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Anritsu MS2721B internal CF card missing
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brainstorm:
I'm also attaching a picture clarifying a bit my thinking while the tracing of the LT1765's SHDN signal (in red, following it via both sides of the board). Since it was 0V on that pin, I tried "bridging" (direct blue line) it with a wire in an attempt to "wake it up" (unsuccessfully). Here's the datasheet for that component:

https://www.onsemi.com/pub/Collateral/NC7SZ14-D.PDF

It just converts that curvy waveform into a square one by the looks of it... thinking it through that was not a good idea since the healthy rails show a 2.5-3.3V **constant** DC voltage fed on that particular pin and IC (LT1765). On the other hand, I noticed that my "injected" 3.3V input signal got attenuated to around 1.9V, so that makes me think that the line is being actively pulled down somewhere else in the circuit, probably via firmware/software.
analogRF:
your lucky day  :D ;)
I found my stash of photos when I was troubleshooting and gathering notes on this device.
Here are the pictures that I had taken and annotated at that time.
Strangely the -15V never comes on. maybe it is used with some options like GPS or something....
anyways, I hope this helps

based on my notes, all rails start right at the beginning except the 24.6V which comes on only a short time after power on (still during boot process)
and -15V never came on.

I think the unit will not boot at all if you disconnect the screen, too. at least have everything connected except maybe just the RF deck

even without RF deck it does not boot as I just learnt from my notes. I was under impression that it booted without RF deck but apparently it didnt
analogRF:
...more pics...
analogRF:
based on what I just posted, I think your PSU rails are probably ok. You are stuck very early in the boot
and most probably the problem is somewhere else...maybe the RAM chips...

what exactly are the symptoms? what do you see on screen when you power it on?
 
brainstorm:
Woah, thanks for those pictures and hints @analogRF!

I went through all your annotations and all of them cross-check right on my end **except** the -6.1V **and** the one that has an annotation down below stating that "comes on after a few seconds of power on. not right from beginning"... I suspect that the latter supplies power to the "RF mini-deck" (not the big RF deck instrument half), so I'm not too surprised now that it's not powering on yet since it might be too early in the boot process.

Also, I went ahead and hot-gunned and substituted the RAM ICs with fresh ones I had on stock (see attached), to rule out faults there... same effect, unfortunately, but at least I can now state that it seems unlikely to be a RAM's fault.

This instrument used to work and indeed it hung up on the "Application started" you point out, with working screen and all. The number of "successful boots" up until that "Application running" point decreased until the current situation: **you can see the pixels of the screen getting "active", but no backlight illumination nor any pattern or image"**... this behavior seems consistent with a slowly dying flash memory perhaps? I've heard Teslas go black screen due to dead eMMCs :P

In any case, all those tests we went through, plus thits last IC swap and your images gives me very valuable information:

1) The fault is **VERY** early on the boot process.
2) The processor works fine, clocks and address lines on the RAM are asserted but there doesn't seem to be data/activity.
3) The RAM ICs do not seem to be the main issue and power consumption remains the same pre/post RAM IC swap, that is ~300mA @15V DC.

For the reasons stated above I'm tempted to believe that the next most likely candidate for troubleshooting could be the Flash ICs (256P30T) since the program might not be successfully read anymore and copied to the RAM perhaps?:

https://www.dataman.com/media/datasheet/Intel/P30Family.pdf
https://www.aliexpress.com/item/32838590465.html

I would like to be extra sure that this is the next step though since a 64 "easy" BGA package can be a bit of a pain to work with :/ So I went ahead and looked at the datasheet above and noticed that there's a silk screen near the 256P30T IC marked as "WAIT" with a VIA, which seems to be a prominent signal for this IC according to the datasheet since on page 20 of the P30Family.pdf it states that:


--- Quote ---In synchronous array or non-array read modes, WAIT indicates invalid data when asserted and valid data when deasserted. In asynchronous page mode, and all write modes, WAIT is deasserted.
--- End quote ---

Probing it with the oscilloscope, it seems like it is asserted (high) all the time at 3.3V... there could be many other glue logic involved here, but I'm trying to think this out loud, feedback welcome as always!
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