[R-390] Help! R-390 Meltdown

[Paul Anderson]

Hi Rick,

A resistor that fries rarely(*) fries because it failed.

Usually, it is connected to something else that developed a short,
such as a capacitor or less likely a coil in one of the transformers.
This is because the most common cap failure is a short, and the most
common coil failure is an open.  There are always exceptions to the
rule, of course.

To diagnose, you'll want to look at the circuit diagrams, and look for
obvious problems close by - another one would be a shorted tube.
Because of the background of your radio, I'd do a physical inspection
- looking for loose debris  - sometimes just a big burst of compressed
air to make sure there isn't anything stuck in there that shouldn't be
can help, too.  A meter will help you track it down - if it fried
twice, it should be pretty easy to spot the other component.

Good luck!  The good news is it is a pretty easy problem to track
down, compared to some... :)

Paul

(*) I had a resistor in a TV-7 that baked itself because it had
physically cracked open, causing arcing, then burning.  Replacing it
fixed that problem.


On Mon, Mar 2, 2009 at 12:17 PM, Rick Popovich <RickP at fndmail.csus.edu> wrote:
> Good morning all,
>
> I'm having a problem that I hope someone out there has also encountered and
> can share their knowledge and experience and hopefully the fix.
>
> I have been working on an R-390 that I acquired as a "parts rig". It was
> only missing a power supply so I substituted a P/S that I had in my pile and
> fired it up just the see what would happen. To my surprise it actually
> worked.
>
> After using it off and on as a test mule it quit working and luckily I was
> right around the corner and was able to shut it off as smoke started to come
> out of the RF deck. While I had never removed an RF deck before I dusted off
> the manual and pulled it out of the receiver.
> Since I saw where the smoke was coming from it was easy to spot the failed
> component - it was R 220 which I've traced to part of the 3rd IF circuit.
>
> Assuming it was probably just a resistor that was tired & out of tolerance I
> replaced it and put her back together. No sooner did I get it back together
> and fire her up when up in smoke she went - same problem.
>
> I have stated to check voltages etc. and have yet to find what is causing
> this. HELP!
>
> Any experience/insight or solution would be appreciated.
>
> Tnx, Rick
>
> rickp at fndmail.csus.edu
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