Hi All,
I have a strange problem with my o-scope power supply that I'm hoping someone can help me provide some ideas to look at or maybe even a schematic.
I've been working in Africa, and had the scope with me. Input power was ~240VAC 50hz. The scope was working fine. I recently moved back to the US, and after shipping, the scope would not power up with ~120VAC 60hz input power. I opened up the scope and found that the 15VDC inhibit voltage was 0, which explains why it won't turn on.
I did a little more tracing out of the circuit, and this is where things get weird. Input to the bridge rectifier was ~119VAC. Output of the bridge rectifier was only 110VDC. (I was expecting ~170VDC.) Nothing on the board is getting hot/warm, so I can't figure out what could be dropping the voltage. I rigged up a cord to plug it into 240VAC, and the scope powered on normally (so this problem is only relates to the input voltage, not damage due to shipping). With 240VAC input, the output of the bridge rectifier was 218VDC, double the output with 120VAC input. I then moved the input back to 120VAC and the scope also powered up on 120VAC input (same 110VDC voltage output from the rectifier). After allowing the power supply to sit overnight without being plugging into AC, it would no longer power up on the 120VAC input. So the problem definitely revolves around the low DC voltage output of the bridge rectifier.
There is a large AC component on the DC side of the rectifier (~65VAC), but even with a bad/shorted diode in the rectifier, shouldn't I still get ~170VDC? I don't have another scope to check the output of the rectifier to see what is really going on. With the bridge rectifier in place, all four diodes test ok, put I can't be sure without removing (under an EMI shield, so not easy to remove). Does anyone have any ideas or a schematic to this power supply so I can figure out what is going on?
Thanks in advance.