Author Topic: Teardown and Repair of an HP4275A LCR Meter  (Read 4111 times)

0 Members and 1 Guest are viewing this topic.

Offline diegoperezTopic starter

  • Contributor
  • Posts: 30
  • Country: es
Teardown and Repair of an HP4275A LCR Meter
« on: February 10, 2017, 07:05:11 pm »
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
 

Offline Cubdriver

  • Supporter
  • ****
  • Posts: 4201
  • Country: us
  • Nixie addict
    • Photos of electronic gear
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #1 on: February 10, 2017, 07:20:54 pm »
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
If it jams, force it.  If it breaks, you needed a new one anyway...
 
The following users thanked this post: diegoperez

Offline PA0PBZ

  • Super Contributor
  • ***
  • Posts: 5259
  • Country: nl
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #2 on: February 10, 2017, 08:20:09 pm »
Well done! Grats on the repair and geez that thing is huge!

Keyboard error: Press F1 to continue.
 
The following users thanked this post: diegoperez

Offline Cubdriver

  • Supporter
  • ****
  • Posts: 4201
  • Country: us
  • Nixie addict
    • Photos of electronic gear
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #3 on: February 10, 2017, 09:23:35 pm »
Indeed - I have its sibling, a 4274A, and it's a big honker!

-Pat
If it jams, force it.  If it breaks, you needed a new one anyway...
 
The following users thanked this post: diegoperez

Offline bitseeker

  • Super Contributor
  • ***
  • Posts: 9057
  • Country: us
  • Lots of engineer-tweakable parts inside!
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #4 on: February 11, 2017, 03:35:13 am »
Nice job, Diego. Congrats on the repair. Now you just need some feet for it (unless you're putting it in a rack).
TEA is the way. | TEA Time channel
 
The following users thanked this post: diegoperez

Online wn1fju

  • Frequent Contributor
  • **
  • Posts: 583
  • Country: us
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #5 on: February 11, 2017, 06:57:41 pm »
Must be a common failure mode.  I fixed an HP 4274A a while ago with the same symptoms - alternating series/parallel LEDs and apparent
lockup of the unit.  Same bad part, the comparator on the A/D board!
 
The following users thanked this post: diegoperez

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 2766
  • Country: ca
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #6 on: February 11, 2017, 07:08:23 pm »
Hi,

It is not clear, are we talking about assembly A5 U22, LM361N ?

Regards,

Jay_Diddy_B

 
The following users thanked this post: diegoperez

Offline diegoperezTopic starter

  • Contributor
  • Posts: 30
  • Country: es
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #7 on: February 11, 2017, 08:35:49 pm »
Hi,

It is not clear, are we talking about assembly A5 U22, LM361N ?

Regards,

Jay_Diddy_B

Yes, that's exactly right.

Must be a common failure mode.  I fixed an HP 4274A a while ago with the same symptoms - alternating series/parallel LEDs and apparent
lockup of the unit.  Same bad part, the comparator on the A/D board!


Nice 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).


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

Yep, 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 comments  :)

Schematic detail of the assembly 5 (page 283 of service manual) https://drive.google.com/file/d/0BwElEpRLIAVNMGNXRXpzYlV4Wjg/view?usp=sharing
Link to the service manual https://drive.google.com/file/d/0BwElEpRLIAVNQnBmNXlpMmlxeUE/view?usp=sharing
 

Offline banjoman

  • Newbie
  • Posts: 1
  • Country: it
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #8 on: June 12, 2017, 05:56:25 pm »
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

 

Offline denny_cheng

  • Newbie
  • Posts: 2
  • Country: cn
Re: Teardown and Repair of an HP4275A LCR Meter
« Reply #9 on: September 17, 2020, 08:15:31 am »
Can you send me hp4275 service manual?
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf