that I'm not sure what it does?
The power transistors are somewhat expensive so I figure it's best to ask here before I do anything. Are there any, "traps for young players?"
There are a couple of multi-turn trimmers that I guess are for biasing of the transistors.

/Erik

[G0HZU]

The last two power stages all have a short between source and drain, no visible damage.

The power transistors are somewhat expensive so I figure it's best to ask before here before I do anything. Are there any, "traps for young players?"

If the power stages use a depletion mode device then the drain and source will appear to have a short across them (shows just a few ohms when tested with a DMM) when the amp is unpowered. So this symptom would be normal and not a fault.
If the amp does use a depletion mode device then there should be a negative voltage at the gate when the amp is powered.

[Yansi]

The component circled in blue might be attenuator. Seen those before, were labeled for example "AT" in silkscreen and marked in dB value on the package. But I might be wrong with this one.

[G0HZU]

The component circled in blue might be attenuator. Seen those before, were labeled for example "AT" in silkscreen and marked in dB value on the package. But I might be wrong with this one.

I think that it is likely to be a thermopad. Eg something from EMC Technology.

http://www.emc-rflabs.com/Passive-Components/Thermopad

[eriklidgren]

Wow, fast replies.
@blueskull
Both 50 ohm resistors near each 90 deg coupler measure 50.2 Ohm at DC. So they should be ok.

 @G0HZU
Ok, the last stage uses FLL120 transistors. Without them mounted, there is -1.15V where the gate should be. So if I'm lucky they might be ok?
There's a PNP transistor in the bottom right corner. I've not studied its function that much. With the power transistors mounted, it would limit the voltage to the regulators. I started with a low current limit on my supply and  increased it to see if anything would happen. I chickened out when the PNP-transistor was dissipating 20W. Maybe I need to start with a higher current limit for that not to happen. Or maybe something to do with the missing pre-regulators.

@Yansi and G0HZU
Ok. It has reference designator R7 in the silk screen.

I'll order the pre-regulators and do some further testing.

Thanks
/Erik

[eriklidgren]

The spare parts I ordered arrived today. I put everything back together and it works. Thank you all for your help.
Regards
Erik