Where to repair an HP3478A in Melbourne for cheap? - Page 1
Forum section: electronics / repair

author: gwpt
author_rank: Newbie
timestamp: March 25, 2014, 09:52:13 pm
content: Hi,I just picked up a very cheap HP3478A but it has the error "AD Slope Err" which I have read could be a blown comparitor (or something like that)Does anyone know a reasonable place in Melbourne (or Australia wide, I could post it) which could fix that and maybe do a calibration?Any idea what that might cost?I Am just a hobbist without much cash...ThanksGuy

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 25, 2014, 10:15:12 pm
content: Quote from: gwpt on March 25, 2014, 09:52:13 pmI feel ya, but that probably implies that your best bet is to fix it yourself! I haven't finished reading the service manual, but you can probably find some test points or procedures in there surrounding the digitization circuitry and maybe you can isolate the problem that way. There's a good chance that if you've got a blown discrete chip, you can find a replacement.

author: lowimpedance
author_rank: Super Contributor
timestamp: March 25, 2014, 11:36:13 pm
content: An AD slope error is more likely the AD hybrid or its controller (a micro) and rather 'un-obtainium'. As alex. said your pretty much going to have to look into it yourself. Download the manual and read through the section 7-d-38 and flowchart d (unfortunately they use an signature analyzer to test, but it gives you an idea of whats going on!). Look at the circuit, U403 is the AD and you may find something amiss around here.Check the DC supply voltages for correct values and no mains ripple etc (you have another multimeter or CRO?), at least to eliminate bad power supplies. Check the AD reference is good U461.Is the PCB clean, has this meter been abused, since it was cheep.Does the error message all that happens, no other functionality possible?.Good luck

author: PaulAm
author_rank: Frequent Contributor
timestamp: March 26, 2014, 01:52:28 am
content: Welcome to the swamp of repairing old test equipment. You really do have to do it yourself, you can buy a replacement for a tenth of what it would cost to have somebody else fix it. You probably want to buy a spare anyway so you have parts available.You'll really want to get a signature analyzer. HP was really into that during that era. The prices are reasonable since most people (ie, those without broken HP gear) have no idea what they're for. I picked up a dead one for $20 US that had a bad PS cap.Just keep telling yourself you're having fun, you might just convince yourself

author: VK5RC
author_rank: Unknown
timestamp: March 26, 2014, 03:55:57 am
content: Repairing old gear can be very difficult, a 3322 with a bizarre switch on fault has beaten me (for a while anyway) but the real joy I got when I fixed the ac v range on a 34401 keeps me doing this as a hobby.I always start with the 'low hanging fruit' and check out the voltages and power supply (a really common fault area and not too hard to fix), don't always believe the fault messages. :-)

author: gwpt
author_rank: Newbie
timestamp: March 27, 2014, 12:46:35 am
content: Thanks for the all feedback!Yikes, sounds like it might be a bit tougher than I originally though.Oh well, here goes nothing

author: PaulAm
author_rank: Frequent Contributor
timestamp: March 27, 2014, 01:26:57 am
content: Jim Williams was a big fan of repairing test equipment.The manuals are usually pretty good (at least for HP and Tektronix), you can learn a lot trying to figure out why the circuit isn't working and, in the end, you wind up with test equipment that's far better than what you can afford to buy new. Oh, there's also that great feeling you get when you find the problem; it more or less helps you forget the hours or days of frustration getting to that point.One of these days I'm going to add up the list price of all of the equipment I've acquired and repaired.

author: echen1024
author_rank: Super Contributor
timestamp: March 27, 2014, 03:25:08 am
content: I recently bought a 3478 with a broken power switch. I have full service manuals (paper) that I can scan if you'd like.

author: casinada
author_rank: Frequent Contributor
timestamp: March 27, 2014, 06:03:10 am
content: It seems that everybody have a 3478A around hereMine doesn't work in the OHMs setting but everything else works perfect. It could be the input protection or part of the Hybrid chip that is unobtanium. In your case start with downloading the manual from here:http://www.ko4bb.com/manuals/index.php?dir=HP_Agilent/HP_3478A_MultimeterGo trough the troubleshooting steps starting from the power supply as suggested by others.Look for signs of bad parts as blown resistors, capacitors, etc.Join the HP yahoo group:https://groups.yahoo.com/neo/groups/hp_agilent_equipment/infoGood luck

author: echen1024
author_rank: Super Contributor
timestamp: March 27, 2014, 04:34:22 pm
content: The 3478A is really a nice bench meter. No frills, cheap, easy to use. I bought mine broken and the only issue was a stuck power button.