Re: Ageing components

Dave Casey
 

I can vouch for that. I've got at least one variable cap that's done that
in a 7A22.

Dave Casey

On Wed, Mar 15, 2017 at 7:40 PM, 'Dennis Tillman' @Dennis_Tillman_W7PF
[TekScopes] <TekScopes@...> wrote:



The bearings in the variable capacitors of the 7A22 plugin fail and the
rotors fall out. Often many fall out at once. My best guess is this is
caused by exposure to something in the environment. Other people have had
the same thing happen to their 7A22 variable capacitors.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Wednesday, March 15, 2017 3:24 PM
To: TekScopes@...
Subject: Re: [TekScopes] Ageing components

The plastic covers on the 7K series backplane connectors can become
brittle and crack/break, and it's not always obvious. Since they exert
force on the contacts, they can be a source of intermittent problems.
Ta caps (dipped) don't always explode- I've seen them grow a "third leg"
that would have looked normal to someone looking at the PCB casually; not
even darkened on the outside.
I have a plugin somewhere in which all the silver plated leads are black
with growth (looks like spiky black hair) This was obviously an issue
caused by the environment it was in, but you never know what you're going
to find.
-Dave

From: "g4sra g4sra@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Wednesday, March 15, 2017 1:30 PM
Subject: [TekScopes] Ageing components

Could anyone please advise on what can be generally expected from ageing
components in equipment from this era. I am after some general rules to
apply to help identify circuit faults due to partially failed components.

My current understanding is..

Electrolytic capacitors (aluminium can) loose capacitance and ESR rises,
whereas tantalum's tend to explode (obviously failed).

Carbon resistors tend to rise in value.

What commonly occurs to polymer capacitors, metal oxide resistors, bipolar
transistors, zener diodes, etc?

Thanks in advance...
Charlie



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