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A Little Off Plumb
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Discussion Starter · #1 ·
Ran across a manual for a Merc-O-Tronic Model 98 tester that was quite common and used in small engine shops from the 1950's to the 1980's. The tester was capable of testing condensers, ignition coils and other components and I can remember using one back in the mid 1970's for setting points and timing on snowmobile engines in a small engine shop where I worked for a couple of years. Thought I would do a post on it since I was able to find a youtube video that shows one being used to test ignition condensers.

Been looking through the manual and I see that there are resistance and current specs for a lot of magneto ignition coils by brand and part number as well as spec ratings for condensers that were used on small engine, snowmobile and marine applications.

Thought I would share in case other members are interested. Looks like the company has a facebook page but the last post regarding the testers appears to be about three years ago so not sure if the company is still in business or not. Small engine shops that have closed down or may close in the future that have been in business for quite awhile may have a similar unit sitting on a shelf somewhere and to people that are not acquainted with the tester it may appear to be of not much value.

Attached is a link to a picture that looks very similar to the one that was used at the small engine place where I worked.
Merc-O-Tronic Model 98 Ignition Analyzer Tester. Tests Ignition coils and condensers. | Small engine, Engine repair, Repair

I have also attached a picture of the meter face with the various scales
 

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Well that's a neat tool Stew. Would be handy to have.

Noel
 

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A Little Off Plumb
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8,433 Posts
Discussion Starter · #3 ·
Well that's a neat tool Stew. Would be handy to have.

Noel
Yes Noel they were a handy tool in the days before digital multimeters were available - wish I had taken the time to read the manual and learned what all it was capable of doing. Being a young 19 year old kid I thought I knew a lot more than I did and didn't need to learn much - hindsight that comes with age and maturity sure is a great eye opener.
 

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Old, but not dead -- yet!
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The tester was invaluable for testing magneto coils. ....You could see when the coil would begin to fire, and if it was "breaking down" after awhile.

If I remember correctly, coil testing was done using the internal lantern battery. .....Condenser testing required the tester to be plugged in to 110VAC.
 

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A Little Off Plumb
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Discussion Starter · #5 ·
Been doing some more research and found that another company offered a similar tester for small engine applications - Graham Lee Electronics. Look like they offered a model 31 and a model 51 - the 31 did not have a meter and appears to be an economy version. Looks like there is a specifications book in the manuals section for use with the model 51 tester - for those interested here is a link to it - Graham Coil,Condenser and Ignition Tester Specifications
There is an operating manual for the model 51 as well on the site - Operating Instructions for Graham Lee Testers
For those interested there appears to be a bit more information available on the internet as to how the model 31 operates and how to repair one.
Operating manual - https://wrcoutboards.org/wp-content/uploads/2020/03/GrahamLee_Model_31_Users_Manual_r.pdf
Repair information - https://wrcoutboards.org/wp-content/uploads/2020/03/Troubleshooting_Graham_Lee_Model-31_r.pdf
While the testers may not be readily available the manuals contain valuable information on a lot of manufacturers ignition coils and condenser specs for those who wish to know resistance specs on the coils and capacitance values on the condensers.
 

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random thought

do they make coil ignition testers anymore?
if not, I take it there was some build-you-own type plans in either popular mechanics, popular electronics, mechanics illustrated or similar?

the restoration manual talks about how the tubes are non-existant and that the SCR isn't made anymore, with some sub-optimal suggestions for compatible SCRs
 

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A Little Off Plumb
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Discussion Starter · #7 ·
random thought

do they make coil ignition testers anymore?
if not, I take it there was some build-you-own type plans in either popular mechanics, popular electronics, mechanics illustrated or similar?

the restoration manual talks about how the tubes are non-existant and that the SCR isn't made anymore, with some sub-optimal suggestions for compatible SCRs
Coil testers are still made but I doubt few, if any, are made to specifically test a magneto style coil coil that is triggered using points and condenser and the current generated by a magnet passing over the coil post. Most modern day small engine coils are designed with a SCR inside them that gets triggered with the magnet as it builds up an electrical current in the coil. Not sure about plans - might be something to check if someone had the time to look through the back issues that can be found online.

Tubes are hard to come by but I believe that some are still made and can sometimes be replacing using transistor circuits. The SCR T106M (4 amp 600 volt) listed in the schematic is shown as obsolete. T106M1 Littelfuse Inc. | Discrete Semiconductor Products | DigiKey

Digikey do show some a substitute (700 + in stock) that has a similar rating and would probably work if the SCR in the tester was bad - S6004VS2TP Littelfuse Inc. | Discrete Semiconductor Products | DigiKey For $1.54 I would take a chance on it working. If it didn't I imagine the triggering circuit could be modified so that another SCR that required a little more current to trigger would function. Mouser Electronics also show a part number that would probably work in place of the T106M1 https://www.mouser.ca/ProductDetail...=/ha2pyFadujuNmHmi5CvVcgJDhnFCOUsSLzR58iLEx8=

There are a few reasons I posted about the Merc-O-Tronic and provided the links for the Graham Lee units. Some members may possess or come across a unit in the future and not realize what it is and what all it can test when it comes to older small engine ignition circuits. The manuals contain testing information for resistance and other values on a lot of older coil and condenser part numbers - some of which was never published in other manuals. I am currently working on cleaning up a manual for the model 98 Merc-O-Tronic that I am hoping to upload to the site. The manuals show how to test the majority of coils for resistance values and also things to watch out for when testing. In todays world a multimeter that can measure capacitance are readily available for testing the components but without knowing the original values a coil or condenser may appear to test ok but may actually be out of spec. The actual powering of a coil to test a suspected defective unit could probably be accomplished suing a 555 timer circuit to trigger a SCR, a spark plug to observe the spark produced, and a heavy duty wire wound variable resistor to limit the current using an amp meter in the circuit to measure the current value to see when the coil starts to produce a spark. If someone watched the video that I linked to in the original post they can see how a capacitor may pass a test using just an ohmmeter that shows it is not open or shorted but can still be out of spec resulting in the points pitting or in the case of an intermittent condenser an engine that will run not bad at idle but stumble, back fire and have poor performance if the capacitor is partially shorted and not holding the correct charge. An ohmmeter may or may not show the capacitor as bad and it will drive an unexperienced technician crazy - spent three days the first time I encountered one when I was 19. Encountered two more in the years since and they were each diagnosed in about 10 minutes given the way the engine was running - some things stick in the back of your mind when they challenge you.
 

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A Little Off Plumb
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Discussion Starter · #8 ·
For those interested in how the tester was used as well as the coil and condenser specs that are contained in the manual I have cleaned up an owners manual for the Model 98 version of the Merc-O-Tronic tester and uploaded it to the manuals section. Here is a link to it - Merc-O-Tronic Model 98 Ignition Tester Manual
 
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