[jrblasde]

Good morning, folks! This time I am working on an AC-only model, so it's a bit different than my AC-DC model 49-906. I'm looking at the schematic from the Philco library, and I notice that there is a connection to the chassis ground on the primary side of the power transformer.

   

    The line and neutral connections from the AC cord are connected, via C100A and C100B, to chassis ground. My question is, why would that be? In, addition to stepping voltage up or down, isn't part of the purpose of a transformer to remove any DC reference which exists on the primary side? Granted, I know that the intention of C110A and C100B is to remove any DC offset, but I also realize that real-world capacitors do have some minute leakage.

    So far as the power supply section is concerned, I also plan to replace the ripple filtering capacitors (C104A and C104B) with fresh Y safety capacitors. Additionally, I plan to install a 1A fuse socket on the primary side of the transformer. This is a 70W radio, meaning that normal conditions will draw 0.583A at 120V. Thus, I feel that a 1A fuse will be a great safety addition. I also typically replace all capacitors, resistors, and wiring in my restorations. I just feel like it's a good idea to freshen up the insulation on wires and it replaces any brittle or out-of-tolerance components.

[RodB]

Revisiting the safety cap, C100A&B should be replaced with Y safety capacitors. This is a line filter intended to reduce noise on the power line. C104A&B are electrolytic capacitors and should be replaced with similar value aluminum electrolytic capacitors. Be aware that the negative side of the electrolytics may not be grounded to the chassis.

[MrFixr55]

Hi Joseph,

The C100 A and C100B are for RF suppression, to keep motor and switch noises from the radio. Yes, they should not pass DC, but I can attest from strong shocks that these caps leak and break down. These should be replaced with safety caps. Check other experts, but I would install a polarized cord with the neutral on the unswitched side of the transformer and the hot to the switch. Use an X Safety cap between neutral and chassis ground and a Y safety cap across the transformer on the switched side instead of between hot and ground. This is a change from the original circuit, but should be safer and just as effective. Alternatively, if you want to use the same circuit, use a 3 wire cord, but still move the "hot side" cap to the "load" side of the switch.

The 1A fuse is a very good idea.

The filter caps do not need to be safety caps; I don't even think that they make a Y type safety electrolytic but I may be wrong.

[jrblasde]

Ah, I see now. I hadn’t considered filtering on the primary side of the transformer. Very well, these will be replaced. Thanks for the insight!

[jrblasde]

Mr. Fixr, are you able to share a sketch to help explain how you typically rewire the power supply? I believe I follow most of what you have written, but I don’t grasp the complete idea.

[morzh]

As other have said,

There is only one safety capacitor rating, and it is the Y-type.

The X-type is typically is placed across the line. It is not guaranteed to fail either open or short. The formal definition of usage of that cap is "where failure of the capacitor will not lead to the danger of electrical shock".
When X-type fails, it does not expose anyone to the electrical shock. If it fails open, the filtering function of it will not work, and if it fails short, it will simply blow the fuse.

The Y-type is a true safetyy device, as it is guaranteed to fail open, and therefore the failure of it will not expose anyone to the danger of electrical shock. This is why it is placed between the line and the chassis ground (touchable by humans).
The formal definition of usage of Y-caps is:"where failure of the capacitor may involve the danger of electrical shock".
Selecting the value of the Y-cap is a compromise; the value is selected such that, while not compromising the filtering function, it does not expose a human to the so called "touch" current, where, if the GND connection were to fail, and a human were to complete the circuit to GND, the current, defined by the AC voltage amplitude and the value of the capacitor, would be below the dangerous level. Which means, that the value willl depend on the AC voltage and the frequency.

One might ask, why X-caps are needed at all, if they are not safety caps? The answer is, the X and Y-rated caps are tested against performance in AC lines, which exposes the capacitor to the AC noise, which includes many various transients with high energy content. X-cap will survive and possibly even continue to work after it experienced some damage, where a regular cap will burn.
On my working table at my company, I have two X-caps with blown off piece of shell and visible damage to the structure, which are still working fine when tested, and exhibit the nominal value when measured. These were a part of TDK plug-in 12V power supply, which went during some very rigorous immunity test; when I opened it (it is not repairable, but can be opened), the fuse was gone, but the rest of the circuitry was OK.

And, the last:

in the old equipment the X/Y caps fulfilled different function than the one they are used for today: back then they protected the device (the radio) FROM the noise, carried by AC lines.
Today they protect the AC lines against the noise generated by the device.
This happened with the advent of switching power supplies.

[jrblasde]

Mr. Fixr, you are right. I looked back at my notes, and it turns out that I did not use type Y caps on my model 49-906's filter caps. For some odd reason I was thinking I had done so.

    Morzh, thank you for the detailed insight into the different types of capacitors. I appreciate that! It's funny--I am a transmission station engineer, and deal with carrier signals and interference on lines all the time. However, the means are a bit different here when we're only working with 120V instead of hundreds of thousands of volts. The means of fault mitigation are different, too! I am realizing I have much to learn.

    I will use type Y safety caps for the two capacitors on the primary side of the transformer (C100A and C100B), and regular electrolytic capacitors for the filter caps. Thank you all!

[morzh]

For the filter caps, for the first rectifier cap my personal recommendation is High ripple electrolytics (Rubycon/Panasonic/Nichichon) or film capacitors. The value should be as close as possible to the original.
This below is an excellent cap for you specific case. It has 560mA ripple current. On top of which it is 10,000 hrs, 105C-rated cap.

For the second cap you could use the same cap, or any general purpose electrolytic cap of good quality (same manufacturers). Also for this cap you could exceed the original value, so 33uF/47uF/56uF are still OK.

[MrFixr55]

Morzh, the Phorum has (maybe mistakenly) graded me as "expert" , but I learn so much from you and other experts.  I am now hearing about "high ripple current" filter caps, as well as the "Solen" film caps that you and others talk about.  I guess that "caps is caps" doesn't apply anymore.
When I got back into this hobby, I thought that more is better in terms of filter caps (drop the PS hum to a very low level.  Heck, if it's good for solid state, then why not for tubes, right?) so I figured that if 14 uF. was a good input filter cap, 100uf had to be better, right?  However, after seeing a neat but almost fatal (to the tube) light show in a nice globe RCA '80, I looked in the RCA receiving tube manual and saw that the max cap for a cap input filter is something like 40 uF.  Lesson learned (the hard way) .

I get confused about X and Y safety caps and would appreciate a treatise on them if there is not one already in the Library.  Seems to be an important topic, as what was put in these radios back in the day could certainly not be called "safety," as I have found my share of shorted or blown (and I do mean like split open) ones, usually with my body being used as an ammeter  (Maybe, we need an emoji that has smoke coming out of it).  Correct me if I am wrong, but X caps are rated to pass a certain level of AC current and Y are the same except that they are also fused internally?

[morzh]

MrFixr

Well, to know exact requirement one has to buy the standards (national standards, to me, must be available for free, but no....those are agencies, and agencies want money) and read them.

For the US those are UL 60384-14, UL 1283 and UL 1414; there are also EU EN 60384-14 and EN 132400; these specify "across the line" applications and touch on the requirements to the caps.

The most rigorous ratings are Y1 and X1.

X-cap is supposed to filter differential noise, and Y-cap filters common mode noise.
Example: the noise on the electric line has differential (current) component and this will be short-circuited from L to N through the X-cap; this cap has to withstand disturbances (that is, a finite number of them, after which it is allowed to fail), which are inherently present in any AC line, industrial or residential. Of course, various things like Transorbs, MOVs, fuses, gas tubes, heavy inductances etc are utilised to attenuate the disturbances before the X-cap and the rest of the circuitry would see them.
The thinking is, well, OK, if the cap fails open, then it filters no more, so, so be it. If it fails short, this is what fuses are for. (it is incorrect that X-cap is required to fail short; it is not).

Y-cap filters common mode (voltage) noise. This noise, usually a higher frequency one, makes things radiate. An example is a switcher: the AC voltage developed across the flyback primary will, through capacitive coupling, induce the common mode noise on the secondary winding. This means, that the noise is present, more or less as the same voltage (hence "common mode"), at any point of the secondary, as it simply acts as the second plate of the capacitor. The Y-cap (in this app it is called "bridge cap") will provide the short circuit between the primary and the secondary (or rather the secondary and the GND), limiting the current flow to a small loop, rather than letting the whole secondary and the rest of the circuitry after it radiate as an antenna.
Across the line, if the common mode noise is present on both L and N, obviously X-cap will not do much; this is when Y-caps are installed from L to GND and from N to GND.
In both cases the Y-cap connects to GND and therefore failure as shorted may lead to electrical shock.

The value of the Y-cap in universal switchers is rarely more than 1.5nF. This is to limit the touch current in case the GND connection from the case to the GND fails, unbeknownst to the user, and the said most unfortunate user were to complete the circuit between the case and the GND. TYpically required max is not to exceed 0.5mA for plugged in equipment.
It is allowed to be 3.5mA for the permanently wired one, with the label warning of the high leakage current.

Example: a 1nF capacitor from L to GND will create 0.045mA of leakage current. However 10nF (used in our radios) would create 0.45mA, and two of them will create 0.9mA, which exceeds the maximum requirements.
A typical switcher with flyback topology uses 3 caps: L/N to GND plus the bridge, so due to three caps their summary value could be uqite high, so it becomes a compromise to find the values that would work for EMI suppression while passing the safety leakage current test.
This is where additional things like common mode chokes etc are popular additional suppression measures.


This is not a treatise, this is an explanation; the only document that can explain every small detail is the agency requirement, as this is what drives the design and the safety marking of the Y/X caps.


PS. As for the Phorums "expert" rating, I personally am no expert at anything, l just know some things at a better than average level, plus I can repair anything electronic, short of the parts that require programming or pick-n-place line for repair; when it comes to old radio equipment, folks like Terry and some others know about 1000 times more than I do.

PPS. Solen (and film in general) became popular because they do not have limited lifespan, as the electrolytic caps do, and they in fact do have a decent ripple current value.
What ripple current does, it heats up the capacitor due to the AC component (charge-discharge) with Joule heat, with the charge current heating the ESR (equivalent series resistance) of the capacitor. Electrolytic caps (the GP ones) are known for high ESR at smaller capacitance values. Higher value caps (hundreds and thousands of uF) have fairly low ESR, but we cannot use them as the first rectifier filter cap, as the tubes have limits of what capacitance they can handle. Old capacitors, however, had huge dimensions and huge mass, and this kep them from heating too badly; todays caps of the similar values are small, and the same amount of heat will make them quite warm. And in case of restuffing, this small cap gets to be sealed inside the older case with no air circulation whatsoever, which does not help either.
So, modern technology came with so called Low ESR caps and High Ripple Current caps. I am still not sure what the difference is, but basically the idea is, a cap with high ripple current rating wil withstand much higher ripple current. Which in turn means that it is either somehow resistant to that heat way, way better than a GP cap, or has low ESR, which makes the heat way, way less at the same ripple current. In either case the cap runs cooler.

Now, the formula that gives us the life of the capacitor involves WV and temperature. And while WV (that is the ratio of the app voltage to the rated WV) is at the base of the power expression, the temperature (that is the difference between the rated Temp and the ambient plus the temp rise) is at the exponent.
That is, your electrolytic cap could outlive multiple generations of people, if it is employed cool and under the rated voltage.
And, to this day, nothing really beats the electrolytic cap when it comes to achieving high capacitance and high working voltage.