Adding chassis earth to vintage receiver

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]

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

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

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]

Hi Mark,
Will reply over the weekend.

John aka Dr John

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

[John Caswell]

Hi Mark,
All you need to do is remove R149 and C103 from cct as shown, then you can connect the chassis directly to the earth pin of the IEC.
As everything else is earthed to the chassis I am afraid you are stuck with the earthing arrangements but it should work fine. It may be worth trying a filter across the line (mains) input terminals to see if that improves RF rejection.
Any other queries stick them here

John aka Dr John

Thanks Dr. John,
I was planning on removing R149 and C103 (which I assume are the RFI filter) and replacing them with Swampy's mains filter on an IEC plug. That way it will be Class I compliant and I'll feel safer. I'll let you know how it sounds when it's done. Thanks again.
Mark

[John Caswell]

Hi Mark,
I don't like the term "replace them"
Remove them completely and do not put anything in their place. Swampy's filter MUST ONLY fit in the power lead live and neutral with of course any earth connections going to the chassis. But bear in mind that there will be a slight ac leak via the earthy capacitor so unless the chassis is truly earthed you may get a tingle from it

John aka Dr John

Dr. John,
Poor choice of words on my part. I've actually already removed them completely, along with the non polarized AC wire, and fuse, and am currently doing the metalwork to install a properly earthed IEC receptacle with integral fuse. I'm also ripping out the two AC jacks on the back and replacing them with proper modern polarized AC jacks. Then I intend to install Swampy's filter directly onto the IEC receptacle's solder leads as the new RFI filter. I will earth the chassis well to the IEC earth lead.
Thanks again,
Mark