Recovery of the relay K101(0490-1914) in HP/Agilent 34401A - Page 1
Forum section: electronics / repair

author: Panfnutiy
author_rank: Newbie
timestamp: June 03, 2019, 01:15:01 pm
content: Hello colleagues!I want to share my research on repairing K101 relays (0490-1914) in Agilent 34401A.I ran into a problem in the resistance measurement mode when the inputs were closed, the device gave readings of 10 or more Ohms. The readings changed upward with the heating of the device and several times amounted to almost 100 ohms. When measuring 4W, this problem was not observed.For several weeks I was reading the internet and trying to find a reason. The search led me to the relay K101 (0490-1914) which switches the connected signal. I finally came to the conclusion that the problem was in the relay after I had short-circuited the relay contacts. Indications immediately returned to normal.A lot of people write about this problem on the Internet. This has been discussed here several times. All came to the conclusion that the reason for the burning of relay contacts.K101 is a reed relay manufactured specifically for HP devices. It is capable of operating at voltages up to 1000 volts and Low Thermo Voltage. The 34401A used relays from two manufacturers COTO and MEDER. They differ from each other only in color. COTO red MEDER blue.Inside the relay are two reed switch connected in series. There is no marking on them, but apparently every reed switch can withstand up to 500 volts. The coil is wound with wire 0.01 mm. 11 parallel wires.I tried to find a replacement for a long time, but I did not find anything similar. In MEDER products have a relay with two reed switches and capable of working with a voltage of 500 volts (BT05-2A75), but as it turned out you cannot buy it anywhere.I tried to contact Keysight, but they said that it was impossible to buy a relay now. It is possible to make only the replacement of the instrument with a refurbished one for $ 1040Almost completely in despair, I decided to carefully disassemble the relay and try to replace the reed switches. MEDER datasheet says that the relay is filled with polyurethane. Polyurethane becomes soft at a temperature of about 180-230 degrees. I armed myself with a scalpel, a hair dryer and patience and began to carefully remove the fill. I also carefully worked with a very thin blade around the perimeter of the metal case to release the coil. After some time, I felt that the relay can be removed.After extraction, I regretfully discovered that I still damaged the coil windings. After removing the fill from the ends, I soldered the contacts of the reed switches from the findings and removed the reed switches.At this point, one kind person responded to my request and sold me a relay from a non-working device. Sighing calmly, I decided to explore the wreckage. I measured the resistance of the reed switches and lo and behold, it turned out to be 0.07 ohms on my portable device !!! At first, I thought it was an accident, but after several experiments and examining the contacts of the reed switches under the microscope, I was convinced that the reed switches were completely functional !!!Reflecting on this, I remembered that in one discussion they wrote that after heating the relay contacts, its working capacity was fully restored.It dawned on me. The design of the connection of the contacts of the switches with the terminals of the relay is made very strange. They are interconnected by a cross and simply filled with solder. No mechanical connection is present. Contacts are leaning against each other. This prompted me to the obvious conclusion that it was a matter of degradation of the place of soldering. For many years, microcracks appeared in the solder and because of this the resistance of the relay changed.I have repeatedly encountered this problem during the repair of old equipment.From this it follows that it is sufficient to warm each relay contact with a soldering iron until the solder melts at the junction of the terminals, after which the operability of the relay will be restored. I conducted an experiment with heating. A temperature of about 300-330 degrees is enough to melt the solder in this place. If you warm with a higher temperature begins to melt the body in which there are conclusions.Unfortunately I can’t check my assumption because the old relay has been disassembled and now I have a replacement relay. After replacing the relay in my Agilent, I took the measurement of a 4W reed switch to calm it down. The device has not yet been calibrated after repair, so a small error is possible.I hope my long story will be useful to you and you will be able to bring your 34401A back to life. Please report the results of your repair attempts.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 03, 2019, 02:01:04 pm
content: Interesting!I took all my old relays apart and do not have a bad one to test your theory on.But it will be interesting to see, how long this heat treated relay will stay stable.

author: elianto
author_rank: Contributor
timestamp: December 01, 2021, 01:35:11 pm
content: Hello,i had the same problem with my HP34401A.Re-soldered the relay for 10 seconds at 380 ° C, calibrated the offsets, now everything is fine.

author: philwong5176
author_rank: Unknown
timestamp: December 01, 2021, 09:13:37 pm
content: There's a previous discussion about the K101 relay over here (with vague subject line):https://www.eevblog.com/forum/chat/help-me-find-a-replacement-relay/with useful part numbers.

author: nedrysoft
author_rank: Contributor
timestamp: April 11, 2023, 08:26:40 pm
content: Sorry for bumping an old post, but I figure this may ping people who've done the procedure here and I'm wondering how successful it was long term.I have two 34401A which both have faulty K101. The first unit I tried heating the contacts in circuit, this seemed to resolve the issue temporarily for nearly a week, but then the issue returned. The issue on this unit was that DC measurements would start going random after some amount of time powered up.I managed to grab a used K101 from eBay, I thought for 35 GBP it was worth a punt and when I replaced mine with it I found that it had indeed resolved the problem, the unit has been powered up for several weeks now as a test in min/max and no further bad readings.I grabbed a second 34401A and this was also showing a similar issue, I thought it may have been the switch as there was an ohms reading on dead-short, but shorting the contacts didn't resolve it and I moved over to the other side of the board and shorted K101 and the ohms reading dropped to zero.I don't have a replacement K101, so I grabbed the COTO 3501-05-511 relay mentioned on the board here, it's contacts are only rated for 200V, but upon replacement the ohms reading was correct and the random readings on this unit that happened from start up were resolved as well.So I have my original unit working with the replacement K101 and I have the second unit working with a lower spec relay but I'd like to get the second unit back with a working K101.So I'm back here asking how successful people found the procedure in this thread? Are your units still working?I see people mention that the relay is taken out of circuit to apply the soldering iron directly to the contacts, is this the critical step to "refreshing" these relays?Thanks

author: robert.rozee
author_rank: Frequent Contributor
timestamp: April 12, 2023, 01:07:55 am
content: after looking at the photos, i'm inclined to suggest the following method to repair the relay itself:1. unsolder the relay from the 34401A, and then resolder it onto a scrap piece of PCB material measuring perhaps 10cm x 4cm. you want the relay securely anchored to this piece of scrap PCB.2. suspend over a heat source with the relay casing downwards, heat the casing until the polyurethane potting material begins to melt and the casing falls away. i would imagine that there will be some loss of polyurethane potting material, but this is a good thing when it comes to reassembly.3. you should now have access to both ends of the two reed switches, so you can resolder the 3 solder joints. you should NOT need to extract the reed switches from within the coil to do this.4. test the 'naked' relay. once satisfied it works, solder the 'naked' relay back into the 34401A and further test. add kapton tape to the inside of the metal casing then slip it over the body. secure the casing further as you see fit.from those who have taken one of these relays apart, does this strategy sound reasonable/workable?cheers,rob :-)

author: daveyk
author_rank: Frequent Contributor
timestamp: July 26, 2023, 04:41:40 pm
content: With the relay I have, I was able to get the metal case off without destroying to relay, but the potting material did not melt. Even with extreme hot air, the potting doe not melt. You can chip some away.On-line Radwell shows inventory, but I am waiting to hear back from them.Dave

author: Paul297
author_rank: Newbie
timestamp: September 09, 2025, 12:05:04 pm
content: I was going to junk my 34401A because of K101.However looking at the pictures I can see where the internal connections are.My unit would show random ohm readings with the probes shorted when I tapped K101 so clearly a bad connection or contact internally.So I removed the relay, cut off the contact end of the metal case with an angle grinder and carefully picked out the potting material with a hot soldering iron until it looks like the photo.At this point the coil is not affected and wont be damaged.The potting material would not melt for me but it crumbles away with the soldering iron.After clearing enough material I could re-solder the reed connections.I energised and tapped the relay to test and it was rock solid so I did not bother with the other end.Refitted and the meter now works fine.You can solder a metal plate on the end to cover it. I used some shielding from a GPS antenna.