Fluke 5440A repair

[MegaVolt]

Another sign that the A2 board processor is working will be the FAULT error LED lighting up some time after switching on.

[Rax]

Another sign that the A2 board processor is working will be the FAULT error LED lighting up some time after switching on.

If a delay from actuating the power knob to the moment of the FAULT LED coming on is the critical aspect here - I can confirm there's no delay at all. The FAULT LED on A2 is turning on at the exact time the instrument is turned on. At least to my eyes.

[MegaVolt]

I checked the diagram. This is the correct behaviour.
Immediately after the reset, the LED lights up. If all is well, it should turn off.

But for this, both boards, A2 and A16, and the connection between them must work. For that reason, I wouldn't count on it.

[Rax]

Thank you, MegaVolt.
This far, given enough run time, it seems those interruptions are inevitable. I probably should investigate if there's any sign of trouble on that specific data path, hardware-wise, that may corrupt it.
But from the data accrued, for the healthy segments, were you able to decipher anything useful?...

[MegaVolt]

But from the data accrued, for the healthy segments, were you able to decipher anything useful?...

I'm sorry ... From the data collected, there is little that can be said.

I see that sometimes it turns out to collect good data. Sometimes not. I don't really understand what this is about. Possibly bad contact? Or the influence of probes.

The device shows signs of life. But something gets in the way.

[Rax]

I will seek to collect the data at different points in the chain, maybe at some particular step it gets corrupted (in the hardware, I'm thinking).

[Rax]

Taking a closer look at this I observe what I didn't notice before - U20 seems to be missing on my A2 PCB. Is this normal?
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Later: U20 is noted as a "SPARE" on p7-9. So maybe it missing from my board is not out of order?

[MegaVolt]

Taking a closer look at this I observe what I didn't notice before - U20 seems to be missing on my A2 PCB. Is this normal?
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Later: U20 is noted as a "SPARE" on p7-9. So maybe it missing from my board is not out of order?

This chip is not installed. At least for my device, its absence does not prevent it from working.

[Kleinstein]

From the footprint is looks like U20 would be another RAM chip. It could be place just in case the SW may need more memory than expected or alternatively use samller size RAM chips and more of these (e.g. 3 x 1 K instead of 2 x 2 K).

[Rax]

From the footprint is looks like U20 would be another RAM chip. It could be place just in case the SW may need more memory than expected or alternatively use smaller size RAM chips and more of these (e.g. 3 x 1 K instead of 2 x 2 K).

The SM makes this a bit confusing - while the schematic calls it "RAM" (p.7-9), the parts list calls it a CD4514BE on p.5-12 (actually, a "CO4514BE," but I don't think that part exists), which would obviously be a CMOS Latch/Decoder. All things considered, I think it's a memory chip that's not being used, like you point out.

[Rax]

Grab KiCAD and create a simple board that passes from an edge connector to whatever you get from Newark.
TonyG

TonyG - just wondering if you're aware of footprints in KiCAD for card edge connectors with 3.81mm/.0150" pitch. I have not found any and maybe they exist somewhere. I'll seek to create them, but I'm not entirely sure how involved that'd be.

[Tony_G]

I couldn't find any - I had to create my own for use in some 8350B plugins.

https://www.eevblog.com/forum/kicad/kicad-0-1-card-edge-connector-definition/msg4192591/

TonyG

[Rax]

Proto extender boards just about ready to be tested. EDAC makes these in pretty low order runs, but it takes a couple of months or so, so here we are.

I need to fix a couple of things - one of which (DUH!!) being that I only provided corresponding notches for one of the positions (and, expectedly, they are all placed at a different locations for each given module position) - and that I need to figure which version (I have a "short" and a "long" one - the latter being the same size as the existing boards, the former being a bot shorter, such that boards plugged into it would partially insert in the vertical guides for a bit more rugged operation).

I will make these available, one way or another, once tested and vetted.

[Rax]

Just a pic or two as to why I made a "short" version (as opposed to the "long," which is as long as the actual modules). It seems to be working pretty well.

I need to fix the boards (a couple more bugs as I look at it closer), and will make a more permanent version soon.

This would at last allow me to troubleshoot a bit more easily (and apply SM procedures).

[Rax]

While still waiting for revised boards, I thought I'd give a run to my new rework setup - a Pace SX90 w/ an MBT PS (which I'm just in love with, BTW... Highly recommended).

Recapping is the obvious use for this rig, and so I started looking at the boards a little closer, and noticed that C6 on A5 (Output/HV Control) was leaked and corroded a bunch of resistors in its proximity (see pic). I haven't noticed this before - my bad. Damage was so bad, R9 essentially seem to have had one leg almost severed (not not glaringly obvious, at least until I tried to remove it, and one leg just came off). I went ahead and replaced C6, R9, R10, and - why not? - recapped the entire board. The SX90 IS that much fun.

Upon putting it back, I see the same warning lights and all, and think - darn, no progress... - but as I'm thinking this, C18 literally blew up. Brand new cap, correctly polarized, voltage rating exceeding original... What gives? I feel this may be hitting at the core of at least one issue with this unit, though what that is exactly, I am still processing.

For instance, why is C18 (and C17, for that matter) only rated for 35V? (see enclosed pic) They seem to (potentially?) be exposed to 175V... Is there a floating ground aspect here I need to factor in (reducing exactly how much those caps are "seeing?").
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I ran the unit with A5 pulled and it seems to exhibit the same behavior as before (same blinking LEDs on the controller board, no PS errors LEDs on, same "8000000" on the left screen and "squares...1/squares...0...." on the right screen). Back to the drawing board.

[Rax]

[...] C18 (and C17, for that matter) only rated for 35V [...]

I just ran the unit with C18 out, and it's seeing about 17V... Looks good to me. Except that - unless my eyes deceive me - it's reversed polarized?... I'll need to double check this later, I am unsure this would be possible (other than that the "float" doesn't elevate enough and the -175V ends up "below" the "S"/"P COM"?...).

Let's keep in mind that, though they don't seem damaged, R102 and VR1 are very "toasty" (see charred PCB in pic).

Sorry for my undigested thoughts, but I'm hoping someone else with knowledge of this unit could think with me on this and help clarify what's going on here.

[Rax]

Well, mystery solved (??).

I'm pretty certain I've experienced a... triple whammy! The schematic, the type of pad on the board (where square equals positive leg), and even the actual capacitor placement for C18 was reversed... No wonder it failed, though it failed far less spectacularly than the cap I used as a replacement. The original seems to just have dried down and leaked very inconspicuously in its misery.

But spending a minute with the schematic makes it obvious C18 and C17 cannot be placed back-to-back like that (assuming that "S" is a module ground and voltages coming in are polarized as per schematic). But measuring over C17 and the C18 (unpopulated currently) shows that indeed these two caps are standing "atop" each other and so the placement as done by factory is incorrect. Oh, and BTW - what I thought is +-175V needs a dot after the first two digits... I am also pretty sure that's supposed to be 17.5V. I got tricked by the HV word in the module name, and, in my defense, I get bits and pieces of time for this currently so it's more like trickle progress.

Not the first PCB and manufacturing error, but still pretty surprising. Can anyone possibly confirm my conclusion above? I don't think I've seen this mentioned anywhere. Maybe I have an early production board and this was addressed later.

[bdunham7]

Not the first PCB and manufacturing error, but still pretty surprising. Can anyone possibly confirm my conclusion above? I don't think I've seen this mentioned anywhere. Maybe I have an early production board and this was addressed later.

Can you verify the actual voltage and polarity across the capacitor under power with an operational circuit?  There have been examples of PCB layouts being wrong before.  And are you sure of how the original (?) capacitor was installed?

Edit: After looking at the schematic, yeah that has to be wrong...

[Rax]

Can you verify the actual voltage and polarity across the capacitor under power with an operational circuit?  There have been examples of PCB layouts being wrong before.  And are you sure of how the original (?) capacitor was installed?

That was my "sanity check" - turn this on with C18 out (it just decouples the rail...) and measure polarity and voltage across C17 and (C18). It checks out.
I am just about positive the cap that was there (and leaked) was original - no reason to suspect it's been replaced.
Also, I always take good pictures prior to taking parts off, for this kind of thing happening. The picture clearly shows how they were back-to-back against that ground.

Edit: After looking at the schematic, yeah that has to be wrong...

I've occasionally ran into various degrees of messup like this (HP, Tektronix), but this one beats my prior encounters by being a "triple whammy!"

[Rax]

And are you sure of how the original (?) capacitor was installed?

Pic enclosed.
I also got confused between the leaked C6 - having nothing to do with this - and C18, which seems to have been able to withstand reverse polarization with no aggravation for a long time. Maybe also died an early, quiet death?... I'm not sure I can still tell which one's which in the small pile of removed caps to check its health on my meter.

[Rax]

Continuing my work on this while still waiting for the extender PBCs.

I found quite a few leaked capacitors across different boards last night. For instance, on A14: C28, C29 (I also replaced the affected resistors). I used 1% resistors, but I don't think any of those is of critical value (affecting calibration, I mean by that).

MegaVolt - my unit, overall, exhibits similar symptoms to yours (having watched your video, pretty much except the front panel board light being continuously on, and no lights on the PS boards, as I fixed those). Can you please elaborate how you investigated U22 on A14 to realize it's faulty? I wonder if mine could be at fault also. Oftentimes, the same parts fail in the same units due to a variety of conducive conditions (marginal design vs. specs, etc.).

[Rax]

Here are videos of my issues - troubleshooting lights and front panel displays.

https://youtu.be/IORA5uATvQo

https://youtube.com/shorts/DxgEM9nHz4A?feature=share

[Rax]

I finally went ahead and did what you asked a while back, MegaVolt - I pulled all boards within the INSIDE/GUARD cage (A4 through A10) + A14 from outside of it. I am getting the exact same error lights from A16 and A2.

I guess this confirms what we knew all this time - that the issue resides in one (or both) of these modules. But it seems to eliminate the possibility that any of the other boards is at fault. Which also means it'd make no sense to try putting them back in a sequence (as obviously there's nothing to observe with that). I feel this is essentially the first major step forward with my unit throughout this saga this far.

I am considering doing the following next:

• go through the fault module isolation steps at 4-28
• depending on what I find the above (though maybe not in this order) try swapping the spare microprocessors I have in A2 and A16
• possibly try replacing the UART IC that you, MegaVolt, found defective
• try reading the memory chips (though I'm apprehensive about deleting them as I try that, so this is a last resort step)
• .......?

[MegaVolt]

Can you please elaborate how you investigated U22 on A14 to realize it's faulty?

My block worked a little and gave out a "communication error" between the boards. There is a communication cable between the boards. Communication protocol TTL_RS232.

I observed the exchange but one of the bits was stuck and did not change. At the input of the microcircuit, the data was correct. So it's a chip failure.

And my topic has examples of the exchange protocol. You can check it against what you see.

Regarding other errors:

1. Judging by the video, the A16 board starts working and successfully checks the bootloader, memory and interrupts and stops on the exchange with the front panel.

2. The front panel seems to be starting to start, but for some reason it does not have time ...

I would do the following:

I would also extract the A16 board and deal only with the A2 board. It should start up on power up and give an exchange error.

You can check the reset circuits, the capacitors in these circuits.

[Rax]

Tried one other thing - replaced the A2 microprocessor (U16), to no avail. It was worth a shot.