[ARC5] Receiver AC Power Supplies

[Mike Everette via ARC5]

This is slightly off topic, but what you mentioned about the National receivers reminds me of something I've observed with my NC-173.
Using a metered Sencore variac/isolation transformer, I have seen a "sweet spot" at about 105 to 107 volts AC input where the receiver sensitivity peaks noticeably.  Above that voltage, even at 110 volts, it falls off a bit.
I haven't monitored any internal supply voltages to see what may be happening; however, this is readily apparent just from listening to the audio and incoming signals..
73
MikeWA4DLF

      From: DSP3 <jeepp at comcast.net>
 To: arc5 at mailman.qth.net 
 Sent: Saturday, December 13, 2014 9:49 AM
 Subject: Re: [ARC5] Receiver AC Power Supplies
   
Corollary to this discussion, I have recently had the occasion to work 
on three BC-348 receivers I acquired this past summer.  All three 
(unfortunately, perhaps) were previously modified for a.c. operation.  
Going over these supplies, I found that virtually all of them suffered 
p.s. issues, most often the iron was not up to the task.  Heat, 
noticeably high heat, were present in all three units at the on-set.  I 
believe that these receivers were modified probably in the 50's, judging 
by the type and manufacture of the p.s. components used.  Aside from the 
filter cap issues, voltages were not what was expected.  Using a variac 
and adjusting the input to about 110v things got better.... a lot 
better.  Using silicon diodes, vice a tube rectifier, also helped 
un-load the small, over-taxed power transformers, a bit.  The 
aforementioned issues were not new ones.  I generally run the "Vintage 
Bus" here at a nominal 110v, via a Sola regulator unit.  Receivers such 
as the National NC-183D and others run ever so much cooler and voltages 
are generally at spec.  A lot of older components are apparently really 
pushed to the limit(s) by current "normal" line voltages.  Just after 
the above activity,  I went around a bit with my local provider (First 
Energy/Potomac Edison) about high line voltage, at about 125.8v.  Turns 
out, they had mis-applied some line comp capacitors (they actually 
informed me of same).  After removal, things are steady at a comfortable 
120v.

We think that the older stuff was made better than today, and in some 
cases it may be.  However, things like better insulating materials and 
advanced core design have made a big difference.  The thing is, though, 
most of the old stuff _is_ just fine, operated in the environment for 
which it was designed.

Jeep - K3HVG
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