2445A CH1 attenuator


Fred Schumacher
 

Hi All,
I got a 2445A Tek scope with a CH 1 problem. Applying a square wave to the input of this channel gives me a square wave on my display with an overshoot on the trailing edge, which isn’t present on all other channels.
Switching from 1 mV/div. To 50 mV presents a perfect square wave but switching to a higher attenuation the mentioned problem is present.
Connecting the CN1 attenuator output to the input of the CH2 pre amplifier and connecting the CH2 attenuator output to the CH 1 pre amplifier input gives me the mentioned problem on my CH2 display.
Also I interchanged the connecting rc network between the attenuator and pre amplifier. Checking the internal resistors of both attenuators showed no difference between them. The attenuator showed correct attenuation.
This mean the problem is in the CH 1 attenuator.
So my question is, is it possible to repair this unit or do I have to replace it as a unit.
Part number of this attenuator is 119-2342-01.
Thanks for your advice.


Roger Evans
 

Looking at the construction of my spare attenuator module (removed from a 2440 part 119-2342-10), you should be able to remove the black plastic cover that holds the four latching relays, carefully remove the plugin relays (they have very thin connecting wires) and then you have access to clean the relay armatures and the area where they contact the ceramic substrate. Overshoot is very commonly due to poor connections giving an effective parallel RC circuit in the signal path.

You will almost certainly need to remove the attenuator from the main board to do this.

Regards,

Roger


Fred Schumacher
 

Hi Roger,
Have you physically opened the attenuator to see the contacts? To my knowledge the reet contacts are operated within a glass capsule. So cleaning is impossible?
Thanks for your info .
Fred

Verstuurd vanaf mijn iPad

Op 16 mei 2022 om 12:51 heeft Roger Evans via groups.io <very_fuzzy_logic@...> het volgende geschreven:

Looking at the construction of my spare attenuator module (removed from a 2440 part 119-2342-10), you should be able to remove the black plastic cover that holds the four latching relays, carefully remove the plugin relays (they have very thin connecting wires) and then you have access to clean the relay armatures and the area where they contact the ceramic substrate. Overshoot is very commonly due to poor connections giving an effective parallel RC circuit in the signal path.

You will almost certainly need to remove the attenuator from the main board to do this.

Regards,

Roger






Siggi
 

Hey Roger,

These attenuators are serviceable. Here are some pictures of the ceramic
substrate and the contacts from my 2430 (same attenuators):
https://groups.io/g/TekScopes/album?id=12814. Be careful with the "lion's
feet" contacts, they're quite delicate.

Siggi

On Mon, May 16, 2022 at 2:03 PM Fred Schumacher via groups.io
<fred_schumacher@...> wrote:

Hi Roger,
Have you physically opened the attenuator to see the contacts? To my
knowledge the reet contacts are operated within a glass capsule. So
cleaning is impossible?
Thanks for your info .
Fred

Verstuurd vanaf mijn iPad

Op 16 mei 2022 om 12:51 heeft Roger Evans via groups.io
<very_fuzzy_logic@...> het volgende geschreven:

Looking at the construction of my spare attenuator module (removed from
a 2440 part 119-2342-10), you should be able to remove the black plastic
cover that holds the four latching relays, carefully remove the plugin
relays (they have very thin connecting wires) and then you have access to
clean the relay armatures and the area where they contact the ceramic
substrate. Overshoot is very commonly due to poor connections giving an
effective parallel RC circuit in the signal path.

You will almost certainly need to remove the attenuator from the main
board to do this.

Regards,

Roger











Siggi
 

On Mon, May 16, 2022 at 4:19 PM Sigurður Ásgeirsson <siggi@...> wrote:

Hey Roger,
Ooops - that should be "Fred".


These attenuators are serviceable. Here are some pictures of the ceramic
substrate and the contacts from my 2430 (same attenuators):
https://groups.io/g/TekScopes/album?id=12814. Be careful with the "lion's
feet" contacts, they're quite delicate.

Siggi

On Mon, May 16, 2022 at 2:03 PM Fred Schumacher via groups.io
<fred_schumacher@...> wrote:

Hi Roger,
Have you physically opened the attenuator to see the contacts? To my
knowledge the reet contacts are operated within a glass capsule. So
cleaning is impossible?
Thanks for your info .
Fred

Verstuurd vanaf mijn iPad

Op 16 mei 2022 om 12:51 heeft Roger Evans via groups.io
<very_fuzzy_logic@...> het volgende geschreven:

Looking at the construction of my spare attenuator module (removed
from a 2440 part 119-2342-10), you should be able to remove the black
plastic cover that holds the four latching relays, carefully remove the
plugin relays (they have very thin connecting wires) and then you have
access to clean the relay armatures and the area where they contact the
ceramic substrate. Overshoot is very commonly due to poor connections
giving an effective parallel RC circuit in the signal path.

You will almost certainly need to remove the attenuator from the main
board to do this.

Regards,

Roger











Roger Evans
 

Siggi,

Thanks for the extra information, it saves me stripping down my 2440 attenuator. The relays are not reed relays, which are almost always sealed as Fred says, but are latching relays with two energising coils. These require only a brief energising pulse to change state which means that that the power dissipation in the attenuator does not change when the attenuation factor is changed and the temperature can stabilise independent of the relay settings. There is an extreme example of Tektronix attention to this sort of detail in the 7A11 probe head - the x10 miniature attenuator relays in the head have split coils and are always powered for constant heat dissipation. The relay is switched by reversing the polarity of one half of the winding so that the magnetic field goes to zero!

Roger


Fred Schumacher
 

Hi Siggi,
Thank you for these informative pictures of this particular attenuator. Never imagined this was a very fine piece of miniature electronics. Working with latching contacts driven by pulses. Also there are more components involved I was thinking of.
Once again thanks for sharing the pictures with us.
Fred


Verstuurd vanaf mijn iPad

Op 17 mei 2022 om 09:31 heeft Roger Evans via groups.io <very_fuzzy_logic@...> het volgende geschreven:

Siggi,

Thanks for the extra information, it saves me stripping down my 2440 attenuator. The relays are not reed relays, which are almost always sealed as Fred says, but are latching relays with two energising coils. These require only a brief energising pulse to change state which means that that the power dissipation in the attenuator does not change when the attenuation factor is changed and the temperature can stabilise independent of the relay settings. There is an extreme example of Tektronix attention to this sort of detail in the 7A11 probe head - the x10 miniature attenuator relays in the head have split coils and are always powered for constant heat dissipation. The relay is switched by reversing the polarity of one half of the winding so that the magnetic field goes to zero!

Roger