Machine manufacturers regularly use us as their parts service company. We received a Giddings & Lewis servo control PCB, part no 501-04303-02, for repair. This PCB had ‘done the rounds’ with other less successful repair companies who said they could not repair or test this PCB. The machine agent sent the board to us for our assessment. The fault reported was ‘servo channel #1 was faulting on diagnostics. From our experience we know that the inputs to these types of PCB’s are very sensitive. Our initial approach was to replace the entire analogue front end. During this replacement we found a number of drifting carbon resistors. These were also replaced. Our next action was to functionally test the sample/hold stages of the PCB. During this operation we found one stage did not work correctly, one of the ic’s, even though functional, was giving a low output. After further fault tracing in this area we found that a sample storage capacitor had become leaky. This was replaced and the output levels were restored. We carried on checking the rest of the PCB with a number of sophisticated test pieces. The processor and associated circuitry were checked and run, the EPROM’s were checked and the data copied for storage. One we were satisfied with the repair the board was cleaned, the edge connectors were cleaned and the PCB was lacquered for protection. The board was shipped back to the machine agent who sent it back to his customer. Once fitted the machine diagnostic mode was selected and the machine performed as expected. This type of repair shows that even without drawings or a full test rig, complex PCB’s can be successfully repaired.