LeCroy Waverunner 6200 Repair

[Alex_twn]

Did you check the diodes as well?
I had one PSU with dry capacitors and a broken diode.
If possible you should get a differential probe to check the primary?

[chubbymonk]

Been quite busy lately so haven't had much time to work on this.
I've checked all the diodes in the surrounding circuitry and they all seem fine. I don't have a high voltage differential probe to probe around the primary side unfortunately.

I desoldered the BIAS/PFC control board and discovered the SMPS chip had been replaced along with a couple resistors. Most likely the previous fault and the previous repair was that the MOSFET driving the BIAS windings of T2 had shorted and took along with it all the current sense resistors and the SMPS chip on the BIAS board.
I tested the BIAS board externally by supplying the supply voltage for the SMPS chip and it seemed to work fine, it generated the 5V reference voltage.
I'm pretty sure it's an issue on the primary side, I probed the output of the transformer T2 on the 5V standby rail (arbitrary, was just easy to probe) and there are large spikes in the output before the rectification on the secondary side. Besides the fact that the noise is present on all rails.

What types of component failure could cause there to be such noise on the output while allowing the PSU to function completely normally in terms of voltage and current supply capability? The only thing that comes to mind are capacitors or some convoluted control loop issue but I can't seem to find either of them...

[shakalnokturn]

Weak capacitor or resistor gone higher value in snubber network?
What topology is the main converter ?
You may gain some time in identifying the noise source by hovering your scope probe around as an aerial.

As for probing without a dedicated HV differential probe you still have the isolation transformer option (on the offending PSU of course...) or use two channels with X10 probes with ground clips removed and subtract them.

[chubbymonk]

- All the resistors check out fine and I don't have a way to accurately measure capacitance unfortunately, especially a few suspect SMD ones. The electrolytics seem to check out fine though. (I do believe there might be something wrong with a snubber circuit though)
- Their is essentially two power supply circuits in here, both using the UC3845B SMPS controller. The smaller one powers the VBIAS and 5VSB circuits and is where I believe the problem is. Topology I believe is a flyback (SMPS's are far from my area of expertise so could be wrong on that).
- Waving a scope probe doesn't help too much in this case unfortunately, as basically everything in this supply switches at 100kHz (PFC, both SMPS).
- The CMRR is a problem when using a DIY solution unfortunately, looking at the quality of a 15V rail or some MOSFET gate switching at ~-100V DC bias doesn't really work.

I believe to have isolated the problem to this area of the circuit. Or the signals which drive this circuit.


Seeing the below measurements, the yellow trace represents the transformer output of T2 for the 5VSB rail while the purple trace represents the output of the 5VSB rail. The switching noise (or some of it at least) is correlated directly to the switching output of the transformer, driven by the bias circuit. The ringing on the output of the transformer makes it seem like a snubber circuit isn't doing a very good job. Particularly C92 - R145 snubber circuit maybe?


And everything is pretty much the same but worse on the VBIAS rail. What I'm unsure about is, is it really the snubber circuit(s) (C92-R145 and C103-R149) made of a MLCC and resistor that's the issue? Or could it be something else? No replacement alternatives available locally, so any ideas before I place a Digikey order?

To note: The resistors measure fine and inspecting the MLCC's from under the microscope they look spotless. The electrolytics highlighted and with a comment next to them have all been replaced. The drive MOSFET and current sense resistors have all been replaced, so has the actual switching IC from the previous repair.

[Alex_twn]

Indeed, very often the PSU fail in this area.
On one WR6100 I fixed I had a broken diode and while it is unusual, I recommend you check them as well.

[jfoster]

Hello, I'm trying to repair a WR6050 power supply but I don't have the schematics. If anyone has them could they PM them to me please?

[chubbymonk]

Manual can be downloaded from here: KO4BB.

For my repair, the power supply noise wasn't really there, was just common mode noise
I'm shelving the repair for now, don't have enough time nor effort to continue digging into it...