[R-390] Coming: Real In-rush Protection / Step Start

[chuck.rippel at cox.net]

Was having an off-list conversation with one of the folks whose radio 
will be here fairly soon for a restoration about the question of current 
in-rush.

Is it a "real" issue ?

If so, how to mitigate the problem safely

In the last few years, I've seen a trend of open and shorted 
transformers.  At first, that was chalked up to a shorted 26Z5W with the 
answer being to install a new supply, pull the tubes and solid-state the 
supply.  The transformer failures have escallated
to the point where they are nearly on par with Mechanical Filter 
failures.  Last October, I attended a DX'pedition with servera long time 
friends.  One of them had an R390A that I completed within the last 2 
years that was intermittently blowing the 3A fuse.  The problem was 
elusive but at the end of the day, we swapped out the transformer, added 
a 3A slo-blow fuse and it look like the issue has been conquered.   I 
forget exactly what I was doing at the time but he has pictures of me 
laying on the floor with his radio working on the failure.

What to do?

I've seen a number of radios come in with a thermistor between the size 
of a nickel or quarter installed in series with the AC input, after the 
line filter.  These always end up getting pulled out and returned to the 
owner with the rest of the replaced parts.  They run very hot and, IHMO 
are a fire hazard.  Not an acceptable fix.

The proper way to address the matter of inrush is with a proper limiter, 
or in this case, a step-start.  That discussion went on right here not 
long ago.  The owner of the radio I'm currently working on has given his 
permission for an inrush limiter to be installed.  The "rules" for a 
modification are that it be 100% reversible, which will be the case 
here.  We're only talking a couple of parts, a relay and power resistor. 
As soon as I find the correct combination, I'll share the design, 
installation pictures and parts sourcing right here.  You may install it 
in your own radios if you wish.

Have a great week !  I'm having fun with this current radio; its really 
going to turn out nicely !


Chuck Rippel