EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: diegoperez on February 10, 2017, 07:05:11 pm
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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
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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
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Well done! Grats on the repair and geez that thing is huge!
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Indeed - I have its sibling, a 4274A, and it's a big honker!
-Pat
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Nice job, Diego. Congrats on the repair. Now you just need some feet for it (unless you're putting it in a rack).
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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!
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Hi,
It is not clear, are we talking about assembly A5 U22, LM361N ?
Regards,
Jay_Diddy_B
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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
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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
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Can you send me hp4275 service manual？