Adding chassis earth to vintage receiver - World-Designs-Forum

#1 17th July 2006, 03:44 AM

Hello all,
I'm reburbishing an early '60s Fisher 500C receiver, (7591 output in push pull pairs). As on most units of the era in the US, there was a reversible two prong plug hard wired into the unit. The input power wires are grounded through a 0.01uF cap paralleled by an 820K resistor.

http://oldtech.net/Fisher/500C/Schem.pdf

I'd love to install a modern IEC three prong with integral fuse for safety, and connect the chassis to the earth lead. I'm concerned that this might negatively affect the sound, though, as the signal earth is the chassis, and there are dozens of soldered connections all over the chassis, so there is no easy way to raise the signal earth off the chassis.

Has anyone done this with an older unit? Any thoughts on safety vs. sound?
Thanks,
Mark

david.rogers · #2 17th July 2006, 12:24 PM

Was that the unit on sale on ebay a while back?

#3 17th July 2006, 07:57 PM

I'm not sure which one you mean. I bought this in December '05 on ebay. They come up all the time here in the US.
Mark

#4 21st July 2006, 10:32 PM

Howdy,
No takers on this one huh?
I was hoping someone had tried it. Morgan Jones in both books repeatedly says how dangerous the old units were because there is no chassis ground, but doesn't suggest what to do.
Has anyone tried a modern three wire IEC on an older unit?
Any degradation of sound?
If I do this I will install a fused IEC outlet, ground the chassis, and replace the RFI filter cap and resistor with modern X2 caps per the old WAD noise filter.
Any opinions are welcome.
Thanks,
Mark

John Caswell · #5 22nd July 2006, 08:49 AM

Hi Mark,
Will reply over the weekend.

John aka Dr John

#6 26th July 2006, 02:50 AM

Thanks Dr. John,
Any thoughts would be appreciated.
It was rather quiet for the few seconds that I auditioned it before ripping it apart to replace all the mylar and e-lytic caps, and the plate loading resistors.
The circuit is earthed to the chassis at every tag board and the chassis has an earth screw for turntable earth wires, so I suspect it would be okay to connect the chassis to mains earth, but before I do some metal work, I was wondering if anyone had tried it, or there was some strong reason not to. Let me know.
Thanks in advance,
Mark