Agilent 34410A vs 34411A: Fan Noise - Page 1
Forum section: electronics / projects

author: abitofhelp
author_rank: Contributor
timestamp: April 17, 2025, 03:38:56 am
content: Hello,I bought an Agilent 34410A on eBay a few months ago, and it is a great DMM, but when I turn it on, there is a significant fan sound -- nothing scary or grinding. A few days ago, I decided to add another DMM to my bench, so I bought an Agilent 34411A, which is supposed to be identical to the 34410A but faster. Everything worked when I turned it on, but there were no fan sounds. I placed my hand on the back of the unit and felt a gentle air movement. My apartment was about 78°F during these tests. I looked at both units' menus but found nothing related to fan speed, sensor, or temperature control.Questions:1) Are these units working properly? Is there some hardware difference between these versions that accounts for the difference in fan sounds? Both of these units have been well-maintained and come from reputable labs.2) What is the best course of action if the 34410A is noisy because of an issue, besides selling it!3) What is the best course of action if the 34411A is not noisy because of an issue?I have seen many discussions about fan noise online, but nothing related to fan noise between these two models, such as my situation. I did learn that the newer models have corrected fan noise and temperature control issues.I'd appreciate your comments and suggestions.Mike

author: KE5FX
author_rank: Super Contributor
timestamp: April 17, 2025, 04:11:21 am
content: I put a (5.1V?) Zener in series with the fan in my 34410A.

author: abitofhelp
author_rank: Contributor
timestamp: April 17, 2025, 05:17:13 am
content: Hi, and thanks for the response.I want to learn whether the difference in fan behavior between the two models is correct.Or should both of them be silent, like the 34411A?Or should both of them be noisyish, like the 34410A?I've seen many discussions about addressing the noise issue for the 34410A, but nothing for the 34411A. I assume HP's/Agilent's claim that they are identical except for the 34411A's higher performance is not entirely accurate.Anyway, I hope to get clarification about expected fan behavior for these two models so I can learn what is "normal" for them and take any appropriate action.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 17, 2025, 06:53:49 am
content: Both have the same fan and the same electronic control for the fan.But Agilent has used two different fan models.In any case, I have used a 100 Ohm resistor in line for the fan of each of mu 34410A and 34411A and they have been working quietly ever since.https://www.eevblog.com/forum/repair/agilent-34410a-repair-attempt-of-2-dead-meters/

author: abitofhelp
author_rank: Contributor
timestamp: April 17, 2025, 08:07:04 am
content: Thank you for your information! I will open both DMMs and see what fans are in them. If the fans are the same, I must replace the one in the noisy 34410A. If they differ, I will do the same but use the same fan as the 34411A.

author: abitofhelp
author_rank: Contributor
timestamp: April 17, 2025, 08:55:40 am
content: Hi @HighVoltage,I read your links about replacing the fan and found that Papst doesn't sell the item you used. I filtered the list of fans that matched your fan's voltage and size. I don't know what flow is required, so I based my selection on noise and mid- to high-flow. What do you think about 612NME? Based on what I read online in rebuild postings, it seems like a good replacement.

author: ivo
author_rank: Regular Contributor
timestamp: April 18, 2025, 02:17:13 pm
content: I bought and installed a HA30101V3 fan from Sunon for my 34411A, and IIRC put a 10 ohm resistor in series with it. You can hear it in a quiet room, but much quieter and more comfortable noise. Also IIRC from that thread (or it was the upgrading 10 to 11 thread), 34410/11As can either be setup for a 35mm or 30mm fan.The stock fan of my 34411A was very clearly audible, you knew it was on.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 14, 2025, 04:58:01 pm
content: Quote from: ivo on April 18, 2025, 02:17:13 pmI tried a 35mm fan but wouldn't recommend it. The fan is not centered over the grill opening and as a result blows partially into the center disk of the grill. This which makes the fan louder inside the case than when it is sitting outside by itself. A 30mm fan blows straight through the openings and the air turbulence isn't contributing to the noise.That said I have a really hard time sitting next to the 34411a with 100Ω series resistance slow turning Sunon fan (HA30101V3-1000U-A99 ), the fan noise drives me crazy.

author: floobydust
author_rank: Super Contributor
timestamp: July 14, 2025, 05:14:33 pm
content: Using a high speed/high airflow bladed fan and throttling it way down- runs it very noisy. Wails like aBansheeI think a lower speed/airflow fan throttled down can actually move more air. But you'd have to look at the vent area etc. to see why this performs poorly.I find it funny the voltage reference is directly inline with fan airflow on this series.We should heat the ref and cool it at the same time lol. Hint: do it like the HP3458, ref located in the far corner out of direct airflow.I imagine most of the heat is generated by the CPU, front panel processor, power transformer.There's no heatsinks on the big IC's, you'd have to find out where the temp sensor is. It might be built-in to an IC that makes heat.

author: guenthert
author_rank: Frequent Contributor
timestamp: July 14, 2025, 06:43:28 pm
content: @MickleT determined inhttps://www.eevblog.com/forum/projects/agilent-34410a-fanless-mod/that it's actually some linear regulators which get unduly hot w/o a fan moving air inside the case. Even though the 34411A is a bit faster, I'd guess the fan's reason d'etre there is the same.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 14, 2025, 07:15:37 pm
content: Thanks for that link, quite interesting, I'm very much tempted to repeat this experiment. A 34411a as silent as a 34401a would be great indeed.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 15, 2025, 01:19:35 am
content: why complicate things, just change the noisy fan for suggested ones, and put an resistor in seriali made theses change for the noisy one in my 34410a now almost inaudible ...

author: hp3310a
author_rank: Regular Contributor
timestamp: July 15, 2025, 07:55:27 am
content: Quote from: coromonadalix on July 15, 2025, 01:19:35 amI did and while it improved things slightly the fan is far too annoying still.Plus I have been outed as incompetent for doing sohttps://www.eevblog.com/forum/repair/34411a-permissible-fan-exhaust-temperature/

author: KE5FX
author_rank: Super Contributor
timestamp: July 16, 2025, 04:46:18 pm
content: What everybody misses in these perennial threads is that the equipment has to be rated to operate under conditions that none of us would ever subject it to. If your lab doesn't regularly exceed 30C, you don't need a fan running at full speed. Or potentially any fan at all.An IR camera will tell you what's going on and what's needed. Keep in mind that in many cases the original equipment designers themselves didn't have access to one. You are in a better position than they were, in that you have both good instrumentation and control over your environment.