Onto a new learning curve…CD ignition

[johnyrude200]

Thank you, very much! I had a great conversation with a fellow AOMCI member, 10 years younger than me, but also been in the club 15 years longer than me (I’m 36…do the math). All the Yankee chapter members know who I’m talking about.

It was really a great conversation about something related to this thread. The meat of the conversation was that, CD ignition conversations in 2015 are akin to magneto conversations in 1985. There is a ‘passing of the torch’ that is a necessary reinvestment to attract new members (30-50 years old). I seek knowledge, and am thankful for the resource that the forums represent from the 5-7 regulars who are here just to help everyone, personal and political views aside.

Thank you everyone for your devotion to keep these motors alive…I have had such an outpouring of folks who have said this to me as I’ve worked on their motors that marinas tell them to throw away, and I consider myself to be a newbie to the repair profession. I’m just passing along the knowledge you all have worked long and hard to accrue. I will say I had a local emergency department ask me for a motor from the 60’s for a rescue vessel. If it were not for the outpouring of support in these forums, the vessel wouldn’t be supplied with a cost-effective motor that once I am done with it, will be (other than cosmetically), as good as new to help rescue folks in the water.

I shake my head at what marinas have said to customers bringing their motors in for normal service orders, only because in many cases the motors require very little to be reliable and serviceable.

[legendre]

As ever, there’s no shortage of lazy, half-assed mechanics. It’s (almost) always easier to work on newer engines, with current factory support.. trouble is, there are far more old motors out on the water than new stuff.

[frankr]

quote legendre:

quote FrankR:

Gotta tell you though, you could actually cobble up your own DVA Adapter. I did. Very crude though, but it shows how little is actually in the real deal.

I can’t image there’s a whole lot needed.. it’s just a peak hold circuit. A fast, high-voltage diode, a small value low-leakage cap..? What else is in there?

Why can’t a standard DVM with peak-hold feature be used?

ETA: I just found a discussion on another site. Apparently, I was correct – nothing more than a cap & diode in there. For some reason, the builder also used a 1M resistor in parallel with the cap – I have no idea why. All that would do is cause the “held” reading to fall off to zero more quickly. He also used a 2.2uF @ 450V electrolytic cap – I’d have tried more like 0.22uF – 1.0uF first (in a low-loss film type) before venturing into electrolytic territory.

I believe the resistor is to bleed down the capacitor after you finish with it. To make ready for the next test and so it isn’t sitting there like a rattlesnake ready to bite you.

[legendre]

quote FrankR:

quote legendre:

quote FrankR:

Gotta tell you though, you could actually cobble up your own DVA Adapter. I did. Very crude though, but it shows how little is actually in the real deal.

I can’t image there’s a whole lot needed.. it’s just a peak hold circuit. A fast, high-voltage diode, a small value low-leakage cap..? What else is in there?

Why can’t a standard DVM with peak-hold feature be used?

ETA: I just found a discussion on another site. Apparently, I was correct – nothing more than a cap & diode in there. For some reason, the builder also used a 1M resistor in parallel with the cap – I have no idea why. All that would do is cause the “held” reading to fall off to zero more quickly. He also used a 2.2uF @ 450V electrolytic cap – I’d have tried more like 0.22uF – 1.0uF first (in a low-loss film type) before venturing into electrolytic territory.

I believe the resistor is to bleed down the capacitor after you finish with it. To make ready for the next test and so it isn’t sitting there like a rattlesnake ready to bite you.

It would certainly have that effect, but I’d also think it would tend to cause the reading to fall-off more quickly than desired.. before it can be read, and so forth. Basic math, T=RC etc.

And it’s not like just crossing the leads won’t eliminate any remaining charge. If I built one, I’d leave off the drain resistor.. but I’m still not convinced that a general-purpose meter with a peak-hold function wouldn’t suffice. Maybe I should set up a simulation, and take some data,,? =)

[fleetwin]

I’m pretty sure they do make affordable adapters these days that can read peak voltage for those of us who don’t have a strong electronics background.

[frankr]

As far as I can understand, a DVA is another name for a peak-hold voltmeter. Accumulates the short voltage peaks and stores it in a capacitor to give you time to obtain a reading. More technical explanation is that electrons will flow into the capacitor until the voltage across the capacitor equals the highest (peak) source voltage. The diode is a one-way door to keep the electrons from flowing back out. The system is compromised a bit by the escape path as already discussed (parallel resistor, meter load, etc.)

Unless somebody can explain to me why it isn’t.

BTW, a cheap analog multimeter would present a large escape path for those stored electrons. In other words, you have to have a suitable multimeter as generally used for electronics work.

[legendre]

quote FrankR:

Unless somebody can explain to me why it isn’t.

No, that’s all correct.

quote :

BTW, a cheap analog multimeter would present a large escape path for those stored electrons. In other words, you have to have a suitable multimeter as generally used for electronics work.

The key concept is "Input impedance" (of the multimeter), and generally speaking, you want the highest possible impedance. Cheap analog meters tend to have lower input impedances – in the few thousands (5K – 20K) of ohms, though the higher-end analog products like the Simpson 260 have far better specs. Digital meters, on the other hand, tend to have much higher impedance – in the range of one million ohms (1M) on the low end, and 10M – 11M on the upper. Even the cheap digital meters have a far higher impedance than any** analog design.

In short, the higher the impedance, the less burden the meter places on the measured circuit. As impedance falls and burden increases, readings become more skewed.

(** We’re excluding VTVMs and FETvoms, etc.)

[frankr]

Yeah, I avoided using the term "input impedance" here for fear of driving everybody away. We’ve already driven off everybody that is challenged by changing a light bulb. But that is certainly true, and input impedance is the term.

[Buccaneer]

🙂

Q: How many mechanics does it take to change a light bulb?

A: According to the Repair Estimate Manual, the answer is .3 man hours plus parts … if you can bring it in Tuesday before nine.

Prepare to be boarded!

[legendre]

quote FrankR:

Yeah, I avoided using the term “input impedance” here for fear of driving everybody away. We’ve already driven off everybody that is challenged by changing a light bulb.

If true, that’s kind of unfortunate.

It’s not like the discussion is all that high-level.. and if someone needs further explanation, all they need do is +ask+.

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