[R-390] Reduce high line voltage - bucking or auto transformer

[Larry H]

As I said in my previous post of about 2 weeks ago, my occasional F103 blowing was due to high line voltage.   I've been using my variac to reduce the line voltage from 121 to 115 vac.  This has been working great, so I decided the solution I would use is a bucking transformer.  The reason is, I have other equipment that will benefit from using 115, also.  And, I need to use my variac quite often on other projects.  

I know how to hook it up, but I thought I'd look around to make sure I was right about the proper xformer I should use.  On my way, I found an article by Mr. Rod Elliott from 2010 about bucking transformers and autotransformers.  You can see it here: 

http://sound.whsites.net/articles/buck-xfmr.htm 

Section 3 is the relevant information.  He discusses using a bucking transformer as we think about them, but also discusses how to use the same transformer as an autotransformer.  The latter being more efficient (uses less current), but the resultant output voltage is a little different.  Using a 6.3 vac filament xformer in bucking connection, my 121 went down to 114, but using the same xformer in autotransformer mode, it went down to 115.  I'm going to opt for the autotransformer mode.  I'll let you know how it works out in the near future.

Regards, Larry