Anritsu ms2602a Display Unit repair

[sergey]

Hello everyone,

I have a Anritsu ms2602a spectrum analyser which has no picture on its CRT display. The exact symptom is that during the startup there is a single vertical line in the middle of the screen which fades in shortly after power-up, and fades-out after a second or two. It fades out by becoming less and less bright, while staying in-focus.

The unit was sold as working, and I do not have reasons to disbelieve that statement. But, surely, being an engineer I trust the actual measurements than assumptions. Is just something I have at the back of the head that maybe the issue is simpler, before going deep into rabbit holes. But it also could be that something physical broke in the tube itself.

Quite sure the issue is in the Display Unit itself, as there seems to be no anomalies at neither of the connectors leading to the module (it has two of them).

The DISP CPU board outputs proper VSYNC, HSYNC, and the monochrome signal (the 8-pin connector J2 on the A9 board). In fact, with some voltage level adjustment on the signal line I've managed to make picture appear on an external VGA monitor (which recognized it as 640x400 @ 56.4 Hz, 21 MHz pixel clock).

The COMMON BOARD has +12v. It seemed a bit noisy without a load, so I've checked PSU and none of the capacitors seemed bad. The 12v regulator itself worked correct (it is on its own little board, based on STR9012): putting 20v in gave clean 12v out. I don't have schematic of the PSU, and going upstream was a bit of a challenge. At that point I've also noticed that the +12v is pretty clean when the load is applied, so I've stopped perusing the PSU route. The connector I am talking about is the 5-pin J10 on the A11 board.

In the Display Unit there were two main things which made me pay attention:
- An unconnected pin of the tube. Although, it did not seem to me that the pin was ever soldered, and there is no silk screen label for it either.
- The trim on the H.SIZE inductor was all the way in. Seemed a bit odd, so I've tried adjusting it, but it did not lead to any visible change in the behavior.

The HV diodes measure good. Measuring deflecting coils is a bit tricky, as I don't know the exact pinout. The red-blue pair and yellow-gray pairs measured continuity (as in, red measures continuity to blue, yellow to gray).

The symptoms themselves makes me a bit confused:
- Clearly a lot of essentials are working, as there is a moment at which the screen displays a vertical line.
- If the issue is in the horizontal deflection, why does the day fade out.

At this point I wouldn't mind hearing from more experienced people
I've attached some extra pictures to show the overall construction.

If it helps, it is a CDT display unit, MDT732B-1A, by Totoku Electric.

Looking forward hearing from you,
Have a fantastic day!

P.S. Surely I can make an LCD replacement, but I wanted to use the opportunity to learn something new, and better understand things which I had vague ideas of.

[sergey]

Some extra detail which might help guiding troubleshooting and interpreting the symptoms: the board uses AN5790N which is a Horizontal Signal Processing ICs for CRT Display.

Still need to find a safe way of probing the IC to see if it is working as expected.

P.S. Not sure if it is more preferred to add new info as edits to the original post, or to post a new message.

[Manolo Mos]

I see in some pictures the PCB cracked in some places.

[sergey]

Good eyes! I did not see anything suspicious about the PCB itself during its previous inspections, and did not even consider such a possibility, duh. But now when you mention it, it is rather so obvious

Time to get the board out, inspect and repair traces!

[coromonadalix]

yep around the flyback ... 2 places  at least

[sergey]

Yep, that was it. I was checking all those digital part, PSU part, diode readings, and even visual inspection of components and still missed an elephant in the room... Thanks Manolo Mos and coromonadalix!

I've spent some time with enamel wire, and voila!