Products > Test Equipment
Advice please, used 34401a
std:
--- Quote from: Kean on December 09, 2024, 11:30:33 pm ---What do you get if you use the rear terminals?
A common problem is dirty switch contacts for selecting between front/rear terminals, and operating the switch a few times can help.
--- End quote ---
When I short K101 with jumper wire, the readings change by the amount of some ohms.
Now multiple clicks on [DC V]-[Ω 2W] buttons, or multiple pressing [Front/Rear terminals switch] the readings are the same down to 1-2 milliohm.
For rear terminals resistance readings is correct.
For front terminals the readings are biased by 0001.417 OHM. Increasing input resistance on front terminals first reduces readings. After this bias point it starts to increase.
Again, after multiple clicks on [DC V]-[Ω 2W] buttons, or [Front/Rear terminals switch] the readings is stable. Just biased.
(It appears when the K101 relay was broken, someone tried to set the zero offset for front terminals which is now incorrect).
Kean:
--- Quote from: std on December 10, 2024, 02:33:22 am ---For rear terminals resistance readings is correct.
--- End quote ---
OK, well K101 comes after S1 (the front/rear switch) so I would have expected a problem with K101 to show up in both positions.
But as you suggest, someone may have done a zero offset calibration adjustment for the front input while a K101 fault existed.
Out of interest, do you have (or can you make) a 4W short and test zero in 4W mode.
K101 is rather tricky to source. :--
You might want to try this procedure:
https://www.eevblog.com/forum/repair/recovery-of-the-relay-k101(0490-1914)-in-hpagilent-34401a/
Thankfully both of my 34401As have not shown this issue, and 2W short reads about 4.5mohm.
AFIAK the 34970A DMM doesn't have K101 (I have 3 x 34970A and 1 x 34972A).
KungFuJosh:
Radwell sells them for $20 with a 2-year warranty. I've never dealt with them before, so I dunno how they are.
https://www.radwell.com/Buy/COTO/COTO/0490-1914
Kean:
--- Quote from: KungFuJosh on December 10, 2024, 03:40:41 am ---Radwell sells them for $20 with a 2-year warranty. I've never dealt with them before, so I dunno how they are.
https://www.radwell.com/Buy/COTO/COTO/0490-1914
--- End quote ---
The page doesn't mention them being in stock, just an option to request a quote.
ISTR someone asking them and getting a reply that they didn't have any - although that may have been a different COTO model...
Aha, found the post. It was a COTO relay for a K196 - and probably a very similar part.
https://www.eevblog.com/forum/repair/some-help-troubleshooting-a-keithley-196-with-bad-ohms-reading(s)/msg3525264/#msg3525264
The suggested COTO 3501-05-511 is the same footprint as the HP 0490-1914 with low thermal EMF, but has a lower voltage rating.
It is available at DK & Mouser (~30 USD) and should work as a replacement if you avoid measuring over 200V. Usual caveats apply.
std:
Unsoldered K101 (T17231 COTO-9231).
K101 coil DC resistance = 500 Ω.
Inductance = 348mH, open reed switch contact capacitance = 1.2pF.
Next I measured reed switch resistance applying different coil voltages.
Operate voltage 3.33V, release voltage 2.3V.
As in the DMM, reed switch resistance floats between 1.5-15 Ω. With small dispersion in row.
I also found strange thing. When the coil voltage (current and magnetic flux) decreases, the contact resistance does not remain constant. As the voltage decreases, contact resistance increases to hundreds ohms or 3K-7K until it release. Also when coil operated at higher voltage it makes resistance much lower. Of course, relay is clearly faulty. There should be almost no resistance at all. I thought the problem was in bad solder joint, not reed switch contacts (same as here). Now I'm not sure. How bad solder joints would depend on magnetic flux? And if the reed switch itself is faulty, then it's a fiasco. Even if I disassemble the relay, it won't help.
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