The Stovebolt Site Forums The Shop Area The Electrical Bay The Radio Bench Capacitance values of AD noise suppressors?

Has anyone ever really found out what the actual capacitance values of the Noise Suppressors originally used on the AD’s?
Denny Graham
Sandwich, IL

.5uF @ 200VDC Non-polarized.

That's the generalization made on the Motorola link that Tim posted Justin. That is also the value that the vendors have picked up on and are selling. At least a value that is close to .5uF, the standard in this range now days is a .47uF. If the dielectric strength is not given, which it's not on the vendors caps, then the only way to reverse engineer the value is to apply a voltage till you get a breakdown, of course then you've just destroyed an over priced $10 capacitor but you then know what the dielectric strength is on that series of caps for future reference.

To be very specific, what I was looking for were the actual, real, original, honest to god or how ever you want to put it, values of the Part Numbers; 1887829, 1911095, 7257919, 1910147, 1912900. I would like to know what the condenser values are that were specified by the GM engineers for the installation of the 986443 radio in the Advance Design trucks. I haven't been able to find any actual value specifications in any of the references that I have, just thought someone out there in the hinterlands might have run across them somewhere.
To just say, a .5uF cap will work in all locations is like saying that any tire will work on you vehicle, and they will, to some degree. But to optimize the performance of a tire, or an electrical component there are some that are better suited than others. I mean after all, a resistor is a resistor, just put one in no matter what size you really need and that should work, right??? WRONG!!! It helps to have the correct value.
But thanks for responding anyway.
Denny Graham
Sandwich, IL

I've used an old heathkit condenser checker on several "old" (original?) condensers, either out of a distributor, or off a coil or generator... here's what I've found:

At absolute "best" the condensers I've checked were up as high as .5uF. Some were as low as .1uf but I have to take into consideration that these things lose capacitance as age and heat dictates....

This old condenser checker I use also has a 'load' test ranging from 25 volts up to 450 volts. Hooking up a good original condenser that measures between .4 and .5uF, I can get it to charge and hold on 150v all day. When I bump it up to the next load voltage, 250 volts, it takes noticeably longer to charge completely and sometimes, as indicated by this unit's "magic tuning eye" indicator, it would not charge at all... I've also noticed this same trend on the condensers available from our FLAPS.

Short of looking up data in some original GM engineer's handbook I'd have to wonder does it really matter? The simple RF trap, coupled with the condenser's arc-quenching utility is hardly a 'critical' circuit.

In radios particularly, any given manufacturer will use any number of a dozen different values in areas of the circuit which are engineered to filter out noise.... .47uF, .1uF, .05uf ... the values are all over the scale. So given that tidbit and in understanding some background in electronic engineering, I'd have to say that the value of the filter capacitor in these given 'circuits' largely depends on what frequencies the engineer was specifically trying to filter out.

In an engine, we consider that the frequencies that are being filtered out are constantly changing, and this coincides with the engine's given RPM. At varying RPMs the frequency of the points opening/closing is changing, but the generator as well along with what interference is being produced by that. Keeping things simple, I think it was just easy to slap that .47 value on there and call it done.

Because, not only does can a .47uF capacitor provide adequate RF filtering, but also acts as a good value for arc quenching, which is the primary reason for all the RF noise in the first place.... and even though you can't always see it, the points in voltage regulators and the points controlling the ignition are arcing, and it's that itty bitty arc that produces so much noise!

Just some experience and thoughts.

I have several pages of an original GM manual supplement which discusses some of the objectives of their interference suppression schemes. Unfortunately the manual is packed away in storage awaiting our move in the spring. Some time in early summer I intend to scan this information and post it.

In these designs, engineers didn't necessarily take a "that's good enough " approach but tackled each individual aspect separately and also together. That is why some aspects seem so minor in relation to the overall picture. Often the objective in engineering isn't to just find the easiest solution but rather to create a design with the very best result, even if it has a very small impact on the over all picture.

On the subject of distributor condensers, I have seen it stated many times that it dissipates the energy created by the breaking points. The role of the condenser in the ignition is to create an LC circuit with the coil that oscillates at a high frequency and sustains a burst of coil AC voltage. The energy is dissipated by the resistance in the system including that of the coil. When running properly the system creates a high frequency burst with a fairly short decay time. By high frequency I mean relative to the points cycle.

Boy, that info would be pure gold when you'd have time to get that scanned.

This then would seem to indicate that the 6 volt coils would (or at least should) have a different capacitance value than their 12 volt counterparts, since the impedance of those coils would be different....hmm.... I know that in some publication I read about ballast resistors, and the fact that the resistors used in 6 volt systems were a different value than the 12 volt ones... I've just never seen an 'original' 6 volt ballast resistor still installed in a stock truck. By the time I get to them someone else has already been under the hood making their 'improvements.'

Man Fred, I hope you don’t loose that in the move. I for one would very much like to see that. I’m still not convinced that the engineers just simply threw a dart and whatever value it landed on was the cap that they choose to use.
The RF is not simply function of the number of times the points open and close although this being in the audio rang it is imposed on the carrier and we here it at the speaker. The spark generates a multitude of frequencies as it rings down. As you know the spark gap was used in the earliest days of broadcasting to generate a carrier and this is exactly what we are doing with out little spark gap generators in the vehicle. Back in the early days as more stations went on the air, and were closer together the need to assign carrier frequencies became necessary. By then the spark gap had been replaced by the vacuum tube transmitter where the frequency of the carrier could be regulated.
Any cap is going to act as a short to ground for the RF. I would imagine that the size is pretty much determined by the amount of energy that it has to handle. I don’t have an electronics engineering background, just an old mechanic so I have to leave these calculations up to guys like you Justin.
I have been checking a bunch of caps using an HP 4262A LCR Meter, quite accurate and again I am finding them all over the place. And I think I might have mentioned it, he caps from JC were .047uF
Denny Graham
Sandwich, IL

Yes you're right, and I was even thinking of using an old spark gap to illustrate my point.

Perhaps I was a little unclear about my take on the engineers choosing a capacitor; sounding like they just took a shot in the dark, but what my analysis is based on is deciding on one capacitor that would work at clearing up as much noise as conceivably "possible" by only using one simple component... but also based on the premise that the frequencies generated by the spark are in fact changing with engine RPM.... this I don't know.

If the frequencies generated by the spark (not the noise of the "spark" itself) in fact don't change, and resonate at a fixed frequency, then the formula to calculate the filter capacitance for this frequency is rather simple. If it were to constantly change with RPMs, undoubtedly the capacitance required to filter this out varies, and that's where I would have wagered it was one value of capacitor that would have been used as a 'catch all' to prevent the implementation of more complex filtering equipment.

Outfits like Radio Shack carry (or used to carry...) noise suppressors that in themselves are much more complex than a simple condenser... so does this device provide better filtering than a simple cap? I'd say so.... BUT one thing I can think of is that with the advent of transistors also came the availability of the alternator with its silicon rectifiers... I've heard this 'alternator' whine on lots of two-way equipment installed in cars ......


but anyway, getting a little off track, just a few more thoughts.

Well whatever! I'm kind of stuck in the garage till the weather gets a little better. My two rebuilt radios work just fine in the garage and I've had the engine running for a few minuets. Got Carter's .47uF caps on the Ammeter, the generator, the VR and a suppressor in the coil wire. I can't hear any engine noise with this short test. When I get a dry day without all the slop I'll take the old gal out for a road test and see how the reception is and report on it.
I've heard complaints from guys here that the radio couldn't be heard over the trucks rattling. The volume in these two radios is way more than adequate. If you can’t hear one of these radios while cruising driving down the hiway then either your deefff or your truck need a whole bunch of work.
Denny Graham
Sandwich, IL

Radio noise supressors are larger than ignition condensors, as has been widely noted by others. I don't recall a specific value for the noise supressors, but the .47mfd sounds familiar. Delco ignition condensors are in the range of .18-.23 mfd, Ford and Chrysler use larger condensors, but all are based on the inductance of the coil primary winding and the rate of change of the primary voltage when the points open. That rate-of-change is inductance-dependent, with larger inductance causing a slower rise in voltage. I don't remember condensor values for Ford and Chrysler ignition systems, but .23-.25 (Ford/Motorcraft/Ford Autolite) and .25-.28 (Chrysler/Prestolite/Electric Autolite) come to mind. Regarding engine RPM and the points, the faster the distributor turns, the faster the moving contact changes from closed to open as the rubbing block slides toward the apex of the lobe. As that contact moves faster, there is a 'natural' tendency to snuff the arc. (Thats why electrical switches have a 'snap' action- to break the circuit quickly with minimal arcing.) The resonant frequency of the coil primary doesn't change with engine speed or level of magnetic saturation. A frequency change does occur though, when the spark goes out, which causes the coil primary and secondary windings to become one large series-resonant circuit, with a ring frequency considerably lower than the primary winding has during the spark duration. That happens because the 'standard' ignition coil is an autotransformer, with the primary and secondary windings having a common terminal- usually the primary positive terminal. You see this frequency change with an ignition oscilloscope- or any kind of scope that can handle +400 volts input, or a little higher. There are ignition coils that are not autotransformers, but not many in terms of application. Delco HEI coils are one application of that, and there are others.

As far as the noise supressor for voltage regulators, generators and alternators, moving-contact regulators have an arc-supressor resistor that parallels the contact points to supress arcing there, and you don't care especially if the arc is drawn out a little. It is my understanding that the noise supressors in those applications are for dealing with 'ring' frequency of the relay coil that develops when the generator/alternator field current is reduced or interrupted. I think the condensor on the generator armature terminal is more to supress commutator switching noise- watch the commutator on a DC generator with it running at a moderate load- there is continuous sparking at the brushes. For alternators, I think the supression idea is the same as with DC generators, except the output has a fairly large AC ripple component- on the order of several tenths of a volt,depending on the stator winding, condition of the diodes and load on the alternator. And there is no arcing in an alternator output as there is with a generator commutator. The alternator field (rotor)also resonates to some extent when the voltage regulator switches field current to control the alternator voltage.

Enough said- I don't mean to clutter up this whole discussion.

Harvester

No clutter at all, what you wrote sounds like it belongs in a textbook, good reading.
I would not have asked if I didn't want people to respond. What you say makes a lot of sense and my finings verify what you say. I have an HP LCR meter that I can accurately check inductor, caps and resistance with and have been checking every automotive cap/condenser that I can get my hands on. If ya ever turn a cap over and find the value written on the bottom of the clamp you can be sure it crossed my bench.

My original question still remains un answered though, and that was, "what were the actual capacitance values of the Noise Suppressors originally used on the AD’s?"
All we seem to have are part numbers and no values. I’m sure that someone has an original NOS GM AD radio accessory still in the box, after all we see lots of Heathkit's that were never built and still in the original boxes. Even then, I doubt if we could determine what the capacitance was because I'm sure they were paper caps and we all know how good a paper cap is after 50 or 60 years whether it was in service or not.
I had hoped that someone might have some service note or engineering specs that would identify the values but looks like those died with the designers.
Thanks for the comments Harvester.

You don't happen to know my cousin Wally Wiley out there in Tucson do you??? Been a long time since I was out thar, like 50 years, back then you just about knew every body in town, well, at least all your neighbors!!!!
Denny Graham
Sandwich, IL

Denny,

I don't know Wally Wiley, but Wiley Coyote and family still live here. And the Roadrunners, too.

Harvester