Help with HP34401A - Page 1
Forum section: electronics / repair

author: darksky
author_rank: Contributor
timestamp: November 16, 2016, 03:11:31 pm
content: I need help with a HP34401a DVM.Problem I have is when I switch it on the display will fade out. It will always come on when the unit has been switch off for an hour or so.It may work for a minute or two or go off after a few seconds. The display will just fade.I have checked the voltages and all looks OK. The unit is 12 years old but only used for occasionally and been on for a few 10s of hours,All the components look good.I can see what look like the correct waveforms at the back of the display and the keys on the front seem to respond.Any thoughts what can cause this and has anyone seen this fault.Greatly appreciate any help in getting my old friend working again.

author: singapol
author_rank: Frequent Contributor
timestamp: November 16, 2016, 04:27:47 pm
content: Check power supply connector/cable to display,voltage ok? Pull out connectors and reinsert them again,this will scrape off oxidation from the pins. Also check for cracked or cold solder joints. Refer to service manual for details.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 17, 2016, 02:40:36 pm
content: You most likely need a new display.http://www.ebay.com/itm/322328736701

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 17, 2016, 04:47:41 pm
content: Quote from: HighVoltage on November 17, 2016, 02:40:36 pmNope. I'm going with bad solder connections at the display tube itself - specifically the filament connections. Alternatively - check the -30V power supply.

author: darksky
author_rank: Contributor
timestamp: November 17, 2016, 06:37:35 pm
content: Thanks for this. Display will light then goes off. So think display is OK and filament not broken. Will try connectors and resolver joints. Should the filament voltage be 30 volts?

author: Simon Spiers
author_rank: Regular Contributor
timestamp: November 17, 2016, 09:34:37 pm
content: Do a power on test. Hold a key down for 3 seconds while turning on. If display is still up see what it says.I think there should be a dc to dc convertor to supply hv to the display. Is it ok?Where are you?

author: darksky
author_rank: Contributor
timestamp: November 19, 2016, 03:47:01 pm
content: Hi Guys,Thanks for the input.I have soldered the joints where the cables and display are, cleaned the contacts and put it all back togtherAt first the display faded.However, in moving the cable the around the display has come back.It looks like a ribbon cable issue or cracked track.I am based in the midlands -UK.

author: Simon Spiers
author_rank: Regular Contributor
timestamp: November 20, 2016, 09:14:09 pm
content: Check cable continuity. Sounds like a bad connection. I have a few spares if your stuck.

author: darksky
author_rank: Contributor
timestamp: November 27, 2016, 08:03:51 pm
content: Hi Simon,Looks like the cable - please if you still have a spare can I buy one from you?

author: plesa
author_rank: Frequent Contributor
timestamp: November 28, 2016, 12:33:19 am
content: Try to use deoxit or similar oxid removal liquid on white snap on connector. Or connect and disconnect it few times.