HP 54503A DSO: How to repair a faulty channel #1? - Page 1
Forum section: electronics / repair

author: HP User
author_rank: Newbie
timestamp: November 28, 2016, 01:52:12 pm
content: Recently, I got a HP 54503A with a known NVRAM failure (empty battery).After replacement against a new NVRAM the scope passed the self-test (see image 1).After initialization of the self-calibration I encountered the following errors on ch#1, whereas ch#4, ch#2 and ch#3 pass the calibration (see images 2 to 5).The message “Check BNC cable connection” has been verified and cable connection found OK (otherwise the calibration could not pass).The self-calibration page shows the following (see image 6) status summary (the delay calibration was not executable without pass of all 4 channels).The probe calibration menu (see image 7) displays an error: “Attenuation too high or bad connection on channel #1”.The connection to the calibration output has been rechecked and found OK. Channel #1 shows the beam only, but no applied signal. Channel #4, #2 & #3 display signals applied.I assume channel #1 is obviously dead as no applied signal (beam only) is displayed.I disassembled (lifted from the main board) the channel #1 cover with the attached BNC connector and visually inspected the cover inside and the board area for sign of marks or blown components. There was no evidence of any visible damage.Any repair hints or ideas how to fix this problem?Does someone has access to the drawing/schematics of the input circuit for troubleshooting or is familiar with this type of failure?Where to get affordable surplus parts?Any suggestions are highly appreciated.

author: Johncanfield
author_rank: Regular Contributor
timestamp: November 29, 2016, 02:57:42 am
content: Check with eBay seller watronics, he's a retired HP engineer that helped design the 54500 and 54600 series scopes. He sells refurbished 54500 series scopes so he might be able to help. I bought a refurb HP 54542A from him, good scope.http://www.ebay.com/usr/watronics?_trksid=p2057872.m2749.l2754

author: Smith
author_rank: Frequent Contributor
timestamp: November 30, 2016, 01:17:02 pm
content: Every channel has a separate input module. These can be replaced, I believe they are the same for the 54502A and 54503A.Have you switched input modules to make sure it's the input module?On some modules the 50Ohm input resistor is blown, this cannot be repaired. The module has to be replaced.I have heard about the relay contacts of the inputs being stuck, so you might look at them. As I remember correctly you can open up the input module by removing the two clips (one on either side).

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2016, 01:23:45 pm
content: Have a look here:Quotehttp://wonghoi.humgar.com/blog/2016/07/27/front-end-attenuator-autopsy-of-hp-54616b-digital-oscilloscope/

author: stj
author_rank: Super Contributor
timestamp: November 30, 2016, 06:49:35 pm
content: Quote from: Smith on November 30, 2016, 01:17:02 pmis that a challenge?

author: dan3460
author_rank: Frequent Contributor
timestamp: December 01, 2016, 05:56:23 pm
content: +1Given enough money or time, or both I believe everything can be repaired.

author: HP User
author_rank: Newbie
timestamp: December 02, 2016, 08:22:05 am
content: First of all, thank you folks for your inputs.I have swapped the attenuator channel 1 against channel 3. Now ch 3 is dead and ch 1 works fine. It’s the attenuator.In the meantime I have ordered one spare from watronics (thanks’ to Johncanfield for the hint) which will solve my problem.

author: tautech
author_rank: Super Contributor
timestamp: December 02, 2016, 07:18:43 pm
content: Quote from: HP User on December 02, 2016, 08:22:05 amShould you need another parts supplier there's also Sphere:http://www.sphere.bc.ca/test/hp1.html#catalog

author: dogbert
author_rank: Contributor
timestamp: October 25, 2018, 06:49:24 am
content: Quote from: PA0PBZ on November 30, 2016, 01:23:45 pmNot true for 54503A (it's an older attenuator block that has the same form factor, but the substrate is different). The author updated the post.