Teardown and Repair of an HP4275A LCR Meter  - Page 1
Forum section: electronics / general

author: diegoperez
author_rank: Contributor
timestamp: February 10, 2017, 07:05:11 pm
content: Hello everyone,Today I posted a video of a repair and I'd like to share it with you.I think the final result turned out to be a bit too short and I skipped some details.However, any suggestions and questions will be welcomed (prefereably, post them in the youtube comments so as to allow everybody to read them).Thanks for your attention.https://youtu.be/9Fm8tgwsOPI

author: Cubdriver
author_rank: Unknown
timestamp: February 10, 2017, 07:20:54 pm
content: Nicely done. It never ceases to amaze me how much HP managed to cram into those instruments - very little wasted space. Congrats on getting it back up and running!-Pat

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 10, 2017, 08:20:09 pm
content: Well done! Grats on the repair and geez that thing is huge!

author: Cubdriver
author_rank: Unknown
timestamp: February 10, 2017, 09:23:35 pm
content: Indeed - I have its sibling, a 4274A, and it's a big honker!-Pat

author: bitseeker
author_rank: Super Contributor
timestamp: February 11, 2017, 03:35:13 am
content: Nice job, Diego. Congrats on the repair. Now you just need some feet for it (unless you're putting it in a rack).

author: wn1fju
author_rank: Frequent Contributor
timestamp: February 11, 2017, 06:57:41 pm
content: Must be a common failure mode. I fixed an HP 4274A a while ago with the same symptoms - alternating series/parallel LEDs and apparentlockup of the unit. Same bad part, the comparator on the A/D board!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 11, 2017, 07:08:23 pm
content: Hi,It is not clear, are we talking about assembly A5 U22, LM361N ?Regards,Jay_Diddy_B

author: diegoperez
author_rank: Contributor
timestamp: February 11, 2017, 08:35:49 pm
content: Quote from: Jay_Diddy_B on February 11, 2017, 07:08:23 pmYes, that's exactly right.Quote from: wn1fju on February 11, 2017, 06:57:41 pmNice observation.I also noted that there is a capacitor (see attached picture) at the input of the comparator, C70* in the schematic, that is not installed in my unit (older serial numbers).Although adding capacitance at the input of the comparator degrades the performance by slowing it down, I think this capacitor was added to avoid a possible problem of high frequency oscilation of the comparator caused by any small noise in its input, such oscillations would overstress the IC and eventually kill it (that's my wild guess).Quote from: Cubdriver on February 10, 2017, 07:20:54 pmYep, back in those days there were no arduinos or fpgas and everything had to be done at a much lower level of detail using many components for every simple task. Luckily the digital part of the instrument was fine; otherwise, it could be a nightmare troubleshooting it without a signature analyzer.Thank you for all the commentsSchematic detail of the assembly 5 (page 283 of service manual)https://drive.google.com/file/d/0BwElEpRLIAVNMGNXRXpzYlV4Wjg/view?usp=sharingLink to the service manualhttps://drive.google.com/file/d/0BwElEpRLIAVNQnBmNXlpMmlxeUE/view?usp=sharing

author: banjoman
author_rank: Newbie
timestamp: June 12, 2017, 05:56:25 pm
content: Hi,I'm desperately trying to fix an HP4275A.I have hearing problems and in the Diego's movie I cannot understand what signal exactly refers to the last bottom trace . Is it the output (pin 11) of the LM361 comparator?Thanks,Max

author: denny_cheng
author_rank: Newbie
timestamp: September 17, 2020, 08:15:31 am
content: Can you send me hp4275 service manual？