HP 5720SD oscilloscope stuck before selftest - Page 1
Forum section: electronics / repair

author: ar3itrary
author_rank: Newbie
timestamp: June 20, 2019, 01:33:21 pm
content: Hi,theDL0XK ham radio research grouprecently 'inherited' a HP 5720SD oscilloscope mainframe with a HP 54722A module installed.It was given to us because "the display was to dim to read it properly" and I had hoped to able to fix it by just cleaning and tweaking the brightness and contrast potentiometers.Also it looks like it has been hit be a high power laser beam a few times, but that should be purely cosmetic damage.After applying contact cleaner and twiddling the contrast pot for a bit we had nice bright image:A light grey box on a dark grey background, with one of those old-school bevelled edges. (Just like HTML tables used to look.)We waited for a few minutes but nothing else happened, it seems to be stuck somewhere.To eliminate the module as cause for the problem, we removed it from the scope, but still we just got the empty grey box.When we noticed that the diskette drive does not make any noise at all (I expected it to move its head around during power up, similar to what diskette drives in computer do),we tried disconnecting it as well, but it did not make a difference.A longer google search lead us to some old FTP server containing the Service manual for an HP 54710A, 20A 17A and 18A.I've mirrored it tomy serverbecause it was rather hard to find and it might be useful to somebody else.We tried to follow the troubleshooting guide in the manual, which appears to be close enough match to our D unit.In contrast to what one might expect after watching a certain electronics youtuber, it is not a power supply issue.We checked all the voltage test points using a decent multimeter and an admittedly crusty old analogue scope and they all appear to be in spec.In addition to that we also visually inspected the power supply and the CPU boards of the instrument, without finding anything.Take out the other boards requires to disconnect a forest of rigid coax. I'd like to avoid that as long as possible.All the capacitors look okay as far we can tell.The trouble is that service manual differentiates only between "no display", which likely means the CRT is blank,as it recommend checking supply voltages and the video connections in that case (we tried that just for good measure)and "display working" in which case it assume that the display shows the main menu or a least a self test error.Our current working assumption is that the device is either stuck in on the really early stages of its self test or its firmware has been corrupted.Has anyone here worked with this model of scope and can describe how the start up should look like?In particular is our assumption that the diskette drive should make a noise correct?Has anyone seen this kind of fault before?Does anyone have access to firmware disk for this model?It is possible to force a firmware update (from the diskette driver) using dip switched on the back of the unit.This way we could ruled out that the firmware is somehow broken.

author: dzseki
author_rank: Frequent Contributor
timestamp: June 20, 2019, 02:13:54 pm
content: Without know this model. But I see you have a battery on the CPU board, is it still in spec? Depleted batteries sometimes cause unexpected problems...

author: ar3itrary
author_rank: Newbie
timestamp: June 21, 2019, 04:03:08 pm
content: Good point.I haven't measured it yet, but I will take a look on Monday.After sitting on shelf for a few years I'd expect the battery to be fully discharged.Without the schematic it's hard to tell how exactly the battery buffered RAM is supposed to work,but maybe the deeply discharged battery leads to failing RAM accesses or something like that.

author: KG7AMV
author_rank: Frequent Contributor
timestamp: November 17, 2020, 10:35:01 am
content: I am in the same situation with a 54720D From my research its because the battery went dead. I have searched for a couple hours myself for the 4.07 firmware / rom / system-disk just dead links.