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 $ 1040
Almost 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.