I am having a new problem with a previously designed SEPIC switching regulator circuit that comletely perplexes me. I am using the Linear Technology SEPIC switching regulator with recommended circuit from Linear Technology to go from a 4-cell battery supply (4-6V) to a 7.5V output. The design was previously giving me a very nice result. I recently had new PCB boards made with very slight modifications to them... had a handful of boards assembled by a local company for me, and the output of the switching regulator now buzzes with a sawtooth wave of about a 1V amplitude centered around the desired 7.5V output. I've experimented with different output capacitors, which only changes the frequency of this sawtooth, but the amplitude remains at about 1V... I have also noticed that if I remove my 20MHz crystal from the board the buzzing goes away completely. I'm not sure how this could be related... but never had any of these kind of problems with my previous PCB's. I was also forced to change a few components as the ones I had previously ordered were no longer available, i.e. diode, capacitor, etc... I'm now left wondering if the problem is a result of the PCB change or the parts change? Or if I simply received a batch of defective PCB's? They work functionally... but could they just be bad quality somehow causing this problem? Does anyone know what would cause the SEPIC regulator circuit to buzz like this?