The LM393 has a 3.3k pull up resistor to 12V rail voltage on its output, and it DID have a feedback resistor for hysteresis, but removing it seemed to improve stability.... This evening's job is to `scope the output of the 393 and the input of the 4023. The thing is at a fairly late stage of development, changing the 4023 is something i would like to avoid if possible. I have added extra LED's and flip flops to accurately show what fault triggered the amp's shut down, others have this amp working wothout issue, I am *fairly* sure lead length and RF pick up is the issue. Not being too competent with a `scope makes me not too optimistic about looking for noise accurately, we'll see. Thanks for the reply, all opinion very welcome! Here's a couple of photos of it being bench tested, the wiring is of course very temporary and worst case scenario...