HP 3478A - buzzing noise and negative rail out of spec - Page 1
Forum section: electronics / repair

author: No.Mad
author_rank: Regular Contributor
timestamp: September 19, 2020, 04:40:54 pm
content: Hi everyone,recently I decided to change all RIFA caps in my HP 3478A multimeter (I have two) as I had recently one of these caps blown up during power up.Mistake #1: My soldering iron has tip grounded to Earth. Calibration gone. It's going to cost me $$, but I learned my lesson hard way.You don't have to...Mistake #2: I wanted to clean it properly, with soft keys at front as well. The problem is when I took the front bazel off, some gold spongy boi dropped on floor. Does anyone knows what proper orientation of this gold plated sponge is? Photo would be nice.As I wanted to send it over for calibration anyway I cleaned it and reassembled it. When I was checking voltage levels, to make sure I will sent it in mint operational condition, I noticed that -15V rail is out of spec. Like really, out of spec.+15V rail: reading OK, +15.1V+5V rail: reading OK, +4.99V-15V rail: reading -24.89VIn attachment you can see where I placed probes (right on regulator leads). Is it me being dumb as shoe or is it really having issue? Does anyone else had it before?In addition I can hear quiet clicking / buzzing noise next to rear part of mainboard and I can't pinpoint it. Any suggestions?Regards,No.Mad

author: pqass
author_rank: Super Contributor
timestamp: September 19, 2020, 04:58:03 pm
content: The analog section in this meter has a floating GND (not connected to case GND).Measure between JM703 (red lead) and JM702 (black lead). If it says 30 volts then both regulators may be okay. If not, the downstream circuit is probably smoked. Oh well!

author: No.Mad
author_rank: Regular Contributor
timestamp: September 19, 2020, 07:08:02 pm
content: Thanks pqass.I was measuring in regards to floating analogue ground. But as you pointed out this, I re-measured it once again with another multimeter.Good news is now it reads +15V and -15V respectively. 30V between JM703 and JM702, as you said.Bad news is I might have broken Fluke 87The buzzing is still there but the multimeter does not have DC-DC converter anywhere. Could it be bad electrolytic cap?

author: pqass
author_rank: Super Contributor
timestamp: September 19, 2020, 07:31:11 pm
content: WRT buzzing...Maybe it's the transformer. It could be loose? Put your finger on it (light touch to firm and back) and see if it changes tone.Also, there are four RIFA caps; a pair on either side of the transformer. Did you replace all four? There could be some arcing. The two below the HPIB connector are before the power switch so if they are arcing then they would make a sound even if the unit is off but plugged into the wall.EDIT: Does it still "click" if you short the LO and HI inputs while in DCV mode?If the input jacks are left unconnected the relays may click through the various ranges as the readings bounce around. The relays are the red rectangles marked "COTO" near the front/rear terminals multi-pole switch.WRT "gold plated sponge"...I have not taken apart my meter so I don't know what it can possibly be. Can you post a picture of it.

author: Miti
author_rank: Super Contributor
timestamp: September 20, 2020, 02:37:43 am
content: If you have two and one is calibrated, you can calibrate the other one yourself. All you need is a Prologix compatible USB to GPIB interface.

author: wiesl
author_rank: Contributor
timestamp: March 16, 2021, 02:41:29 pm
content: Replaced all of the RIFA caps already