HP (Infinium) 54845A Power Supply Problem

[scoutdriver73]

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.

[Miti]

With a shorted diode you’d get sparks and blown fuse. You have dry capacitors in that PS. Start with the first one after the bridge but I would replace all of them, primary and secondary.

[scoutdriver73]

There is no filter cap after the rectifier. This explains both the AC component and the "apparent" low DC output from the rectifier. Output of the rectifier goes to a switching transformer. There are at least two resistors networks on the output which I assume are for level/refence signal. I have not traced out the entire circuit yet, but I believe the mentioned switching transformer is a boost converter, as I did measure 380VDC across the main filter caps on the board. I found a schematic for a TEK scope power supply which seems to match very closely what I've traced out so far. My best guess is the UVLO is keeping the boost converter from running. Hard to trace because all the control is on a vertically mounted PCB, but at least I have a starting point with the resistor networks.

Any other thoughts and input are welcome.

Thanks   

[wn1fju]

You say, "There is no filter cap after the rectifier." 

Am I missing something?  What about C12 on your schematic?  There has to be a large (relative to the others on the board) cap somewhere close to the switching transistors.

Also, be careful of interpreting DC voltages when there is a huge amount of AC ripple.  I have made this mistake many times.  A DC voltmeter will give you some sort of average reading when a capacitor is nearly open and the DC voltage is fluctuating (typically at 120 Hz).  As a previous poster suggested, check/replace the large cap on the primary side.  And check all of the outputs (with the supply off) to make sure there are no (near) shorts.

And be careful!  I can't tell from your previous posts whether or not you have experience working on switching power supplies or not.  They can hurt you really bad (or kill you) if you don't know what you are doing.  It's best to use an isolation transformer if you are going to be poking inside.