[Collins] Re: KWM-380 Fix additional comment

[David Knepper]

I forgot to mention that prior to the 380 going into that dreaded lockup
mode (beep-beep-beep), I knew something was going on when the radio would
skip a beat, that is, when something would pull the listening frequency just
for an instant.  I knew that it was a matter of time and held my breath - I
didn't have to wait long!

Recommendation:  Before breaking out in a cold sweat that an IC has gone bad
on those two synthesizer boards (A5A1 and A5A2) do what I should have done a
year ago:   check the three test points on the synthesizer board (A5A1) that
measures the voltages coming out of the regulator circuit.

The culpit that I told you in an earlier post was that C61 was shorted to
ground.

I also replaced another tantalum cap at the main 28 volt regulator board,
located outside the enclosure across the feedthrough capacitors.

Heinz is correct about replacing these tantalums to a higher voltage.  You
can tell them by their orange color and bulb-like shape.

Had I had a higher voltage value (50 volt) I would have gone that route.
Mouser Electronics carry these caps and the difference in price betwen a 35
volt tantalum and a 50 volt tantalum is only 7 cents - which is cheap
insurance.  I trust that the larger voltage type would fit nicely on the
circuit board.

The two replacement caps were the older style round aluminum case type
electrolytics not tantalums.    The 380 seems to be working fine for the
past several days.


Dave, W3ST
Secretary to the Collins Radio Association
Publisher of the Collins Journal
www.collinsra.com
----- Original Message -----
From: "Heinz und Hannelore Breuer" <hbreuer@debitel.net>
To: "David Knepper" <cra@floodcity.net>; <collins@mailman.qth.net>
Sent: Sunday, December 28, 2003 5:25 AM
Subject: KWM-380 Fix additional comment


> Hi Dave,
>
> I vaguely remembered an issue about failure of tantalum capacitors
> connected to power rails from a former life. A quick Google search found
> the following reference on the failure mechanism and on derating the
> operating voltage of tantalum capacitors:
> http://www.ncc-matsuo.co.jp/pdf_e/app_tan.pdf
>
> Actually the tantalums in our KWM-380 are working under high failure
> conditions, a 35V tantalum connected to the 24V rail. In this particular
> low impedance circuit operating voltage is recommended not to exceed 1/3
> of the rated voltage.
>
> Better get a few higher voltage tantalums for future use. At least now
> you know where to look when the next failure occurs.
>
> 73
> Heinz DH2FA, KM5VT