• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Safety ground question


2006-07-08 3:06 am
So, a very quick question (that, now that I'm done typing it, no longer qualifies as 'quick'):

I'm in the midst of restoring an old Zenith console radio/turntable, and have just finished recapping it. One thing that I know I need to do before I put any voltage on the power transformer is update it to more modern wiring standards - ie, the chassis (and power transformer's secondary CT) ought to be grounded.

Normally I accomplish this by just grabbing a fused IEC inlet, "making space" for it on the chassis with a hacksaw, and wiring it up in place of the original cord. The ground wire gets soldered to a convenient spot on the chassis to ground it all and I'm done.

However, it looks like in this case there's a 0.0047uF capacitor coupling one of the mains lines (this is from the days before polarized plugs in the US, I guess) directly to the chassis.

So, that's a lot of background to ask: Is it safe to just remove this dangerous-looking capacitor? I can draw a diagram if I've not been descriptive enough, but my MS Paint skills are sorely lacking.
I've done many radio restorations, and there's nothing wrong with a line to ground capacitor (or across the line) provided it's rated for that use. Typically I just replace this capacitor with a modern one and it's good to go. I must admit that I'm more of a purist with regards to radio restoration though :)
Here's some good info on these types of capacitors:
ABC's of safety caps
Dave has excellent prices and hard to find values of caps.


2006-08-27 3:32 pm
jneutron said:
Do not solder the ground wire to the chassis. Use a screw/lug combination.

Make sure to use an "energy storage device" when fastening down the screw/nut. "Energy storage device" is a fancy name for a lockwasher, either star or split.

Cheers, John

ps..agreed, toss the cap..

Not doubting you, just wondering why? More (mechanically) secure fixing or better conduction?


jrevillug said:

Not doubting you, just wondering why? More (mechanically) secure fixing or better conduction?



Questions are ALWAYS allowed..

National Electrical Code 2008...page 70-97

250.8 Connection of grounding and bonding equipment.
(A) Permitted methods. Grounding conductors and bonding jumpers shall be connected by one of the following means.

(1) Listed pressure connectors
(2) Terminal bars
(3) Pressure connectors listed as grounding and bonding equipment
(4) Exothermic welding process
(5) Machine screw type fasteners that engage not less than two threads or are secured with a nut
(6) Thread forming machine screws that engage not less that two threads in the enclosure
(7) Connections that are part of a listed assembly
(8) Other listed means.

(B) Methods not permitted. Connection devices or fittings that depend solely on solder shall not be used.

Cheers, John

ps.. I notice I really didn't answer the question...sorry bout that..

For mechanical strength. Solder tends to ductile creepage over time, it's a connection of dissimilar metals that over time may corrode, no matter how you try to clean, there will be trapped flux within the joint. As a standard practice, never rely on any solder for the mechanical strength of a joint. And never "tin" a stranded conductor before clamping it under a screwhead...that always loosens over time.
AndrewT said:
for mechanical security.
Due to the possibility of excessive heat inside a faulty appliance there must not be a soldered joint that could fail open circuit to defeat the Safety Earth protection.

I imagine the capacitors would already be toast by the time the chassis exceeded 183 degrees C...:eek: ...221 if it's lead free.

Cheers, John
NEXT question...! In attempting to determine the secondary voltages of a more-or-less unknown transformer (Identities of the filament windings, etc, are known; voltages aren't), I measured ~725VAC unloaded across the HV secondary.

Buuuut that's unloaded volts. What's the likely actual secondary voltage of this little guy?

(thanks for all the help up until now, by the way)


2006-07-08 3:06 am
Arright - tinkering anew this evening, I've decided that this is probably a 720VCT brick - probably overkill and a half for the little Aikido linestage that I'm in the process of putting together (from a kit - I don't have that much faith in my own ability yet ;))

This thread is straying further and further off topic - I should probably start a new one - but does anyone have a quick link or pointer on how to get a ~300VDC B+ voltage out of this guy? I've been playing around with PSUD II, but I'm not sure what kind of load the Aikido imposes, and dropping necessary volts across a resistor obviously makes the final B+ voltage sensitive to loading.

Is this one of those "put it together and play with the values" situations? I don't want to blow anything up.