HP 54520A with Dead Display

[Kernel86]

I recently picked up a pair of HP 54520A scopes.  I bought them as not working as the display on neither of them worked and someone had already previously removed a cap and the fuse from one of them which were taped on it in a bag. After getting them home I opened them up and beside all the dust there was nothing obviously wrong with the one besides the fuse on the display driver board being blown and the missing parts as mentioned on the other.  I unplugged the display boards and powered them up and they both did power on fine otherwise.

I do have a similar vintage 54501A which I was hoping I could use the display from to test but after looking at the cable pinouts in the service manuals the video signal appears to be different between the two.  I did find the signal was similar to rgb/vga and after checking the signal levels on a scope I decided to try connecting them to an old NEC lcd I have that syncs to nearly anything and to my surprise it actually worked.  With that I was able to run the self test and feed signals to them and everything else seems to actually be working properly. 

So the big question I was wondering is if anyone knew of any common problems with the display in 54520A/54540A/54522A/54542A or other models that used the same display assembly. Since both displays are not working I figured I'd see if anyone already had some insight before I did too much yet.   I did pull the driver board from one but I haven't done too much poking at it.  After a dusting there's no signs of problems such as ovious failed components or over heating or shorts across the power input.

I do plan on replacing the fuse in one and powering it up to see if anything at all happens, but I've got to wait for the correct fuses for that!

[dzseki]

If it really resembles to RGB interface, then you have a good starting point, since you can be sure they used of-the shelf solutions for the standard signals, therefore it should be easy to track down.
For any CRT based display the high voltage flyback  converter, and the deflection circuits those are the most stressed, so from electronics standpoint I'd try to locate those and check for problems first.

[TAMHAN]

AFAIK - I am now farting in the blind - these units share the display system with the 53310a. Jzon Gellert describes a common failure here:
http://www.gellerlabs.com/hp53310A.htm

Fortunately - I almost typed sadly here - we did not have this problem with our 53310A yet. So I cant say much, but wanted to share - that cap and the fuse are, um, a good candidate.

[james_s]

If the display is blowing the fuse the first thing I'd check is the horizontal output transistor, that's a fairly common fault on CRT displays. Most CRT monitors are pretty similar, there are a handful of faults that show up over and over. These small monochrome displays are very simple.

[coromonadalix]

if the photo was taken without moving   it seems the beam focus is out , it is normally a set of rings on the crt, and you're supposed to have one adjustment on the flyback too, to be adjusted first for the finest dot

as james said  maybe cold joints, capacitors aging, and the horizontal output,  unless you smell a ozone like odor or hear an sparkling in the flyback transformer, they tend to crack and ventilate

[dzseki]

if the photo was taken without moving   it seems the beam focus is out , it is normally a set of rings on the crt, and you're supposed to have one adjustment on the flyback too, to be adjusted first for the finest dot

Nah, he said he managed to hook the signals to an LCD monitor, to see the basic functions of the scope, you can see the frame of the monitor on that picture actually.

[Kernel86]

I did initially check the HOT and the flyback for shorts but didn't find any problems there as that was my first thought.  I also ordered a replacement bipolar cap as that was the part removed from one of them and it also looked slightly bloated on the other unit as well.  The fuse and caps came late last week and once installed the displays both came on.  I had to tweak the three brightness and contrast adjustments to get a balanced output on both units however.  When they first came up the bright/highlighted portions were really bright while you could barely see the rest of the display with the rear adjustment turned up all the way.  Both displays do have some slight burnin over those areas so they were likely turned all the way up for awhile before they had stopped working.

I've run each of them for a few hours so far and there hasn't been any other issue.

Thanks everyone for the suggestions and comments.

AFAIK - I am now farting in the blind - these units share the display system with the 53310a. Jzon Gellert describes a common failure here:
http://www.gellerlabs.com/hp53310A.htm

Fortunately - I almost typed sadly here - we did not have this problem with our 53310A yet. So I cant say much, but wanted to share - that cap and the fuse are, um, a good candidate.

That didn't come up in my searching and it does look like it's a different display driver board, but yeah that does appear to have been my issue as well.

[TAMHAN]

It was my pleasure. Good to see the machines up again, bro!

BTW: if you have the buggers open, I would also recap the Boeschert cartridge. These power supplies WILL croak...

[Kernel86]

It was my pleasure. Good to see the machines up again, bro!

BTW: if you have the buggers open, I would also recap the Boeschert cartridge. These power supplies WILL croak...

Thanks for the tip. I've already got them back together right now but I'll keep that in mind.  I'll probably only be keeping one of them but think my 54501A also has the same supply as well so those might be a good idea to look at in the future.