AC Bypass Resistor, why

[K6JEK]

Fixing an old FM tuner for a friend, a Fisher FM-50, I discovered something I don't understand, an AC bypass resistor. One side of the AC mains is bypassed to the chassis with a .01 cap paralleled with an 820K resistor. The plug is not polarized so this could be either side of the AC mains, hot or cold, 50/50 chance.

What is this resistor for?

[KA2DZT]

Drain off static build-up from the antenna.  Does the chassis have a power xfmr?  If it does, you could replace the AC cord with a 3 wire grounded type and remove the resistor and cap.

Fred

[K6JEK]

Drain off static build-up from the antenna.  Does the chassis have a power xfmr?  If it does, you could replace the AC cord with a 3 wire grounded type and remove the resistor and cap.

Fred

I never thought of that but the antenna is transformer coupled to the 1st RF amplifier with neither side on the chassis. Still maybe it's some sort of the static drain maybe for people walking across the room and touching the chassis or something.

I will put in 3-wire cord. I will also get rid of anything before the switch and the fuse and probably won't put the bypass cap back in even though I do have a tempting supply of X1/Y2 caps.

I do wish I really understood the resistor ...

[W3RSW]

The resistor and cap combo are similar to those installed in AC/DC all American five sets of the period.  All had them to eliminate or vastly reduce the shock hazard of a hot chassis to an outside ground through your body.
The chassis is lifted above ground to high resistance, low current levels. The capacitor simply assures an RF chassis ground if the plug is inserted "chassis hot."

As Fred asked, "is there a power transformer?"  If not then the R/C combo is definitely required for safety.  If yes, then the R/C combo may be used as an FM signal ground to the AC line without an external ground.

Regardless, the parallel combo allows the chassis to be at RF ground with respect to the AC mains if plug is inserted either way, one way direct, the other through the high capacity primary windings of a power transformer (if any.)

[K6JEK]

The resistor and cap combo are similar to those installed in AC/DC all American five sets of the period.  All had them to eliminate or vastly reduce the shock hazard of a hot chassis to an outside ground through your body.
The chassis is lifted above ground to high resistance, low current levels. The capacitor simply assures an RF chassis ground if the plug is inserted "chassis hot."

As Fred asked, "is there a power transformer?"  If not then the R/C combo is definitely required for safety.  If yes, then the R/C combo may be used as an FM signal ground to the AC line without an external ground.

Regardless, the parallel combo allows the chassis to be at RF ground with respect to the AC mains if plug is inserted either way, one way direct, the other through the high capacity primary windings of a power transformer (if any.)

Now this is beginning to sense. It's not an AC/DC set. It has a nice power transformer. I'm in the habit of putting in 3-wire power cords and plan to do so here. That will put the chassis at RF ground if you can call the the third wire RF ground.

Thank you for the explanation.

[AB2EZ]

I would guess that the intended purpose of the capacitor is to reduce the amount of RF from the radio's local oscillator that couples into the building's ac wiring, and is subsequently radiated by the ac wiring. It might also have been intended to provide some protection from spikes on the ac line due to nearby lightning strikes.

The resistor in parallel is puzzling. Perhaps it was intended to leak off DC that might develop across the specific type of capacitor being used due to asymmetry in the behavior of the capacitor in response to the AC across it.

Stu

[K6JEK]

Whoa. I think Fred is right after all. I now have two schematics for the FM-50, the service manual and a Sams. No mystery resistor in the service manual which is the one I was using.  But the resistor looks  factory installed. Didn't look like a mod. I figured it was a rev and was mystified by it.

Well here's the rev. The Sams shows it and another resistor from the antenna connection to chassis which is also missing from the service manual schematic.  I have no idea which schematic is the newer one but I think I finally understand what is going on.  

Thanks, guys.

[KA2DZT]

The 100K resistor on the antenna terminal is for draining off static buildup on the antenna.  Draining it to an ungrounded chassis might not do any good. Hence, the 820K resistor from the AC line to the chassis completes the ground back to the panel ground.  Does not matter which way the plug is plugged in the outlet.  It will either drain to ground from the neutral on chassis or if plugged in backwards, through the primary winding back to the panel ground.

You eliminate this issue by using a 3 wire grounded cord.  No need for the 820K resistor with the 3 wire cord.  Leave the 100K resistor as that is still needed.  The .01 cap probably should be removed.  Hopefully not much RF will pass through the power xfmr to be shorted to chassis ground by the .01 cap.  If the .01 cap is on the neutral line, it is doing nothing.  The neutral line is already grounded.  The .01 cap would have to be on the hot side of the primary to possible do something.  I would just eliminate it as the chance of it shorting is one big reason to not have it there.

Fred

[W3RSW]

I would place a .01 at 1kv or better on both sides of the active line leads for AC transient protection if I were building from scratch.  I would hope that the green wire is truly ground.  The cold side is neutral only for slow events such as 60 cycles through 108 MHz. 

Ar least place a .01 at 1kv on the black, hot AC lead.  This should before any switch or fuse, both of which should be on the hot lead as noted.

You will note that many transient suppressors have three capacitors, one across hot to neut. and from both hot and neut. to green ground.  For a few pennies, can't hurt.

[k6jca]

I agree -- if you're installing a 3-wire cable, leave the 100K for static drain (this seems to be the most likely reason for this resistor), although if you aren't using an outdoor antenna, I don't see much need for it.  And even if you were using an outdoor antenna, I wonder as to its need, given that the antenna is transformer-coupled to the first tube.

As for the 0.01 caps across the input line, given that it's a tuner, you might consider adding a true line filter (with integrated caps and common-mode choke), to filter RF and/or noise (e.g. from switching power supplies) that might enter the tuner on the AC line.  But two caps is a great start.  Just don't make them leaky (having been "tickled" myself, in the past)!

I'm still puzzled as to why Fisher installed only one cap.  Why not two?  Or why any at all?  After all, given that this tuner is transformer powered, this cap is not serving the same function as the cap in an AC/DC all-American 5 radio -- the tuner's chassis is inherently isolated from AC mains.

Also -- in the classic all-American five radio, I'm really unclear as to the function of the resistor across the cap.  Wikipedia says: 

"The hazard was eliminated from later sets by the use of an internal ground bus connected to the chassis by an isolation network. Underwriters Laboratories required the adoption of the floating chassis, as isolation from the mains (the exact circuit and component values were not specified although the leakage current allowed was specified) to limit the shock to a "safe" current level. The chassis was maintained at RF ground (for shielding) by a bypass capacitor (typically 0.05 µF to 0.2 µF) usually with a resistor connected across it (typically 220 kΩ to 470 kΩ, although values as small as 22 kΩ were sometimes used or the resistor was simply omitted)."

But this really isn't a very good explanation, for me.  How is this resistor improving shock hazard?  Or does it have nothing to do with shock, but instead it simply provides a DC path from chassis to ground (via the AC wires), even though this path is passing through this very high resistance?

Thanks!

- Jeff, k6jca

[k6jca]

And even if you were using an outdoor antenna, I wonder as to its need, given that the antenna is transformer-coupled to the first tube.

Strike that thought.  Too much static buildup and there might be an arc-through from the primary winding to the grounded secondary.

- Jeff, k6jca

[W3RSW]

Jeff, one cap either side of old standard line is better than none. As explained before, if plugged in hot way works fine. If plugged in cold way, still couples frequencies higher than low audio from AC line to chassis through the high capacitance of primary winding (in series and parallel with it.)

In the days of intermediate standard ( still good for lamps, etc.) polarized two prong plug, one cap is almost  all you need for most cases of line transients.  Three caps as you understand is even better along with the choke coils you've mentioned.

The resistor from line to chassis might have guaranteed some sort of time constant in some old design engineers mind at the time or may have just been added in some sore of hazy recollection of AC/DC sets.  At any rate, the high value assures voltage equality at small current flow.  In parallel with a .o1, definitely sets a freq. knee.

I'm worn out with this one... 

[k6jca]

Thanks, Rick.  All this discussion is getting my brain cells out of their normally dormant state.

Yes, if the plug were polarized, and if the other side of the AC line were connected to the chassis, I'd agree. 

But what is weird:  the AC line in this tuner does not connect to the chassis, and the chassis is ungrounded (not connected to AC mains earth), so a single cap from one side of the AC line to the chassis doesn't make any sense to me, if its purpose is to shunt transients on the AC line -- there's no path from the cap's chassis connection to the other side of AC, so it can't act as a shunt across AC mains.

And if only one cap is used, I wonder -- why didn't Fisher just connect the cap directly across the AC line, rather than from one side of the AC line to the ungrounded chassis (this would have been trivial in Jon's tuner -- both sides of the AC line are terminated very close to each other).

Also, with different Fisher designs, sometimes the R and C are in parallel, and sometimes the R and C are connected to ground from opposite sides of the AC line (e.g. FM-100B and FM-500B).  If the primary winding of the power transformer looks like a short at RF, then, effectively, they would be connected in parallel.  But I'm wondering what the impedance of the primary really is at RF.  Does the inter-winding capacitance dominate at RF, or the leakage inductance?  (Something for me to check, one of these days.)

I'm beginning to think that the cap is not for transient suppression at all, but instead it provides RF grounding of the chassis to the AC mains (low-impedance at RF, assuming xfrmr primary capacitance dominates, if plug not polarized). 

Why ground the chassis for RF?  I don't know, but it is a tuner.  So Stu might be on to something.  Nothing else makes sense to me.

Thanks again for your explanations!

- Jeff, k6jca

[K6JEK]

I would place a .01 at 1kv or better on both sides of the active line leads for AC transient protection if I were building from scratch.  I would hope that the green wire is truly ground.  The cold side is neutral only for slow events such as 60 cycles through 108 MHz. 

Ar least place a .01 at 1kv on the black, hot AC lead.  This should before any switch or fuse, both of which should be on the hot lead as noted.

You will note that many transient suppressors have three capacitors, one across hot to neut. and from both hot and neut. to green ground.  For a few pennies, can't hurt.

Why before? I've always put them after the switch and fuse on the hot lead on the theory that the switch was unlikely to fail but the cap might. After the switch and fuse it won't be sitting there unfused across the mains all day and all night whether the unit is turned on or not. But I'm just making this stuff up. I don't know if makes sense or not.

[W3RSW]

-combo transient suppressor even when the set is off and switch arc preventer. Closing the switch itself produces a transient that might pop a under voltage capacitor.  Ok, circular reasoning easily takes over from here.
Danger!

Who knows what fuzzy ol' engineers knew?

just seems to be found that way in majority of sets I've seen.  In a HQ110 for instance a couple of .01's to gnd. from both old standard lines before any switching.

I once found both blown at some unknown time.  Set didn't seem to care but maybe it protected the primary winding insulation.

[wa3dsp]

Now-days you should not put a capacitor or resistor on just one side of the line. It can cause imbalances which can play havoc with GFI's. One device might not trip it but it you have many on one circuit there might be enough imbalance to cause a problem. GFI's don't care if the path is from hot or neutral to ground, either will trip it. Most entertainment devices now use switching power supplies which have an LC filter on the line to meet FCC specs. The filtering is across the line and not to ground. If you do put capacitors on the AC line it should be two of the same type from line to ground and neutral to ground. You can also put one across the line. The capacitors should be AC rated.

In many older radios they actually used capacitive coupling to the AC line for antenna pickup. Sometimes this was a physical capacitor and sometimes it was a piece of metal wrapped around the AC cord and connected to the antenna terminals.