[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.