[R-390] R390 Posts: Excess line input voltage Noise Figures

[Barry Hauser]

Hi John & List

Excellent documentation .. just a bit of clarification on my input .. ("B:")


EXCESS LINE VOLTAGE TO R390A
Barry replied by suggesting I use a variac in front and twiddle the knob to 
maintain line constancy.
(tongue in cheek, maybe?).

B:  Yes 'n no.  (no - not a flip-flopper ;-)  Yes, when I mentioned that 
using a variac with a full time voltmeter would provide yet another knob 
(big one) to twiddle and meter to keep (a third) eye on. ;-)  Seriously 
though, I suggested a variac for sustained high (or low) power line voltage. 
For example, during the cool months, we get 127 volts here fairly 
consistently.  However, during summer, it can go as low as 97 volts during a 
brownout -- announced or otherwise.  Many mounted variacs have a range 
switch, usually 0-120 and 0-140 or similar.  Some don't, but I've found 
almost all of the actual autotranformers inside the box have the addtional 
taps (usually choice of two) that would allow you to add a low range.  As a 
safety, to protect agains a slip of the wrist, keep it on the lower setting. 
Also the "120" nominal or "115" can be higher than that as they were labeled 
assuming 115 or 120 going in.

I replied that I had solved this problem by (first) using a servo feedback 
motorised variac with a front panel screwdriver adj.
When this broke down on me I used a constant voltage, ferroresonant 
transformer, (CVT) with sine wave output option
(an Advance Model CVN230A) This gave a very well regulated, transient free 
230v AC constant output
to run my R390A and allowed me to remove the line bucking transformer 
previously installed..

Barrys  reply was that he uses a similar CVT, but that it 'ran hot'  and was 
noisy, which I take to be lamination buzz.

REPLY:  Barry, if your CVT is not actually faulty, thats usually caused by 
not running the CVT at, or close to, its rated output current.
These things are designed to run under a load close to the rated maximum 
output and literally shake themselves to bits when they arn't. !  Not
only do the darn things then run hot, they add a whacking big amount on your 
electricity bill !!!!

B:  Hmmm??!!  Well, might not be so, or might be worse than I'd figured. 
See next comment.

Mine has a max rated output of 230W with a power factor of 1.0. The R390A is 
rated at 225W which matches it pretty well.  My CVT then
runs quiet and cool . You wouldnt know it was there. One of the big 
advantages of running a CVT in front of the R390 is its ability to suck out 
damaging line transients.

B:  The unit I was referring to is a Solar 20 amp wall-mounted unit that I 
bought used and had installed by an electrician.  It's hard wired off the CB 
panel in line with one 20 amp circuit, downstream of a 20 amp breaker.  This 
is in the office and feeds three or four PC's with CRT monitors and a big 
old HP IIIsi laser printer.  The PC's probably draw 2-3 amps each.  The 
laser printer varies depending upon whether it's printing or idling, or the 
fuser and rollers are cycling at idle.  (to keep dry and prevent 
flat-spotting.)  I think it draws about 10 amps when running, so in 
combination, I'm probably running close to the capacity of the CVT.  It was 
installed about 8 years ago -- no problems, but it has always run hot and 
buzzes loudly.  Given the amount of heat and acoustic noise it generates, 
you have me thinking about the possible cost.  Our utility bills run high 
around here.  As I mentioned to John, we're still paying for the nuke power 
plant that wasn't (Shoreham).  It was just brought up enough to contaminate 
itself during testing before it was cancelled to Three Mile Island, and no 
real way to evacuate Long Island..   The Sola CVT I have appears to be the 
same one that Fair Radio lists for about $250.  Looks small in the photo --  
they're big and heavy and boatanchors in their own right.

<snipped>

Barry also replied that servo controlled variacs often fail because of brush 
wear tracking carbon dust over a small segment of the winding due
to motor hunt when stabilizing.     A valid and sensible point , Barry! I 
will check mine out next time I find it. (Its buried neath a near ton
of gear forced into a garden shed.

B:  Before applying power to a used or long-stored variac -- or one that may 
have been played with, new or used, check the wiper track and clean it off 
with a brush.  The wiper fallout tends to pack in between the exposed 
windings.  From what I learned (on this list), they can tolerate shorting 
between two, maybe three windings, but more that than can result in burnout. 
A motorized-servo-controlled unit might be especially prone as the unit 
"hunts" over the same small arc repeatedly.  Even with a regular variac, 
you'll find the buildup is much greater in one small area - where most of 
the adjustments are done.  The plating on the copper may also be worn off 
which makes the carbon stick even more.  The brush straddles two windings a 
at time to prevent intermittents as you are adjusting the thing.)  Once 
there is smoke, it's probably too late for simply cleaning the track. 
Typically, a burnt out and permanently shorted unit will show a band of 
parallel windings with blackened insulation.  Don't confuse this with the 
black potting material that goes all around 360 degrees where the unit was 
dipped.

All this depends on your line voltage and persuasion as to what degree of 
voltage control is needed for an R-390A.  They were designed when mains 
voltage ran about 10 volts lower (in US), like 110 or 115.  I'm looking at 
126-7 most of the time.  John is getting 270 vs 240 full time. He suspects a 
conspiracy with bulb manufacturers based on his illuminatatory experiences. 
I suspect a dastardly plot to make the little wheel in the watthour meter 
spin faster for the same amount of juice.  Of course, they'll tell me this 
theory is at odds with Ohm's Law -- but when was the last time anyone 
dissected one of those things?  Also, complicating matters, at the office I 
have what's called a "demand" meter. It's very demanding.  The deal is you 
pay for the peak wattage draw per 15 minute time slice, or something like 
that.  It's digital, therefore even more suspicious than just the spinning 
wheel dingus.

Barry