HMC307 GaAs IC digital attenuator - Failure Modes?

[NY2KW]

I am repairing a R&S SM300 signal generator.  Originally it couldn't output more than -26dBm but I got it fixed replacing a an RF amp (AGB3303) at the end of the attenuator chain.  Now it all works linearly from  -120 to +13dbM and flat across 0.5MHz - 3 GHz BUT at any power level, it 12 dB low, e.g at maximum 13 dBm it is outputs 1 dBm.   R&S won't share a schematic but the block diagram gives a lot of information (see below).  In the final RF chain, there here are 4 HMC307's digitally controlled (see attached datasheet), the last 3 are shown in the Block diagram as 0/30dB all or none, while the first HMC307 is 0-30 dB in 2 dB steps.   Could a HMC307 fail so that 2 of the GaAs transistors are not switching out both the 4db and 8db paths?  I guess it could also be a problem with the ATT control chip but I cannot see it as there is a lot of soldered down shielding and removing them is above my skill grade.  Any advice appreciated.

[NY2KW]

Here is a picture of the RF PCB:

[chick0n]

Hi

did you ever find a solution.

I had same problem with my sm300. I replaced blown AGB3303, but output is still about 4dB down.