General > General Technical Chat

Teardown and Repair of an HP4275A LCR Meter

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wn1fju:
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!

Jay_Diddy_B:
Hi,

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

Regards,

Jay_Diddy_B

diegoperez:

--- Quote from: Jay_Diddy_B 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

--- End quote ---

Yes, that's exactly right.


--- Quote from: wn1fju 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!


--- End quote ---

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).



--- Quote from: Cubdriver 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

--- End quote ---

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

banjoman:
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

denny_cheng:
Can you send me hp4275 service manual?

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