First just because the source works with a 8515 doesn’t mean its ok as the 8516 requires different source parameters as the 15 ,? generally no if found or phaselock error comes about if the source power is too low to generate a meaningful IF signal from the reference samplers , for the 8516 the power into the testset must be between 0 and 10dBm for the 45MHz to 20GHz range and 10dBm for the 20-40GHz range , you mentioned that you have a flashing light on your source when the error occurs so more than likely your source goes unleveled when you use the doubler !

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This is the link to the service manual ,? check the unratioed and sampler test procedure to make sure your samplers are ok !

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