[R-390] Troubleshooting Question

[Jacques Fortin]

Rodger, you will surely found my advice strange, but...
Just replace the mains fuse with a new one and retry.

73, Jacques, VE2JFE

Hey All,
I have an R-390 (non A) that has developed the following problem.  When I
turn it on, it warms up and works normally for about a minute or so.  Then
without warning, it shuts down.  Lights out, no B+, etc.  It does the exact
same thing every time I turn it on.  Any thoughts on where to begin
troubleshooting?  How to easily isolate the B+ from the modules, etc. ?  For
the minute that it's on after warming up, the B+ reads 185 volts.  
Thanks in advance for any advice.
Rodger