Radio Road

RE: Curious antenna connection on Lafayette LT-D10

69.142.177.2

Eli Duttman

In Reply to: RE: Curious antenna connection on Lafayette LT-D10 posted by 1973shovel on May 07, 2019 at 09:09:22

I read the schematic exactly as you do. That thing is DANGEROUS.

Replace the 1000 pF. cap. with a part rated for both across AC mains duty and several KV working voltage. A polarized 2 conductor or 3 wire safety grounded power cable is indicated too.

Eli D.

Follow Ups:

Dangerous even without the capacitor - Triode_Kingdom 17:26:06 05/12/19 (0)
These old units can leak a considerable amount of current (in terms of human safety) from the power transformer to the chassis. That was mostly OK back in the day, but mixing this with newer gear that's grounded (3-wire power cord) invites catastrophe. It's all too easy to find yourself in-between the hot chassis of an older piece and the grounded chassis of modern gear. The ONLY safe solution is to add a 3-wire power cord to older gear, so that all the various chassis within the system are at the same potential. I would also recommend replacing the antenna coupling capacitor with a gimmick capacitor (many older tuners used this approach) or moving it to the secondary of the power transformer.

--------------------------
Buy Chinese. Bury freedom.
"That thing is DANGEROUS" - 1973shovel 07:50:00 05/08/19 (3)
After I read your reply, my first thought was to simply eliminate the internal AC line to antenna connection. But then I started looking at X1/Y2 safety capacitors at Mouser, and the data sheet specifically lists "antenna coupling" as an application (link to data sheet below) and for less than fifty cents, replacing it would be easy and cheap. However, they don't meet your several K voltage rating recommendation though.

Thanks for your take on this tuner, Eli. Your expertise is always appreciated, especially where safety is concerned.

"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
- X1/Y2 (Open in New Window)
- Not to worry...mostly.... - Steve O 09:46:15 05/08/19 (2)
  ...X/Y caps are typically rated for a nominal AC line voltage, say 250VAC but are designed to withstand KV magnitude spikes and other crap found on real-world power lines. They also have built-in overload protection and a defined and safe failure mode.
  - RE: Not to worry...mostly.... - Awe-d-o-file 14:30:29 05/09/19 (0)
    Al Sekelas parallel AC filter used them for safety.
    
    ET
    
    "If at first you don't succeed, keep on sucking till you do suck seed" - Curly Howard 1936
  - Thanks Steve - 1973shovel 10:49:00 05/09/19 (0)
    My inclination is to just remove that portion of the circuit so that it's no longer a concern.
    
    Some of it depends on how the sound the Lafayette LT-D10 compares with some of my other tuners. The Tuner Information Center gave it fairly high marks for its sonics.
    
    If I'm not impressed, and decide to use it in my garage, the AC power cord as an antenna might be a convenient feature for casual reception, rather than having to fool with an external T. If that's the case, I'll replace the cap with a proper safety cap. Either way, I will replace the unit's plug with a polarized plug, per Eli's suggestion.
    
    "Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon

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