7A29 attenuator fault corrosion issue

Adrian
 

I don't know if this is a common issue (google didn't find it for me anyway) so just in case it helps others this is what I found.

Now that the 7912AD is behaving reasonably well I thought I would take a look at the 7A29 because some of the steps in the attenuator were off and on anything other that 'straight-thru' (10mV) range the rise-time was horrid.

My assumption was that some of the yoke adjusting screws needed tweaking to get the stage switches working right but once I got the thing out and started checking end-to-end resistance I realized that the resistances on anything that used the middle element were not sensible at all and visions of toasty black resistor elements began to swim in my head!

On stripping off the bottom - non actuator side - plate** I was reassured that the resistor substrates looked a normal color and careful probing showed the series resistances on the first and third elements were correct but the center one was giving weird (and slightly unstable) values. I then removed the screws from the other plate and lifted off the aluminum side blocks thus removing the ground connections to all the elements whereupon each individual resistor element on the substrate measured correctly.

It looks as if the ground connections from the resistive elements to the aluminum side blocks are made with some sort of (elastomer?) material and there was a nice white discoloration along the contact face - presumably aluminum oxide - once that was all cleaned up with cotton bud and IPA and reassembled all was good in the world and the attenuator seems spot on! I was careful to put all the actuator pins back in the holes they came from and no tweaking of yoke screws was needed, which is just as well as they seem to have been locked in place with a blob of nail-varnish in a tasteful shade of red but the adhesive properties of two-part epoxy resin!

**TIP - it is important to remove the two SMA connector end plates BEFORE lifting the bottom plate, the center pin of the SMA is slotted and the switched line keys into it, great care is needed to correctly engage this on re-assembly too!

Adrian

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