Technical Points ignition/condenser question

Ha! Not too many people understand that reference.

 

And I don't even play guitar.

 

OK, back to points distributors for a quick question.

Does the capacitor/condenser have to be mounted inside the distributor?

It seems to me you could hook it up to wire coming from the coil at the connection where it goes into the distributor as long as it is grounded, preferably directly to the distributor body.

 

no, it can be outside....the old Mallory dual points had it outside, a big can. But it doesn't really matter. Inside, it's better protected, so it doesn't have to be as well made

 

Jim you are correct on spark jump and air density

 

On the subject of condensers, would connecting one into the hot wire of my radio work to filter the interference from my AM radio?

 

It might help. GM used a condenser on the positive terminal on the coil during the 12v point ignition AM radio era (1955-74)

 

Condensers for eliminating interference on radios is a little beyond my bailiwick, but I have seen them on generators, oil pressure senders, voltage regulators, instruments, ignition coils, and various other places on different cars. My '51 even has devices inside the grease covers on the front hubs to eliminate it.

It seems to be a little more complicated than one might think.

 

It's absolutely more complicated than one might think. There are loads of electronic filters out there, sometimes they use capacitors, often they use coils that the current has to go through (remember, coils hate fast changes in the current, so high frequency interference being very fast changes will have trouble going through).

I can't figure out exactly what you are dealing with, you say you have interference coming OUT OF your radio, what is it interfering with? When dealing with radio frequencies it may easily come out of any wire coming out of the device, using them as antennas, as well as straight through the box if it isn't built well for stopping interference.

 

Well I started poking around for one, then realized there's an App for that! Just have to figure out how to connect the coil wire to the usb port.

Seriously, there are a bunch on CL around here. Does it have to be an automotive one?

 

I posted a picture of a scope pattern earlier in the thread, it's a Tektronix 535a that I used to have. Fully HAMB approved, built in 1959, contains gobs of vacuum tubes, and sucks half a kilowatt just sitting there.

Be careful using modern scopes, that are designed for solid state work...the primary side of the ignition system actually has several hundred volt spikes, and can fry things that are not made to deal with it.

 

No wonder Amelia Earhart could'nt get her SOS out. lol

 

The utility provider has to fire up a peak load electrical generating plant when that thing gets turned on, eh?

I like vintage electronics, and test gear is no different, so vacuum tubes don't scare me. But they also made solid state ignition scopes w/ CRTs well into the 1970s or so for hobby users. Heathkit and others, everybody knows about the big SUN units but there were many less expensive models out there. That auction site will have a bunch of them.

The big thing to look for probably is make sure it is complete with all the cables, and the manuals and schematic are a plus. You want one that works, but that's tough to gauge. The one I bought was a Heathkit built in the 1960s and works very well, I replaced the power supply electrolytics on principle.

 

No, it absolutely does not have to, but you do want a storage oscilloscope that has memory that keeps showing the waveform so you can analyze it after the fact, a simpler oscilloscope w/o storage function will just let it flash by on the screen once and then it's gone - good for identical signals that repeat over and over several times per second or more but close to useless to find that one glitch that shows up once in a while - if you blink when it happens on the screen you missed it.
Don't get me wrong, my 30 year old analog scope w/o any storage functions is still great, and for some jobs it's better than other scopes. But the storage functions open up so fantastic possibilities on top of what the basic scope can do.

You also have to think about the voltage the scope input can handle. Many measure up to 5V/division on a 8 division high screen, so it can show +/-20V on the screen. The inputs can often handle a couple of hundred volts before damaged, but as others say, the voltages in a 12V system can go very far above 12V. I included this same image earlier, it's the voltage over a basic 12V relay when it's turned off. This one gives off a spike of -270V, far above the 100V my (cheap, non automotive) Picoscope USB oscilloscope is supposed to survive.
You use attenuators between the scope and whatever you measure, most basic oscilloscope probes have 10x attenuation as an option, meaning they'll reduce the measured voltage to 1/10, i.e. 200V to the probe means 20V going through to the scope input. Not quite enough to measure primary voltage at an ignition coil (or the shown relay) as that can go far above 200V, but there are other attenuators more suitable. Just something you need to think about before hooking up a scope, unless you're prepared to fry it.



As for the oscilloscope software/apps that use the microphone inputs on a computer and such. Perhaps a fun toy, but as the input is meant for low level audio signals it can't handle DC or low frequency, it can't handle frequency above around 20kHz (painfully low), and the max input voltage before damage occurs may be less than a volt. It's more of a toy to test how oscilloscopes work, rather than something to actually use as a scope.

 

The size of the condenser has something to do with what frequencies it will be able to filter. The smaller it is, the higher the frequency it can deal with. Large ones are good for lower frequency stuff (ie. large electrolytics as filter capacitors in linear power supplies)

 

You're talking my language. The problem is mostly with electrolytic caps that had about a 15 year design life and are multiple times beyond that now. Coupling caps and tone caps can and do get leaky, but you don't change them unless they're causing a problem. Opening up an old amp to see all brand new orange drop caps is a let down, and devalues the amp. Leaky caps won't really roast anything, but they can cause some unwanted distortions or oscillations. The grid of a tube isn't supposed to have DC on it, so that can cause some unwanted noise; as will DC on a potentiometer, will cause it to be "scratchy". But when it comes to vintage gear, if it aint broke, don't fix it.

 

The issue here is stray radio frequencies (RF) being picked up and amplified by the radio. I doubt putting a capacitor on the hot wire to the radio will do anything to help, because the rf is not likely coming from there, it's more likely coming from your ignition system, especially if you have solid spark plug wires and/or non-resistor plugs.

Try putting a loop in the high tension lead from the coil to the distributor. Many years ago I had a problem with noise in the AM radio, the frequency followed with the RPM of the engine. My dad told me to put a loop in the coil wire, which I thought was strange advice, but what the hell, why not try it. It did the trick, no more noise in the radio.