Products > Test Equipment
HP 34401A random function selection / reset problem
wbooth.clearscene:
Hi all,
Great joy, I have just laid my hands apon an HP 34401A. However it has a very odd fault.
After running for a few minuets it will randomly select a different function, eg continuity. After doing this It will flick around more, beeping as it goes, until settling on ohms. Once there the unit will, every few seconds, beep & display the lamp test, returning to the ohms setting.
I've checked the voltage at u553 (in 10v, out 5v) and all the electrolytic look OK. A full self test passes each time I do it.
Unfortunately the service manual does not give any assistance for this type of trouble, so any insights you may be able to provide will be greatly appreciated.
Cheers.
Dr. Frank:
Check the front keyboard.
Sounds as if there's a problem inside that circuitry, or somebody has poured some liquid over it.
What's the history of this instrument?
Frank
wbooth.clearscene:
Cheers for the reply. A dirty front panel was my first though before receiving the unit (I knew it was faulty when I purchased it). But the unit inside and out is in immaculate condition. However, I did remove the front panel last night to check, and everything does look good.
After replacing the front panel, the problem persisted until I decided to test the output of u553 over a "flipping about" cycle. Strange, while I was doing that the unit did not misbehave! Further, it looks like to problem has now gone away completely. I really don't understand what could have caused/solved this, so am not classifying this one as"fixed" just yet.
SeanB:
Tin whiskers on the front panel causing intermittent switch activation? Clean al the boards and the front panel with IPA and dry thoroughly.
Dr. Frank:
--- Quote from: wbooth.clearscene on November 30, 2013, 09:08:56 am ---Cheers for the reply. A dirty front panel was my first though before receiving the unit (I knew it was faulty when I purchased it). But the unit inside and out is in immaculate condition. However, I did remove the front panel last night to check, and everything does look good.
After replacing the front panel, the problem persisted until I decided to test the output of u553 over a "flipping about" cycle. Strange, while I was doing that the unit did not misbehave! Further, it looks like to problem has now gone away completely. I really don't understand what could have caused/solved this, so am not classifying this one as"fixed" just yet.
--- End quote ---
U553 is also the main RESET / brownout circuitry.
If you have measured on its pins with a nedle, you might have bended the IC legs a bit, and perhaps virtually repaired a bad solder junction.
If the problem pops up again, I would visually inspect the junctions of U553, if enough solder is available on each pin, and all components around, esp. output cap and reset cap.
Maybe the IC itself is not healthy anymore, depending on temperature, or so..
Frank
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