[R-390] Help! R-390 Meltdown

[2002tii]

Rick wrote:

>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.

Resistors almost never go up in smoke just because they are "tired 
and out of tolerance."  Smoke is a sure sign that something else has 
failed and is drawing way too much current.  One of the primary 
techniques in troubleshooting is "find out where the current is going."

In a 390, R220 is a 2.2 kohm B+ feed resistor to V205, the third 
mixer.  Something is causing excessive current flow through it.  The 
only ground paths from R220 are (1) the bypass cap to ground, C325; 
(2) a [mechanical] short to ground inside T207: or (3) through V205 itself.

My vote is for C325.  With the unit off and R220 removed, measure the 
resistance to ground at the ungrounded end of C325.  To smoke R220, 
you will probably measure less than 4 kohm (when the radio is fixed, 
this point will measure open -- at least in the megohms).  If the 
resistance is significantly above 4 k, then the fault current is 
going through V205, but this is unlikely.  Disconnect C325, and 
either it or pin 5 of T205 will be low resistance.  If it is C325, 
replace it.  If it is T205, open it up and see what is shorting.

Now -- if it was C325, all similar caps in that radio (there are lots 
of them) are suspect.  If it was V205, its cathode resistor -- R219 
-- is most likely also damaged.

Best regards,

Don