help - troubleshooting an old scope - Page 1
Forum section: electronics / repair

author: nitzane
author_rank: Newbie
timestamp: November 14, 2017, 12:09:22 pm
content: Hi guys, newbie here.I recently bought a used hp 54645A off ebay.It boots ok with no error messages, shows both traces and properly shows the square test signal.Theres one issue with it - the display seems to freeze when i toggle traces on and off, and when i try to toggle vectors.After a hard reset the splash screen shows "PANIC - DSP COMMUNICATIONS TIMED OUT".Also - sometimes the display would boot up jumbled. I reckon the jumbled display is related to the freezes.no joy with the hp/agilent documentation.Iv'e included some photos, hopfulley they would make up for my non native/professional english.Has anyone seen this before?Thanks,Nitzan

author: capt bullshot
author_rank: Super Contributor
timestamp: November 14, 2017, 12:30:04 pm
content: Don't know the innards of this particular scope, so can't help you in detail.Your problem looks like a intermediately bad internal connection, maybe warm up of the scope influences it. So check for any kind of board-to-board connections, re-seat them. If none applicable or cleaning the connectors and re-seating doesn't help, check for supply voltages to be correct, and search for bad solder joints. If documentation is helpful in defining and locating circuit blocks, I'd search for the failure somewhere on the data busses from the Main CPU to the DSP part (as I said, I cannot help you in details, you'll have to find this out for yourself or wait for others to join this thread).Don't know if this scope suffers from that, but check for leaking SMT electrolytic caps - if there are any, remove, thoroughly clean the board and put new ones in.

author: nitzane
author_rank: Newbie
timestamp: November 14, 2017, 08:15:40 pm
content: Thanks captain, ill reconnect all the cables!what does the fact that the freeze happens only when toggling vectors and traces tells us?Nitzan

author: nitzane
author_rank: Newbie
timestamp: November 19, 2017, 06:17:10 pm
content: ok, a little update.i have opened up the scope, disconnected all cables and reconnected them. nothing changed...i have noticed i can toggle traces on and off, it freezes when i try to do this rapidly (about 3 seconds). the freeze always occurs when turning vectors on and off.i have managed to put my hands on the scope's schematics herehttps://archive.org/details/HP54645NCLIPImgsi noticed theres a sheet named DSP BUS(sheet2), but everything is still very complicated to meany usual suspects?

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2017, 12:28:36 am
content: Quote from: nitzane on November 14, 2017, 08:15:40 pmIt's very difficult to say without a deep understanding of the way the hardware and firmware works. A scope like this is essentially a computer so you could have a problem somewhere say with an address decoder circuit where only certain addresses are affected and that may happen only when you access the memory location where the trace/vector toggle status happens to be stored. Unfortunately problems like this can be extremely hard to track down though it can help if you have access to a second unit to swap boards between so you can determine which board has the fault.

author: poot36
author_rank: Frequent Contributor
timestamp: December 02, 2017, 05:37:34 am
content: I would check the power supply for ripple as these scopes are known for the caps in the power supply going bad. Also re-seat any socket chips as well.

author: oventech
author_rank: Contributor
timestamp: December 05, 2017, 03:13:22 am
content: Hello nitzane, found this to download the Owners and Service Manual plus any other information they have I believe.https://www.keysight.com/en/pd-1000001437%3Aepsg%3Apro-pn-54645A/2-channel-100mhz-deep-memory-megazoom-oscilloscope?pm=PL&nid=-32416.536880205&cc=US&lc=eng.Hope this helps. Do like HP gear.

author: don.r
author_rank: Frequent Contributor
timestamp: February 28, 2018, 12:14:38 am
content: My 54645D is suddenly showing similar symptoms. Traces freezes soon after startup or disappears completely. Fails vertical cal although cal signal is present. I have yet to test rails for ripple but the +-15V rails are +-15.8V. I fear it might be the ADC which unfortunately I believe is a BGA.edit: Looks like the ADC is a QFP. Might be able to remove the overhanging epoxied heatsink and check for bad joints. When the scope is cold I can see a trace for maybe 15 seconds and after that occasionally capture a wave form but it usually freezes after a second or two after capture. Once warm there is no trace.

author: tombi
author_rank: Regular Contributor
timestamp: February 28, 2018, 02:02:20 am
content: Urghh... The format of the CLIP (Component Level Information Package) is horrible. A good copy of the schematics plus a block diagram or theory of operation would help a lot.It's possible that it is not the actual DSP itself but something has gone wrong with one of the bus transceivers or address decoders etc.If you can figure out what signal is generated when the trace is turned on or off then you can find the register where this comes from and you can check this is correctly getting decoded off the bus.Other ideas are to check oscillators feeding systems like this. I presume you have another scope to troubleshoot this with?Tom