[R-390] Help! R-390 Meltdown

[2002tii]

All references to "T205" in my previous message should read "T207":

>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