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Can anyone point me in the right direction to fix this? The scope recognizes the 10x probe when connected to the other three channels.

Thanks!

On Thu, 15 Nov 2018 14:32:56 -0800, you wrote:

Can anyone point me in the right direction to fix this? The scope recognizes the 10x probe when connected to the other three channels.

Look at the ring on the probe. It's likely connected to some
circuitry that forms part of a voltage divider. The other part of the
divider is the resistor in the probe itself.

Try taking a number of resistors and run them from the ring to ground.
If you get no readout change then the mesure/compare circuit isn't
working, or the feedback from the circuit to the readout circuitry
isn't happy.

Harvey

Thanks!

Good point.

Each of the four probe rings is connected to -1.25V by a 10k resistor so with the scope powered off you should be able to measure the resistance between each pair of probe rings at not more than 20k. If the pairs involving Ch1 give different results to the other pairs (ie Ch1-Ch2 is different from Ch2-Ch3 etc) then there is probably a break in the wiring of the Ch1 probe ring. There is a three way ribbon cable emerging from the Ch1 attenuator, if you unplug this you should get zero ohms from the probe ring to pin 3, the orange wire.

The next check is resistance from the Ch1 probe ring to pin 1 of the analogue multiplexer chip U2601. This should read 10k, if not measure from pin 1 to either side of R2704. If all the resistance checks prove OK then as a last resort measure the voltage on pin1 of U2601 and see if it changes when you plug the 10X probe to channel 1. To compare with a working channel measure pin 5 of U2601 while plugging the probe into Ch2. If you don't find anything wrong by now you probably have a dead U2601 since everything after this is common to all four probe channels.

Roger

Thank you for the very thorough reply. I previously measured 1.25v between the ring and ground side of the three working channels. Ch 1 showed a varying voltage, and was clearly not correct. With your advice, I’ll have another look and report back.

Greg

Ok, it appears that the bnc jack on the channel 1 attenuators is bad. The connection to the probe ring was loose and intermittent. The bnc jacks alone are not available for any reasonable price, so I picked up another attenuator.

Btw, the component numbers you referred to were for the 2465b. My 2465 has the same circuit with different component numbers.

Thanks again for the pointers.

Greg

Hey Greg,

I replaced the BNCs on my 2467 a couple of years ago. See message
/g/TekScopes/message/115057 for the kind gentleman who
sourced replacement BNCs for me at a very reasonable price. Maybe he has
more of those.
I'd think the precise attenuation ratios of your attenuators get baked into
the calibration, so you'll likely stay closer by replacing the BNCs than
the entire attenuator. If your scope hasn't been calibrated in the last
couple of decades or so, it may not make for a hill of beans, though :).

Siggi

If the replacement attenuator works acceptably, I’ll use it. If not, I’ll pull the bnc from the new one and use my existing attenuator. Appreciate the suggestion re the possible source, though.

When I got this scope, for free, about two months ago, the ch 1 bnc was covered with duct tape and the statement that channel 1 didn’t work. Since then, I have visually inspected all of the caps, blown out the dust, verified the lvps voltages as correct and determined that the channel 1 issue seems to be cured by chilling U600. I suspect an intermittent terminal on U600 and will clean those contacts. All channels now work correctly, and the calibration seems pretty good still. Just working on the probe sensor ring and planning a future re capping right now.

This forum has been a great resource for me.

Thanks again for the offer re the bnc.

Greg