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Tektronix 492A input attenuator repair ????


Bent Andersen
 

Hello Groupe

I have this 492A spectrum analyzer and I have found that the input attenuator (0-60DB) i defect.... is it still possible to get spare parts for this attenuator
or is there a better way to repair it ....

I have bypass the attenuator and I can se signal on the display, so I hope that the rest of the spectrum analyzer is working...

OZ1CT Ben


 

Unfortunately, there are no parts, except from another Tek attenuator. Is it blown, or are the contacts "dirty"? These were often damaged by DC or excessive input power. If it is damaged, you need to check the first mixer for damage as well. You may be able to find a good attenuator on eBay, perhaps elsewhere, but these can be difficult to find in good condition.
It is possible to disassemble these and make one good unit out of two. The thin film resistive elements can also be inspected. Usually the damage is readily apparent under magnification.
It would be possible to fit a different attenuator, with the correct steps, and frequency range, altering the drive signals as required.
Be sure to take anti static precautions around the mixer.

Good Luck,


Tom Lee
 

If it does turn out to be a blown attenuator, perhaps the best option is to use an HP attenuator instead (they are much more available). A small bit of logic is needed to remap control signals and space is tight, but it can be done. I've fixed two this way after giving up any hope of fixing the original attenuator. Essentially 100% of 492 attenuators found on eBay are blown (keywords are "untested", "as-is" and "from working unit").

There is an extensive set of instructions online for the mod. I don't have a link handy, but a minute of googling should lead you to it.

Good luck!

Cheers,
Tom



Sent from my iThing, so please forgive the terseness and typos.

On Nov 25, 2020, at 14:31, "Bob Koller" <fourdesigns@comcast.net> wrote:

Unfortunately, there are no parts, except from another Tek attenuator. Is it blown, or are the contacts "dirty"? These were often damaged by DC or excessive input power. If it is damaged, you need to check the first mixer for damage as well. You may be able to find a good attenuator on eBay, perhaps elsewhere, but these can be difficult to find in good condition.
It is possible to disassemble these and make one good unit out of two. The thin film resistive elements can also be inspected. Usually the damage is readily apparent under magnification.
It would be possible to fit a different attenuator, with the correct steps, and frequency range, altering the drive signals as required.
Be sure to take anti static precautions around the mixer.

Good Luck,





ulf_r_k
 

Once I had the same issue with the 492 attenuator. Taking it carefully apart, one of the wafers was burned. I was able to manufacture a wafer myself, using very thin teflon laminate and 0402 SMD resistors. The result was acceptable up to about 8 GHz.

Cheers

Ulf Kylenfall
SM6GXV


Bent Andersen
 

I have now disabel the attenuator and inside I have clean it whit Alcohol + Glasfiber brush so now it is working....but it looks as one of the pad is not getting contact whit the gold-spring ???
but I have uploade som close-up picture and write down the Resisten I found whit my DMM.