Trying to fix a HP 3478A(FIXED) - Page 1
Forum section: electronics / testgear

author: TedStormrage
author_rank: Newbie
timestamp: June 01, 2014, 11:41:49 pm
content: I got myself a HP 3478 from eBay. The seller said that self test passes and buttons are responsive and yes self test does pass and buttons are responsive but the meter is not fully operational.The lower dc volt ranges work fine. If I put 1 volt to the input it will show 1v in the 3v range and it will show 10v in the 30v range and 100v in the 300v range. If I put 5v into the meter it will show overload on all ranges.ohms seem to be working correctly.dc amps is just floating. It doesn't seem to matter whether I'm putting current to it or not.What I have done so far:Checked the voltages. Everything seems to be OK.Checked the fuse and shunt resistor.I was measuring the input relays and found something interesting. On all ranges on all measurements k101 is always getting 5v on its coil, so it's on. k102 seems to be never on. k102 is the relay which should let in the higher voltages. k103 is on when measuring ohms and it stays on when switching to amps, but if you select volts it goes off and if you go back to amps it stays off. So something strange happening there.After reading the service manual I have come to the conclusion that u102(1QF7-0067 Hybrid Input Board) must be broken. Are there any other possibilities?I don't think it's the logic because it passes self test and buttons and menus work no problem. Also the higher voltage ranges not working and amps not working would suggest it has to be that chip. I would love to hear your thoughts on this before I start to hunt for the unobtainium.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: June 02, 2014, 02:14:52 am
content: Looks like an attenuator problem! See the voltage in any OPamp (+-) look what happens when you try to measure the problematic ranges.

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: June 02, 2014, 03:21:10 am
content: I have a kikusui oscilloscope with a similar problem.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: June 02, 2014, 03:22:09 am
content: The service manual suggests that it is possibly U462 or U102. I did find another U102 related issue here ...https://www.eevblog.com/forum/testgear/hp-3478a-ovld-2-wire-ohm-test/Maybe there is an issue with the connection to U102?

author: TedStormrage
author_rank: Newbie
timestamp: June 02, 2014, 01:40:55 pm
content: Completely forgot the A/D controller. I was reading about it and would have needed a signature analyzer so I just ignored it like a dummy.I moved some jumpers like it said in the manual and measured the signals with an oscilloscope. I'm not quite sure how the signatures work but at least there is something happening in the data lines between u462 and u102.The manual says signatures should bepin 32: 7ACApin 33: 20F0pin 34: 666HIs there a way to tell if the u462 is working from this data?

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: June 02, 2014, 05:41:55 pm
content: I don't have any experience with signature analyzers. If all the ohms ranges are functioning, you can probably rule out the communication between U462 and U102.The only other thing I would look at is K101. It's default state is closed. This would bypass any of the attenuation if it did not open for the higher voltage ranges. I have not opened my 3478a in a while, but if you can verify that 5v is measured across K101 and 0v across K102 while manually changing the DCV ranges, then it is indeed looking like U102 has failed. I think you may be able to reseat U102 based on some of the pictures I have seen.Also, the 3468A repair manual may have additional info even though they use different U102 parts.

author: TedStormrage
author_rank: Newbie
timestamp: November 17, 2014, 12:08:36 pm
content: I finally got my hands on a HP 1QF7-0067($15 + $15 s&h) and my multimeter is now working. I hope if someone has the same problem this will help.

author: GOR3
author_rank: Newbie
timestamp: October 28, 2015, 07:03:34 pm
content: TedStormrage,I need this part too. Where did you find yours?Thanks,George

author: TedStormrage
author_rank: Newbie
timestamp: October 29, 2015, 06:54:20 pm
content: I found mine on eBay (http://www.ebay.com/itm/HP-3478A-IC-CHIP-PROCESSOR-1QF7-0067-4588-/231371078496?ru=http%3A+www.ebay.com%2Fsch%2Fi.html%3F_from%3DR40%26_sacat%3D0%26_nkw%3D231371078496%26_rdc%3D1&nma=true&si=6ILCKecz7LEuzomUDhm2GG1JGyg%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557)It took me several months to find one.Maybe keep an eye on eBay to see if you get lucky or find a donor unit.