Fluke 8840a rescue

[vibram]

Hello,

I'm not a professional, I'm just trying to learn and rescue a damaged fluke 8840a.
Long story short : it has been abused in the US (missing button front /rear selection and some glue on the PCB) , imported to Europe et someone tried to plug-in in 240v with the 110v jumper.

The AC conversion extension has at least 4 resistors burnt but this is not my priority.

Here is the situation :
When plugged in, I push the start button and I have all my references voltage working (30v +/-15v and 5v). The 4.5v AC to vfd is also fine.

When I push the power button, the relay k301 make a small noise.
Then nothing, nothing on the screen, no noise if I try to change the mode, nothing happens.

I don't really know what should I test next?
Is it possible to be stuck because I missed the rear/front button?
Here is the picture of missing component

Thank you for your help

[vibram]

And here is the picture of the vfd with strange withe marks.

What is strange to me, if the vfd doesn't work it should still make some switching noise when changing mod right?

I don't want to change the vfd if I'm not sure it will work after..

[zike]

Presuming you've found a service manual online (free pdf is out there), you'll see RV401 and RV402 should be there.

These 400V varistors protect the ohms current source from high input voltages (look at schematic page 9-7).
Their removal suggests two possibilities to me; a) your unit suffered unspeakable overvoltage at some point, or b) it was previously scrapped and used as a parts donor. 

While unclear specifically why you have no display, this and the missing f/r switch could suggest it may not be practically recoverable (at least, not without another parts donor or two to cannibalize).

I could be wrong; the manual is generally excellent, maybe there is a more hopeful explanation to discover.

[golden_labels]

The white marks are getters. These are metals sprayed on glass to capture stray gas and maintain vacuum. They’re a sacrificial material. Shiny when fresh, change appearance as they get used up. The photo suggests getters are completely worn out. Either from prolonged use or because the tube is no longer sealed and air seeped in.

Which means: this VFD lost its V and turned into Fluorescent Deceased.

If the seal holds, VFDs with worn out getters may still work for some time. But they’d be on their last lap to death. So don’t throw it away yet: the fault may be elsewhere and that display will survive long enough to help with debugging. But as the initial diagnosis assume it’s the VFD that’s dead. And even if it no longer can display anything, the filament should still be working, so even in that condition it should help with debugging.

[vibram]

RV are both here.
But what is unclear to me is the use of the front /rear button.
Because I guess it should make a contact somewhere but I don't know.

Even with a cooked vfd, I should be able to ear the relay when I change mod right?

[Agent24]

If the VFD has white marks inside it's dead and lost vacuum.The VFD was probably cracked in shipping.
Those white spots should look silver. They turn white if exposed to air.Read here: https://en.wikipedia.org/wiki/Getter

[vibram]

OK it's clear thank you.

Its 50 bucks and I don't even know if it will work after.
I don't know how can I check..

[Kleinstein]

The front / rear switch should not hold up measurements. It can still cause trouble when measuring high voltage or poor contacts, especially for 2 wire ohms.
Even without the display one should still be able to operate via GPIB interface.
One should be able to see if the ADC is running by looking at some internal signals and the input sider to ouputside isolated data interface.

If it is badly damages one might indeed consider this a parts unit. There are a few difficult to replace parts.
The recirculation reminder ADC can be tricky to adjust and is a bit noisy even if working properly. It still contains a high grade reference.

[vibram]

OK clear. There are not many of them in Europe, at least I don't see them often so I will put it in a box and wait few years before the scrap bin as many projects.

Thank you for your help it's appreciated

[Paceguy]

Sounds like U202 (Zilog Z8 861108PSC) may be faulty. If this is the case, your 8840A is good as a parts unit since this part is very hard, if not impossible to find. They are masked and cannot be read or copied.

[vibram]

Interesting, do you know how to test it?
There are some clone on ebay but not cheap either

[Kleinstein]

To test if a CPU is still active one can look at some of the data / address lines. There should still be some activity, even with µC internal ROM.
It there is no activity one could also check the clock and reset part.

There more more things that just that µC that could be bad, at least in theory. There is also an ouput side µC.

[vibram]

OK thank you. I already tested the reset which was fine