HP 3456A / have some stuck (on) segments and leds - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: September 24, 2015, 10:51:53 pm
content: Just picked up a used 3456A and it has 4 stuck segments on the digit panel and 10 stuck randomly distributed LEDs near the keys. Is this a common problem that can be easily fixed? The keys near the stuck LEDs work properly and register correctly, so this seems to just be a display issue.The stuck segments/leds remain stuck through the all on/all off test cycle that can be run via the Test button on the left side near the power switch.Thanks in advance.

author: pelule
author_rank: Frequent Contributor
timestamp: September 24, 2015, 10:57:43 pm
content: I would check the power supply on th outguard board, especially the big cap. looks llike a supply problem.

author: SharpEars
author_rank: Unknown
timestamp: September 24, 2015, 11:01:51 pm
content: Quote from: pelule on September 24, 2015, 10:57:43 pmOK, I'll try that but why would a supply problem cause certain (and same every time) LEDs to always be lit?

author: Vgkid
author_rank: Super Contributor
timestamp: September 24, 2015, 11:17:29 pm
content: Along with the psu check the led control circuitry. I dont belueve the self test can test the display. In class, cant read manual.

author: Vgkid
author_rank: Super Contributor
timestamp: September 24, 2015, 11:34:07 pm
content: Fig 8-71 part 10.U24,28,29,30 check those.

author: SharpEars
author_rank: Unknown
timestamp: September 25, 2015, 01:34:53 am
content: Quote from: Vgkid on September 24, 2015, 11:34:07 pmYup, good advice, that is almost certainly where the problem resides, one of those four chips!

author: SharpEars
author_rank: Unknown
timestamp: September 25, 2015, 01:54:04 am
content: Here are some pictures, perhaps someone will see a pattern to help isolate the IC responsible:Lights stuck on:All lights on, the lights stuck on are much brighter than the rest (as if a current limiting resistor isn't being applied or something):Note that the bright 8 at the end of the numeric display just looks anomalous in this picture and not in real life(i.e., it is not really brighter when viewed, I must have captured it mid-refresh or something).

author: SharpEars
author_rank: Unknown
timestamp: September 25, 2015, 03:30:40 am
content: Update:I got out the scope and got the test equipment ready, took off the cover, plugged it in to be able to measure and lo and behold, the problem is gone. Put it back together and it's as good as new (well the scratches are still there) - no display anomalies whatsoever. I must have run it through the test cycle some twenty times in utter disbelief (well and to make sure that the superbright segments didn't get some "burn" when they were malfunctioning).The only thing I can think of that might have been the cause of this are one or more of the four rack screws, two of which look like they were pretty long and screwed in "a bit deep." I didn't put those back when I reassembled, since if one or more of them were in fact causing some sort of display short, I sure as heck did not want to reproduce it.In any case, everything works as it should and the thing is damn accurate. It was the most recently calibrated of the three units I have (2010) and two of my three units are less than two counts apart on most Vdc readings (the third is about 4-8 counts off depending on where you are) and all are very close on the resistor readings. All in all I am very pleased and have far more confidence in the readings, since all three came from different companies.I apologize for the missing troubleshooting session that was almost sure to follow, but if anyone has similar issues, try removing the side racks and their screws first!

author: Vgkid
author_rank: Super Contributor
timestamp: September 25, 2015, 04:36:16 am
content: Glad you figured out the issue. I was just getting ready to post that section of the schematic as well as adding for you to check U25 as well.

author: VK5RC
author_rank: Unknown
timestamp: September 25, 2015, 06:53:54 am
content: Any win is a win, well done.I repaired a HP 5343 frequency counter, which can also get into trouble with case screws if not careful, but thankfully someone else alerted me to it so I avoided it, I have since written a note on the bottom of the instrument to remind me next time!

author: SharpEars
author_rank: Unknown
timestamp: September 25, 2015, 01:14:44 pm
content: Quote from: Vgkid on September 25, 2015, 04:36:16 amThanks! Sorry for the "disappointment" that this repair became, but boy am I am glad that the device works as it's supposed to it.

author: cncjerry
author_rank: Unknown
timestamp: September 25, 2015, 05:42:10 pm
content: If I remember correctly, the upper bolts on the left side facing the display are shorter than others. If the bolt is too long it will short against wiring on the inside.

author: Vgkid
author_rank: Super Contributor
timestamp: September 25, 2015, 06:22:10 pm
content: I will make a note of this, I'm thinking of picking up another 3456A.

author: cncjerry
author_rank: Unknown
timestamp: September 26, 2015, 11:26:44 pm
content: I have two 3456a meters and think they were the most stable meter built back then and are hard to beat to this day. I think they are better meters than the 3457a which I also have. I especially like the red led display which is so much easier to see.The 3456a is also easier to calibrate than a 3457a as you can dial in the noise average on the lower levels whereas the 3457a will grab whatever noise value it has when calibrated.

author: GreyWoolfe
author_rank: Unknown
timestamp: September 26, 2015, 11:56:22 pm
content: Glad the 'repair' was easy. It looks nice but really big!