[ARC5] BC453 repair

[J Mcvey via ARC5]

I've never encountered "tin whiskers" in the arc-5 series before. It was a pretty strong whisker at that, able to sustain the plate current without burning out.
The cautionary tale here is to NEVER ASSUME that all variable gang caps in receivers are connected to grids! I've fixed many antique receivers, and I don't remember ever encountering a "hot" variable cap. Then again, maybe I never had to deal with that circuit area for repair... I learn something new every day...



On Saturday, August 23, 2014 2:02 AM, Tim <timsamm at gmail.com> wrote:
 


The dreaded Tin Whiskers?  Been there, done that...LOL




On Fri, Aug 22, 2014 at 7:07 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:

I finally got around to fixing the "new to me, 28V virgin" BC453 today . It started out pretty straightforward with a quick ESR cap check showed the usual suspects  needing replacement ,such as the .22X3 ,etc. I replaced the 10W diver resistor that someone has removed , lubed up the dynamotor and applied 24 volts with anxious anticipation of hearing some beacon stations. All I got was a brief cackling, then silence- NO JOY! 
>
>I pulled the 12K8 grid cap and was encouraged when I was able to get noise by touching the cap. When the cap wire was put back, it went silent again. I thought it was the RF stage but everything checked out.
>
>It turned out that the oscillator was dead, in fact I was surprised to find the osc plate at 2.6 ohms to ground! Why, might you ask? They plate tune the oscillator stage, so there is B+ on the gang variable! There must have been a tiny speck of metal shaving in there somewhere which I could not see, before or after blowing it out with air which cleared the short.
>
>Once that was done the receiver was howling away with a 50 microvolt signal lead laying on the bench about 3 ft away and no antenna.
>I've got some more cleaning and tweaking to do, but I'm a happy camper now, knowing that the radio is basically in very good shape.
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