[cctalk] Re: Rainbow H7842 PSU Fault

Thanks for the various replies. Here are my responses and further observations. I was asked if I had a load on the PSU while bench testing it. The answer to that is no. I did check the two diodes around the -12V current sensing resistor (PSU Sheet 3), but as was pointed out the current sensing resistor is very low value (51 ohms). The two diodes measure the same, in circuit, on the working and non-working PSUs. I suppose I may need to lift them to check them properly. I am not sure about the comment suggesting that the bench test conditions could result an overcurrent being sensed on the -12V output. This is because I use the same bench test conditions on both the working and non-working PSUs, and on the working one an overcurrent is not sensed. I hadn't noticed the connection to -12V on the non-inverting input of E1b. However, I don't think this can account for the 0.6V because in both the working and non-working PSUs the non-inverting input to E1b is 4V. However, when I was checking this, I noticed that I must have made a mistake, because the -12V output actually measures +0.4V on the working PSU (not zero as I first thought), but the non-working PSU measures +0.6V. Also, I noticed that the -12V output on the working PSU rises more slowly to +0.4V than on the non-working PSU where it rises more quickly to +0.6V. I still don't understand where this +ve voltage on the -12V output can come from though? Whether on the working PSU or the non-working PSU. Thanks Rob