TEK 2465B attenuator repair

[rfdes]

Hi -
I have an upcoming repair project with my 2465B.  The Chan 1 AC coupling cap has become leaky.  I never noticed this until recently when I attempted to look at ripple on a 100Vdc supply and noticed an undesired DC offset with the AC coupling activated.  Chan 2 works as expected in AC coupling.  I am guessing that Chan 1 was abused sometime in the past.

The question is whether anyone has specifications on the AC coupling cap used in the attenuator and if there exists photos of the attenuator disassembled.  Any tips/hints as to how best to make this repair?
Thanks
Jim

[mawyatt]

Not sure about the cap value or specs, should be available from the schematic. Here's a thread on fixing the input hybrids, think the AC coupling cap is located in the input hybrid (large SMD).

https://www.eevblog.com/forum/repair/tektronix-2465-input-issues/msg3054430/#msg3054430

Anyway, hope this helps.

Best,

[rfdes]

Hi -
Thanks for the link.  The attenuator sub-assembly was sold as a unit, so the value is not available.
Take care.

[mawyatt]

Yep that's it!!

You could carefully remove the suspect leaking cap and measure it with a good LCR meter. Another option would be to measure the good channel hybrid cap in place, might be able to get a good reading.

Regarding voltage rating, the input max DC rating should be considered and would try and find the best ceramic formula that fits the required size within the voltage rating.

Edit: You could measure the low frequency high pass AC coupled response of CH2 and then compute the required capacitance based upon a 1Meg input resistance.

Best,

[bdunham7]

I measured one in a spare attenuator assembly.  The capacitor appears to be 22nF. 

This might work.

https://www.mouser.com/ProductDetail/KEMET/CKC21X223GDGACAUTO7210?qs=sGAEpiMZZMvsSlwiRhF8qtsGU%2FCnaNeQpMrwezP%2FFCWD8Iw%252B55sPcQ%3D%3D

[rfdes]

Thanks for everyone's input.  Now to get the scope opened up and get started.

[rfdes]

Yikes ... another no-stock part from both Mouser and DigiKey.  Also min buy of 1000.   I'll need to pursue another source.

[bdunham7]

OK, this should also work:

https://www.mouser.com/ProductDetail/KEMET/C2220X223JBGACTU?qs=5%252Be0R4ntX6lCxULXW8Z7PQ%3D%3D

I measured approximately 6.0 x 5.4mm, so that seemed to be the 2220 size.  It's pretty low profile as well.

[rfdes]

Appreciate you looking for the part.
Take care -

[rfdes]

I have another question related to the attenuator repair project.
I've removed the attenuator with the issue but before I remove the magnetic latching relays, I want to attempt to do some additional testing with the attenuator external to the scope.

The question is whether it is possible to manually flip the state of the 4 relays without actually using electrical pulses to force the state change??
Thanks
Jim

[David Hess]

I measured one in a spare attenuator assembly.  The capacitor appears to be 22nF.

0.022 microfarads was going to be my guess.  That is the value Tektronix has used for decades.  If needed the value can also be calculated from the low frequency cutoff given in the specifications.

[rfdes]

I managed to remove the Ch 1 attenuator and disengage the latching relay stack up.  Applying 32V directly into the BNC connector and monitoring the hybrid output pin with a multimeter, there was definitely leakage and was measuring about 140 mV (140 nA across 1Meg).  With the relays removed the only current path was through the input cap.

After I did some careful cleaning with 100% alcohol around the input cap, I remeasured and the result was worse.  Measuring about 4V (4uA) leakage.  I ended up removing the capacitor and tested the capacitor outside the hybrid and got the same results. So the cap was bad, which seems strange to me.  The capacitance did indeed measure 22nF, so that was verified.  The new caps should arrive sometime next week and we'll see how it goes.

Curious if others have experienced this behavior before.