The ADRET 7200 V H F /U H F signal generator is a modular instrument comprising boards which plug into a mother board and plug-in modules in light alloy enclosures. This form of packaging is particulary used for HF circuits to maintain spectral purity specifications in spite of the high operating levels. Modular internal structure offers a number of advantages some of which are relevant to maintenance : - the sub-assembly swapping facility avoiding instrument downtime, - the circuit accessibility for the servicing, - the faster check of running signals by the control points grouping. Maintenance is further simplified by the inclusion of a self-test system controlled by the inbuilt microprocessor. This system, enabled while troubled shooting, gives for each point tested whether the signal is correct or not. The self-test results are not sufficient to produce a definite diagnosis of a subsystem malfunction, but are needed to enable that part of the instrument which has failed to be identified and located.