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Hi , new member here. And new to Tektronix scopes after just upgrading from my Tenma 20MHz to a very nice (and mostly functioning) 2465.

Does anyone have a board layout for the Attenuators for the 2245 scope?
Specifically, I am looking for the location of the 50 Ohm resistor inside the attenuator.

I have the service manual which shows the electrical schematic for the attenuator, but I cannot locate the 50 ohm resistor on the attenuator ceramic board.

My channel 1 input impedance setting works for the 1 Meg-ohm AC and DC settings, but the signal flat-lines on the 50 ohm DC setting.

I suppose the problem could be the latching relay. But still, I'd like to check the resistor.

Channel 2 is fine.

Cheers!

Correction, I am only referring to the 2465 scope.
I incorrectly mentioned a 2245 which I do not own.

Hey Neil,

I believe the 50Ohm resistor is in the top left corner of this picture:
/g/TekScopes/photo/12814/163755/Ceramic%20substrate.

Siggi

On Mon, Jan 27, 2025 at 9:51?AM NEIL YUENGER via groups.io <nyuenger=
[email protected]> wrote:

Correction, I am only referring to the 2465 scope.
I incorrectly mentioned a 2245 which I do not own.

On Mon, Jan 27, 2025 at 1:06?PM Sigur?ur ?sgeirsson via groups.io <siggi=
[email protected]> wrote:

Hey Neil,

I believe the 50Ohm resistor is in the top left corner of this picture:
/g/TekScopes/photo/12814/163755/Ceramic%20substrate.

I should note that the easiest way to diagnose these from the outside is to
measure the DC resistance from BNC center to shell. In DC coupling, 1M it
should be a megaohm within 1%. In 50Ohm coupling it should be 50Ohm.

The GND coupling on these attenuators is "faked". Essentially this engages
AC coupling, 50Ohm termination and 100X attenuation, which attenuates the
input signal through the pre-charge resistor against the 50Ohm terminator,
and then by a further 100X.

It is possible that what you're observing is that the first relay is not
making contact to DC couple the signal into the attenuator, in which case
you'd see an effective infinite DC resistance.

Thé résistance is a thick film on ceramic substrate forming the attenuator

It includes a thermistor to sense overload

Abuse of the input fries both 50 ohm and thermistor

Place ohms meter on BNC, switch all input coupling, check 1M, 50 Ohm, open, short.

Thermistor...Check R on 3 pin cable out of attn housing, should be 20 k at room temp.


Unfortunately Burned or broken attenuator resistor or board cannot be fixed, need to replace the module

However the relays can be cleaned, see procedures in threads about 2465 attenuator relays

Bon chance

Jon

Thanks Siggi for your response,
Just to confirm, on this image /g/TekScopes/photo/300382/3881289?p=Created%2C%2C%2C20%2C2%2C0%2C0 ,
is the 50 ohm resistor number 1? (top left) Do you know what the component marked number 2 is?

Either way, I cannot measure 50 ohms on my BNC. So perhaps the resistor is open, or perhaps the relay contact is not making connection.

Next time that I open up the scope I will play around with the relays.

And as has been mentioned in this thread, I will study the cleaning methods for this attenuator elsewhere in this forum.

Cheers!
Neil

Thanks Jon!

On Fri, Jan 31, 2025 at 8:04?PM NEIL YUENGER via groups.io <nyuenger=
[email protected]> wrote:

Just to confirm, on this image
/g/TekScopes/photo/300382/3881289?p=Created%2C%2C%2C20%2C2%2C0%2C0
,
is the 50 ohm resistor number 1? (top left) Do you know what the component
marked number 2 is?

No, I don?t believe so. I believe the 50Ohm resistor hugs the left-hand
side of that picture. You can see it's been laser trimmed.
Component marked 2 is the AC coupling capacitor.

Either way, I cannot measure 50 ohms on my BNC. So perhaps the resistor is
open, or perhaps the relay contact is not making connection.

But do you then measure 1MOhm? As many have pointed out, this should be the
default DC resistance of the attenuator, and by your description, for
reasons I mention in my earlier post, I believe your attenuator is stuck in
AC coupling.