3478A 30v Range Chirping and other oddities. - Page 1
Forum section: electronics / repair

author: SirAlucard
author_rank: Regular Contributor
timestamp: August 31, 2020, 03:40:20 am
content: So I just purchased a 3478A off FleaBay, and overall the condition of it is good. Did my checks the Battery is good, but the rifa caps on the input power needs to be replaced. Measurements look good, pretty accurate and on point. Was able to verify a couple of ranges with the USA Cal Club references. However as the title shows I'm experiencing a chirping with the unit when I plug in my probes. I'm guessing it's the relay switching into the 30v range when I do. As it's throwing the meter above ±3v. What I'm wondering is why would it be at ±3v with nothing connected? Also why does it only chirp for the 30v range and none of the other ranges? I would assume it should be around 0v not floating at 2.7v nearing 3v, but this is my first big boy meter, and want to be sure. You put any other sort of resistance on it, even say another meter it goes straight down to 23mVDC as I would expect it to be in the first place.Anyways the other thing I noticed which seemed odd. As I was reading through the 3478A Service Manual it states.QuoteHowever when checked U405 pin6 is -10.23V, U404 pin6 is -10.23V, and JM201 is +10.23V. The Zener diode u461 should show me +7v, I get -7.04v.Yet later on down in the manual it shows a diagram with U405 pin 6 being -10v, U402 pin 6 being +10v and the zener diode being -7v. Which would all be correct.Sooo I'm lost. If all seems well then great! If not let me know what you think.Also I'm looking at these for my rifa cap replacements.https://www.mouser.com/ProductDetail/EPCOS-TDK/B32022A3223M?qs=%2Fha2pyFaduhDd66L0KefaR5dp143EcXkFI5dzp77CY4P4zuMDC0wwA%3D%3Dhttps://www.mouser.com/ProductDetail/EPCOS-TDK/B32021A3102K000?qs=%2Fha2pyFadughoIb%2FOpAtVzl%2FXOvmVp2GKlY13wf3B92cwoG5csnYQw%3D%3DShould be ok? 2619A is the first few of my serial.

author: Nx-1997
author_rank: Regular Contributor
timestamp: August 31, 2020, 05:15:05 am
content: The voltage rising when input is not connected to anything is normal. This is because of input impedance. As long as self-test passes then meter should be ok. The voltage rises up to 3V then the meter switches range to 30V, this gets rid of the floating voltage and the meter switches back to 300mV range. These meter should not be left with inputs floating while powered on as it wears the reed relays that are inside. My meter floats around 2V. Completely normal.

author: SirAlucard
author_rank: Regular Contributor
timestamp: August 31, 2020, 06:22:05 am
content: Is there any reason why only the 30v range relay is the only one I can hear? I mean it seems all ranges work, so why is it only the 30v range chirps.

author: Nx-1997
author_rank: Regular Contributor
timestamp: August 31, 2020, 08:13:34 am
content: I can hear the 30V relay switching too on both of my 3478a, so probably normal. I guess the 30V relay might be different that the other relays. The relays are custom hp parts, replacements are hard to find.

author: SirAlucard
author_rank: Regular Contributor
timestamp: August 31, 2020, 10:48:38 pm
content: Sounds good then, any thoughts on those capacitors?

author: Nx-1997
author_rank: Regular Contributor
timestamp: August 31, 2020, 11:05:13 pm
content: They look fine to me. I haven't replaced the capacitors on mine. These caps shouldn't affect meter performance.

author: SirAlucard
author_rank: Regular Contributor
timestamp: September 01, 2020, 01:53:46 am
content: No, but they are cracked and are known for starting fires, so I'd like to get rid of em.

author: wiesl
author_rank: Contributor
timestamp: March 16, 2021, 02:45:33 pm
content: Replaced all RIFA caps, very happy

author: Miti
author_rank: Super Contributor
timestamp: March 17, 2021, 03:39:33 am
content: Quote from: Nx-1997 on August 31, 2020, 11:05:13 pmThey don’t until they spill their guts all over the board. Or worse, start a fire. I replaced those in my two meters.