Electronics > Repair

Fluke 5440B repair

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Rax:
With the replacement A9 A8 board, if I try to output a "previously error condition triggering output" (say, 100V), I'm still getting the "UNDER VOLTAGE" error. If I try to output, say, 10V, I'm getting an "OVER VOLTAGE" error. Outputting 10V was successful with the original A9 board. So there seem to be new errors of sorts with the replacement board.

I think I'm just going to have to take a very close look at the trouble unit's A8.

Rax:
This project's still active. I've not had a ton of time to spend on it, but during the winter holidays there may be some additional time so I am getting this back to the front of the docket. I am hoping for some input on what's going on here and where to head next.

A quick and probably partial recap and status report.

          1. The unit initially was completely dead, which I tracked to A19 not being supplied with 120VAC. Which meant all outguard modules were essentially off.

          2. Once the above was corrected, the unit started largely working. Except that, when I attempt to output more than about 60V, the unit initially successfully outputs the requested voltage (say, 70V), but just for a few moments, then it drops to about 57V, which triggers the fault mode "OUTPUT LIMIT FAULT. OUTPUT UNDER VOLTAGE."

          3. My next step at this point was to swap modules (A4, A5, A13/12, and I may have tried others) from my fully working (successfully repaired) 5440A with inconclusive results.
 
Then I went ahead and tracked the symptoms above (the lower than set output voltage) backwards through the output board and back to A13 FILTER B board. Technically, I think the issue resides on what Fluke calls the "HV Loop," and they have some pretty contorted and not illuminating (at all, to me) steps in section 4-23.

I've done this by going by section 4-19, where I've applied a DCV proportional to the set output vs. full scale between A4 (OUPUT board, NOT A5, as per Service Manual, so please note the error in the manual) A4TP5 and A4TP13. For instance, a set 70V out would want this applied DC to be about 112mV. I did this with my HP8904A, which has the great convenience of being able to key-input voltage settings directly. This step is in order to disable the monitoring system, so an out-of-bounds condition (more than 5% difference between the output and the set voltage) would not reset the unit and further troubleshooting would be possible to make at length.

Under these conditions, the events at #2 above occur and stay active past the few seconds it'd take the unit to actual its reset. To me, this confirms the monitoring is operating correctly and the issue originates with the generation of the HV.

       4. I looked at +TSV and -TSV by probing A13TP1 to A13TP2, and I am seeing the voltage go up to about 5V (as Vout stabilizes to 70V), then dropping to about 2.15V (as the voltage out is falling from the 70V set to the 57V is settles for), and then going back up a bit to something like 2.5V or so. I do see some ripple riding on top of that (per my Fluke 189), though it varies pretty wildly (it starts low, maybe mV, but as the fault occurs, it jumps to maybe 250-300mV, then goes down in the tens of mV and stays there). I am not 100% sure what I should be seeing or looking for at A13TP1 and A13TP3, so I don't have a satisfying analysis of my own of what this means.
       5. I am not seeing anything significant happening over R20 (the +TSV .33 ohm) when the fault occurs. Barely in the mV range if even that. So there seems to be no protection actuation on the HV that could be causing this.

MegaVolt:
As I understand it, the problem is with the voltage drop at the output in the range of 275V.

First, take a multi-channel oscilloscope (4 channels) and study the picture "Figure 2-8. Sample String PCA, Simplified Circuit Diagram"

Put the oscilloscope probes in many points of the device at the same time and see in which node the first voltage drop occurs.

1. DAC output.
2. PREAMP SUMMING JUNCTION point
3. PREAMP PCA OUT
4. OUTPUT

Looking at these graphs, you can understand which of the nodes breaks through first. This will greatly narrow the search area.

Rax:
Thank you for thinking on this with me, MegaVolt.

Yes, the 275V, but also the 1100V. So everything over about 60V seems to trigger this fault state. Apparently the unit is unable to output more than the about 57V + 5% for fault to be triggered, but that seems to be the case.

Right now, I'm trying to get a four channel digital scope at my bench (it just so weirdly happens I don't have a fully working one right now, just one that's a project...). Before I do that, I'll be trying a few other things, but if they go anywhere, I'll post here with results.

Rax:
Well, project's done... Hugely gratifying and a gianormous project in a few different ways for sure.

I hope I'll not miss anything, but this unit was inflicted with more separate and possibly disconnected issues than anything I've seen before. Attempt at a summary:

* The HV issue tracked back to A13, where HV gets cooked. That's because the TS +/- lines coming from the digital drives in seemed correct. Almost accidentally, I discovered that voltages over 876V were output successfully, so that put the TS1, TS2, and TS3 triac drives under suspicion (TS4 deemed good). Replacing the corresponding triacs (Q2-4, 5-7) restored operation of the corresponding range for TS2. No luck on TS1 and TS3.
* Given A13 seemed to be the origin of all evils, I went ahead and replaced the optocouplers on the faulty paths. Also, all bridges (including TS4, while I had my hands dirty). This way, those paths had essentially all new parts in them. No luck.
* Further investigation revealed the TS AC inputs into those triac drives were present at A11 and missing at A13. Stripping the unit bare revealed that A11/J3 and A13/J6 were not talking to each other. Taking a magnifier to A3 (MB) revealed the MB card edge connectors had broken pins. This included TS5 (magnifier was a literal must to figure this one out) which wasn't a suspect before, but a perfect tracking of both + and - sides may have been explained by this third AC path being broken. IMHO, the mechanics of this is a design flaw - the massive transformer attached to A11 should be fastened to the A11 board with plastic washers, or something else that would provide a tiny bit of leeway. The massive transformer "manhandles" A11 during insertion into the MB connectors during servicing; or any shipping event, for that matter (even if the transformer is fastened to the chassis, ultimately). Not very smart of you, Fluke.
* Once the edge connector was fixed, the unit had one issue left, namely an "INSIDE GUARD FAULT. CHECK GUARD CTRL BUS" error, which became more and more persistent.
* This was a tricky one, and close examination of the GPOP line revealed some odd handling of the +5 TS AC signal by A10/U13. Replacing C31 fixed this in a climax I have not frankly anticipated.Exceedingly happy camper. Thanks again, MegaVolt, for your kind pitching of thoughts on here. Effusive kudos for your availability.

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