HP 1661C with some issues - Page 1
Forum section: electronics / testgear

author: Maxl123
author_rank: Newbie
timestamp: June 07, 2024, 08:08:42 pm
content: I just got an HP 1661C logic analyzer for cheap, the first 20 minutes or so, it worked like a charm. However, it suddenly started making some crackling noises, and it smelled like burned electronics. I immediately turned it off and when I turned it back on again, it made clicking noises and the CRT only showed a single line. With a quick Google search, I was able to find the service guide, which had a guide for troubleshooting the PS. From what I could find on the Internet, it is apparently a weak point of this machine. The first time measuring voltage rails under load showed the same problem, my multimeter jumped between the 5V it should have and 0V. After powercycling, it started working perfectly fine, every voltage rail was perfect. After connecting everything back up, the logic analyzer booted up just fine.I am not quite sure what the problem was or is. My first guess would be some bad caps however, none of them look bad. Does it make sense to do a complete recap of the PSU and the CRT control board when I am at it? I was not able to find a replacement PSU on eBay, so I have to work with what I have.Thanks in advance!

author: TheDefpom
author_rank: Frequent Contributor
timestamp: June 07, 2024, 08:28:53 pm
content: Recap it.What you probably had fail though is a RIFA cap.

author: Maxl123
author_rank: Newbie
timestamp: June 15, 2024, 04:23:54 pm
content: So I got the chance to unsolder every electrolytic capacitor and I noticed some cordoded spots under the caps. Unfortunately, one spot is really bad, it seems that even the PCB itself has dissolved.I am probably going to clean it with isopropanol and put some UV soldermask on top in order to protect it.I would greatly appreciate it if you could share your opinions on how to repair this.

author: tggzzz
author_rank: Super Contributor
timestamp: June 15, 2024, 05:06:27 pm
content: Quote from: Maxl123 on June 15, 2024, 04:23:54 pmScrape off the soldermask, solder a piece of wire across the corroded part. Ensure all relevant tracks are connected/not connected. The worst possibility is that the through hole has been damaged and inner copper layers can't be reconnected.If there is a danger a replacement capacitor will contact that wire incorrectly, with thin plastic cut a circle the same diameter as the capacitor, make a couple of holes for the capacitor's leads, thread plastic circle over leads and solder the capacitor in.

author: wn1fju
author_rank: Frequent Contributor
timestamp: June 16, 2024, 12:05:43 pm
content: This is probably a Boschert (spelling?) supply used by HP in many of their 54xxx scopes, logic analyzers, modulation analyzers, etc. They are notorious for leaking caps. I have come across three of these supplies in the past - all three failed from bad caps.Also, look around for any damage from leaking electrolyte. There is a trimpot nearby that can also be destroyed.There are many discussions about these supplies on eevBlog as well as on the HP/Agilent/Keysight groups.io forum. While no official manual seems to be available, there was at one point a hand-drawn schematic on the web from a guy who reverse-engineered these supplies. Search around.