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 Post subject: A question about safety caps
PostPosted: Jan Fri 27, 2017 1:09 am 
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Location: Milwaukee,WI
I am restoring an old tube type CB radio. Its a 5 piece Demco Satelite system. 3 of the units receiver, transmitter and modulator are all AC powered and connected together between audio and interface cables. All 3 chassis grounds are connected to each other through these cables. All 3 are using unpolarized 2 prong cords. Only the transmitter has safety caps from each end of the line to chassis ground. The other 2 units do not have any and none are shown in their schematics. From a safety standpoint or introducing any loops would it be best to install the same pair of Y2 caps in all 3 units or am I overthinking things? Any worries about having those AC line cords plugged in with opposite polarity? I think I have some extra polarized plugs laying around and will put them on if needed.

By the way, I didn't mention anything about using 3 wire power cords because I don't have any good ones around here except for those thick bulky black ones used for most computers which would not fit through the grommets on the chassis.

I don't know why my post was moved to this area. It concerns the repair/restoration of a radio. Not a conversation about old tube cb radios. I hope the right people will still find this post and help me out.


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 1:57 am 
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I usually take the non-diferentiated 2 prong plug and using a pair of dikes, split one of the prongs that will then always be the neutral. I split it back about 1/4" or so and fill in the split with solder. Check to see that the split spreads the end enough that it will only fit the neutral opening. I then file a bit to clean it up and I have a 2 prong plug that can only be plugged in one way. I like this especially for original plugs that I want to keep on the radio for looks and utility. Some have fuses in the plug etc. that can't be replicated with a new plug (at least that I am aware of).

Norm

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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 4:55 am 
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Those caps are there to try to bypass the majority of "crud" that tries to come into the set via the line and also to prevent stuff generated in the set from making it back to the AC line. The safety part of the cap simply means they are rated for line service AND if they fail are designed to do so in a non-spectacular fashion. Caps connected to the AC line see significant voltage spikes and those not designed for this service won't do well here. Several years ago I had to replace the power transformer in a RME receiver because the previous owner didn't bother to replace these bypass caps and when one failed it caught on fire and also baked the transformer. Modern line rated caps won't fail in this manner. Adding line bypass caps to the other pieces of gear won't make them safer but if the chassis aren't connected to a good ground once you put multiple pieces that are tied to each other then you are also putting the line bypass caps in parallel and with enough there will be significant potential on an ungrounded chassis. This is likely why the other units don't have these caps in place and there are some vintage receivers with large value line bypass caps that will definitely create a "tingle" if the chassis isn't grounded.

These should all be transformer operated units so plug polarity shouldn't create a major safety issue but in some cases could lead to increased hum. Good safety practice is to have the plug set up where the fuse and power switch are in the hot/load lead because neutral is never to be switched or fused. With an ungrounded chassis there are several failure modes that would allow a switched or fused neutral to create a significant shock hazard.

With few exceptions gear going across my bench gets a 3 wire cord wired according to code with the switch in the hot lead. The hot/load side line bypass cap is on the protected side of the equipment fuse so that if it fails it will quickly blow the relatively low current fuse rather than pulling sufficient current to trip a branch breaker. If you decide to go 3 wire you should be able to find smaller gauge 3 wire cords from Mouser or one of the other major parts houses.

Rodger WQ9E


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 5:42 pm 
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Location: Milwaukee,WI
Thanks. I was worried more about a possible hum problem than the safety part but figured I would ask about both. The one unit that had the caps called for a pair of .005uf from both lines to ground but a previous owner installed some whopping .1uf caps instead. Since I had just bought a bunch, I installed a pair of .0047 caps in each unit while I waited for a response here. I have a small 3way power splitter that I will use for these units. I'll identify and mark the hot and neutral end of each plug and make sure they are plugged in identically. If I notice much of a "tingle" I'll remove the caps from the other 2 units.


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 6:04 pm 
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Quote:
If I notice much of a "tingle" I'll remove the caps from the other 2 units.


There's a more definitive and safer way to test this: measure AC voltage with a meter between the chassis or case and earth ground on the power receptacle. That would be the current path for you to feel the "tingle." There shouldn't be any significant voltage; if you are using unpolarized plug, do the test with the set plugged in both ways.

Best to test your sets one at a time (not when all are plugged in).

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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 6:20 pm 
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Location: Champaign IL 61822
If you install two caps, one from each side of the line to chassis, and
measure (two wire cord) chassis to line ground by a multimeter or VTVM,
you will get 55 volts. At 0.01 uF each cap it will tingle.

For SW use its almost never useful to add caps of significant size
(say > 200pF) in this manner. At most add a .0047 or .01 uF across the line
and, if you want to use the line cord as half an antenna, add 470pF from the
cold side of the line to chassis. Otherwise, adding capacity to chassis
has a 25% chance of being helpful or unhelpful and 50% chance of being
neither. Of course, this presumes a power transformer. For and AD-DC setup
with no direct connection to chassis, you might need .01 uF to ground
to make IF work properly. This latter will bite bad ... for this case and a metal case
boatanchor (or any set), you really really should use a three wire cord, truly
grounded chassis and authenticity be damned.


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 6:38 pm 
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Location: Milwaukee,WI
I forgot to mention in the first post that all 3 units have power transformers.

Seems odd that a radio maker would purposely introduce 55 volts between chassis and ground by using a cap from each line to chassis. But I guess they had a different way of thinking back then. Safety concerns were not what they are today.

Your post gives me more to think about. I may try to find 3 wire power cords that are thin enough to fit through existing grommets or just go to a single cap across the line. If others chime in I might get a consensus. :wink:


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 7:25 pm 
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I omit line to ground capacitors on radios with power transformers and install three wire cords. Grand Brass now has nice fabric-covered cords with molded-on plugs. Many radios perform better with that ground connection.

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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 8:13 pm 
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If you look up the equations (or online calculator), capacitive reactance varies with frequency. You can calculate the reactance at 60 hz for any capacitor. You then use the reactance to calculate current flow at 117V. It's that current leaking through that could hurt you.

Lesson learned: Don't assume anything when safety is involved. AC voltage you detect on the chassis may not always be leakage from those line bypass capacitors. Many things could be wrong with vintage sets. Detach the capacitors and measure again to ensure you don't have a fault elsewhere. It only takes a few minutes.

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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 8:57 pm 
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Location: Santa Rosa, CA
If the original units did not have caps connected to the power line, don't add any. You are not increasing the "safety," merely increasing the chances of a shock or tingle.

Rich


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 9:02 pm 
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I agree with AJJ's point that it is a good idea to check sources of leakage to the chassis in vintage gear. Power transformers can develop leakage to the case and I have replaced more than one power switch that had leakage to its mounting hardware. Of course if the chassis and case are properly grounded this fault will result in a open fuse instead of a fused owner.

The vast majority of vintage gear should have the chassis and case grounded. AC/DC sets that require the chassis to "float" are somewhat of an exception but realistically that issue should be addressed by the vintage gear collector/user. Very few of these sets were ever run on DC and the primary reason for their AC/DC design was nothing but cost reduction since a power transformer was a major part of the total component cost. This cost motive doesn't apply to collectors so adding either an internal or external isolation transformer or a polarized power plug with a properly wired socket is something that ought to be done. Or at least convert it to a "safe" AC/DC circuit like some of the later Hallicrafters S-38 series which isolates the power feed from the case/chassis allowing a normal grounding setup. This is particularly true because of the way that we use most of these radios today since the set is probably operated in a group of other vintage sets most of which DO have grounded cases and chassis setting up a very dangerous hand to hand shock path if the owner touches the ungrounded and grounded sets at the same time. This scenario was far less likely when the set was initially sold with the intent for it to sit by itself on a table or mantle with a low likelihood of well grounded objects nearby. There was a major manufacturer of small appliance motors whose production process created a possible defect resulting in the appliance having full line potential from the case to ground but their study at the time showed the likelihood of fatal exposure was fairly low (i.e. touching the appliance and a grounded water faucet at the same time) and the likely victim was perceived to have low economic value (1950s housewife) so their internal evaluation showed the better economic choice was to avoid a costly redesign and pay for fried housewives at the then fair market value.


Rodger WQ9E


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 9:10 pm 
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rsingl wrote:
There was a major manufacturer of small appliance motors whose production process created a possible defect resulting in the appliance having full line potential from the case to ground but their study at the time showed the likelihood of fatal exposure was fairly low (i.e. touching the appliance and a grounded water faucet at the same time) and the likely victim was perceived to have low economic value (1950s housewife) so their internal evaluation showed the better economic choice was to avoid a costly redesign and pay for fried housewives at the then fair market value.
Rodger WQ9E
Which manufacturer and which products? Sources?

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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 9:29 pm 
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Stephen,

Sorry but under the terms I was given access to this material by the current corporate owner while developing some risk management studies no manufacturer identification is allowed. Although this type of wording appears crass given our current views this basic safety cost/benefit analysis is still part of product design and always will be. Although the bias is typically more towards safety now there is still a long way to go. Recent examples are the Takata airbag recall and a couple of auto manufacturer brake recalls that started out limited to certain regions where the environment increased the likelihood of the fatal flaw appearing but the recall was later enlarged to cover all regions.

It would be wrong to try to judge the 1950s manufacturer by today's standards. Like many members of this forum I grew up sleeping in the back of a moving station wagon on family vacations and the only belts we wore held up our jeans. If I had allowed my daughter to ride this way the state would likely have custody of her. I did allow her to drive my new Corvette Z06 at the age of 12 but that was only from its detached garage to the main driveway :)

Never fear, our best practices of the first decades of this century will be viewed as barbaric in a few short decades.

Rodger WQ9E


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 9:35 pm 
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Rich: My reason for the caps was not a safety concern but for noise or rf elimination.

It gets more interesting. I took the earlier advice about checking for alternative sources of leakage on the receiver. No line caps installed. From electrical outlet ground to the chassis I measured 97 volts and 2 volts depending on which way I plugged the radio in. With the radio on those voltages were 77 and 20. I unsoldered the power switch and nothing changed so no leakage there. That would tell me the power transformer has leakage. So I measured resistance with my DVM from chassis to power cord prongs. I expected some very high but measurable reading. I had infinite resistance from both. Its a head scratcher unless the upper range of my DVM can't measure the super high resistance leak.


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 10:33 pm 
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capacitive coupling (in the power transformer) with no load will result in high measured voltage between chassis and ground. If you do a google search for leakage measurements you can quantify this and I expect you will find that the leakage current is extremely low.

Rodger WQ9E


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 10:44 pm 
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Location: Milwaukee,WI
The no load thing you mentioned might explain why this leakage measured less with the radio on. I know its not very scientific but for grins I took my LCR meter and measured 580pf between power plug and chassis ground. I'm sure chassis wiring layout had some effect on the reading but I was just curious to see what it was.


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 Post subject: Re: A question about safety caps
PostPosted: Jan Fri 27, 2017 11:07 pm 
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580 pf @ the 60 hz line frequency represents a bit over 4 megs of capacitive reactance. A modern DMM will have a 10 meg or higher input resistance so it will read a significant voltage because of the very low current flow through the measurement instrument but if you made the same measurement using a classic VOM the measured voltage would be quite low.

Depending upon your plug orientation the load/hot side of the line is either connected to the open power switch or the transformer winding and when the plug is oriented so that the load side is connected to the transformer winding with the power off the indicated voltage to ground will be higher due to the capacitance to ground of the transformer.

Rodger WQ9E


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 Post subject: Re: A question about safety caps
PostPosted: Jan Sat 28, 2017 12:03 am 
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You are correct. Using my VOM instead of DVM I get no significant voltage between chassis and ground. Since I don't feel like going through the hassle of buying three new 3 wire cords and chassis grommets, instead of the pair of caps per unit I'm going to put a single cap across the line of each unit and plug them into my outlet strip observing polarity of the plugs and call it a day.

Thankyou everyone for your input.


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 Post subject: Re: A question about safety caps
PostPosted: Jan Sat 28, 2017 12:13 am 
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Kudos for sticking with the OEM configuration :D

Chas

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Last edited by Chas on Jan Sat 28, 2017 2:35 am, edited 1 time in total.

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 Post subject: Re: A question about safety caps
PostPosted: Jan Sat 28, 2017 12:53 am 
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Laziness is the bad sister to the mother of invention.


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