[R-390] [Fwd: Reconditioned Amelco R-390A Part Failure]

[Bob Camp]

Hi

Magnetics do indeed fail just as you describe. It's not uncommon in  
older gear. My guess is that eventually we will start to see power  
transformers fail in the same mode.

Bob

On Mar 22, 2008, at 2:44 PM, Tom Frobase wrote:
> Last fall I had the rare opportunity to pick up 2 R-390A's at the  
> Belton, Texas hamfest.  I rebuilt the nicest of the units first, a  
> circa 1962 Almalco,  with the goal of putting it in my shack.  I did  
> the normal, completely  dissembling and cleaning the radio.  I  
> completed the radio figuring it needed to run in the shack for a bit  
> to discover and anomalies not found in its reconstruction.
>
> After being in the shack for a couple of weeks the radio failed  
> outright.  I pulled it apart this morning to find the 2200 resistor  
> R508 to be smoked.  This is the dropping resistor that feeds the 12  
> MH choke L505 to the plate of the 1st IF driving the mechanical  
> filters.  I always change C558 which connects to the filters, you  
> all have warned me many times!
>
> My first thought was to check the .05 ceramic to ground, wasn't it,  
> then I pulled the tube in case it was shorted, it was not.  I  
> thought of the irony of having a defective cap which I replaced to  
> protect the filters but indeed it was good as well.
>
> The problem ended up being the 12MH inductor L505, shorted dead to  
> ground.  Go figure in thirty years of repairing these things I have  
> never seen that before, has anyone else?
>
> tom, N3LLL
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>