12 VDC Inductive Ignition Timing Light: Need Help Diagnosing

Thread Starter

cadman777

Joined Dec 23, 2022
39
Hi Guys,

I am sort of reviving an old thread: "12 vdc Inductive ignition timing light bulb weak flash?".

The other day I took out my old Sears timing light #244.2138 to time a guy's 1963 283cu engine. The thing didn't light up. So I searched the internet for troubleshooting and found a good thread in here (see above link). I did the things suggested and didn't find anything wrong. So then I tried testing the capacitance on each capacitor, since I figure that capacitors are good for about 15 years, and this is much older than that. At first I didn't disconnect a leg and just touched across the capacitor legs to test them and that didn't work. I haven't done electronics testing in over 2 decades, so forgot everything I once knew about it. Anyway, when I touched one side of one of the trigger circuit capacitors with the neg lead connected to the neg side battery lead, the neon light began humming and lighting up. So I put it back together and tried it but nothing happened.

So then I began testing each capacitor one at a time by first disconnecting one leg of each and testing across both legs. All capapcitors tested OK, at least using the DVM capacitance testing function. If memory serves, that doesn't test for them under load, but I remember how to do that. Anyway, in the process of disconnecting one leg of each capacitor (one at a time), I found one of the capacitors with a loose connection at the trace solder joint. Unfortunately, I don't recall if it was C2 or C4 that was loose:

1671994223978.png

Anyway, after I checked all the capacitors and soldered the loose one back to the board, I decided to re-solder every connection to the board just to be safe, since some of the smaller solder joints looked cheezy and weak. After that, I plugged it back into my 12VDC/3A power supply and it still didn't work. But something did happen. The Q1 transistor began to heat up and smoke. Yikes! Now I dune it!

Anyway, I just hate throwing out a perfectly good timing light that just needs a simple repair.
Any chance someone can guide me through troublshooting of this board?

Thanx...Chris

PS: Parts breakdown diagram (got it from another thread in this forum)
1671994564458.png
 

MisterBill2

Joined Jan 23, 2018
18,909
If the flash tube fired a few times then that part was OK. If the transistor burned up then it must be replaced, after fixing what made it bur up. Which is probably a wrong connection or some wrong connections made when closing it up. And it looks like battery positive connects to the case directly. THAT assures that problems will arise. if either the transistor base or collector short to the case the damage will be fast and serious.
 

Thread Starter

cadman777

Joined Dec 23, 2022
39
Thanx MisterBill2.
Thing is, the PCB wasn't in the case when I tested it, so nothing was shorting against the case.
Also, I didn't mess up any of the connections, b/c I only de-soldered one side of each cap before testing, and then re-soldered it back where it was afterwards.
So:
1. How do I size the transistor (or is it a mosfet?) b/c it's no longer available and I can't find it on the internet anywhere and don't know how to find an equivalent based on the part number?
2. How do I troubleshoot what cased it to burn up?
3. Is it possible that the board had to be in the case before testing it, which caused the component to burn-up?
4. Physically, the transistor is nowhere near the case, so I can't see how it can short out that way.
5. One thing that concerns me is there is no heat sink for the transistor, b/c it's basically riveted to the board and soldered in place.
This doesn't look like a very good design, but what do I know?!
Thanx for your help...Chris
 

MisterBill2

Joined Jan 23, 2018
18,909
OK, and I was thinking that the thing was all put back in the case, and the case closed up.
In the era that the timing light was produced, probably mosfets were not yet common , nor cheap enough. The transistor may have a type number visible on the case. What is shown on the circuit drawing is a medium power NPN transistor. What made it burn up was excess current from collector to emitter. That was probably caused by excess bias current to the base.
A guess as to what caused the excess base bias would be leakage in C3, the 4 mFd polarized capacitor in series with the 4.7 ohm resistor, which would not limit the base current very much, or less likely the 0.47 capacitor, C1 in series with the 47 ohm resistor.
But a breakdown of C5, the 2 mFd capacitor on the high voltage side could have reflected an overload, possibly.
So my suggestion is to hold onto the timing light for a while because somebody else may have more information on the transistor. My other suggestion is to use a power supply with current limiting for your testing. 3 amps and 12 volts can provide enough power to do a lot of damage.
 

Thread Starter

cadman777

Joined Dec 23, 2022
39
Thanx for the details MisterBill.
I probably should've just replaced all the caps before trying to fix it the cheap way.
So now my options are limited.
In the past I've done a total cap re-place on PCM units from cars of the 80's which revived them.
So maybe that would have worked on this little unit?
I just have to make sure I get the right kind of cap for each one in this unit.
Also, do you think it's possible that I destroyed any of the other components, such as the diodes and resisters?
Thanx...Chris
 

MisterBill2

Joined Jan 23, 2018
18,909
Replacing the transistor will be the first step, and I suggest replacing those two capacitors also. Limiting the current is very important to avoid a replay of the destruction. Variable resistor R3 sets the bias and if it had changed during handling that could have contributed to the problem. R2 plus R3 set the DC bias level while C4 plusR4 set the feedback. Basedon the low values of resistance it seems that the original transistor had fairly low gain. Hopefully iit will be possible to find an actual type number for the transistor.
 

MrChips

Joined Oct 2, 2009
30,920
I agree with the analysis provided by Bill.
I cannot locate my timing light to give you a definitive answer. Try a TIP31 transistor for Q1 for starters.
Show us photos of both sides of the board so that we can confirm the pinout of Q1.
Replace both C1 and C3. Check for shorts in your solder joints.
 

Thread Starter

cadman777

Joined Dec 23, 2022
39
Thanx guys.
The thing that makes this kind of project a PITA is the wait for shipped parts.
Used to be I could drive 5 minutes down the road to a RadioShack and get the parts that I need.
If they didn't have it I could go an extra 1/2 hour to the local electronics parts supply house.
The other kick in the pants is I can't remember basic electronics that I studied 40 years ago.
So it's like starting from scratch.
I'll give the TIP31 a try and the caps you mentioned.
I'll also check for shorts in the solder joints.
Thing I don't get is, how can C1 & C3 be bad if they checked out good with the DVM?

1672082278094.jpeg

1672082380548.jpeg
 

Thread Starter

cadman777

Joined Dec 23, 2022
39
Was just thinking about what to do with it.
Thanx for the good advice.
I may have found a local supplier of old electronics components.
When checking solder joints, other than poking before and after the joint and doing a resistance check, is there a better way to do it?
 
Last edited:

MisterBill2

Joined Jan 23, 2018
18,909
The failure mode I am suspecting in the two capacitors is some degree of high voltage breakdown. I doubt that your meter tests for that condition. The test is to apply a higher voltage and measure the leakage current at the higher voltage level. So doing the test requires a higher voltage source.
 

Thread Starter

cadman777

Joined Dec 23, 2022
39
That's what I figured.
Is there a procedure for doing that using a DVM, or do you need one of those special testers for that?
 

MisterBill2

Joined Jan 23, 2018
18,909
The test can be done with a voltmeter , a suitable variable voltage source, and a resistor. The variable voltage can be simply a fixed voltage supply and a variable voltage divider ( a potentiometer)
 

PaulEE

Joined Dec 23, 2011
474
Gents,

Perhaps these pictures of a more modern (but just as primitive) timing light will help.

It looks like pressing the button causes an oscillator to generate the hundreds of volts to run the light, and the SCR is used to jerk the trigger transformer and pulse the strobe.

It appears that this light I'm attaching pictures of works the same sort of way. There are more recent P/N's that may help you...if indeed the transistor or SCR are the culprits.

If you hook this to 12V and press the button, do you get a few hundred volts downstream on the capacitor on strobe side of transformer?

The SCR is an MCR-100-8 and power transistor is an MJE3055, which is similar to the TIP31 suggested above. The SCR is apparently a Motorola part, which is long gone availability-wise, but there are alternates if you do a Digikey or Mouser search. SCRs are alive and well, just not this particular P/N... S6X8ES or BT169G,112 may be possible alt's, but I DID NOT REVIEW DATASHEETS. They came up in Mouser search and appeared to have the same max voltage rating of 600V. FYI.

Good luck, hope it helps.

Paul
KI5VNH
 

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Thread Starter

cadman777

Joined Dec 23, 2022
39
MB2, thanx for the info. I have all of those. I have a BK, a variac with a home-made rectifier (runs my drill press w/a Baldor DC motor that sounds like a jet engine!), and another heavy one from the 50's. I also have plenty of POTS, not sure if they're high enough voltage rating. Anyway, if you can link me to a procedure I'll read it. Like I said, I'd been a while, and I'm losing my memory these late years in life.

PaulEE, thanx for the info. Your Actron looks like it's even simpler than mine. You sure it's not a raygun? ;)
I used to be able to create simple circuits and do all the calcs on them, but who can remember that. Don't use it, you lose it! Anyway, I've been on DigiKey and Mouser doing just the thing you said. I also downloaded a Spice app and am trying to model these circuits to see what the V, I and R values are at various points. That's the only way I can think of to figure out the specs on replacement components. Thanx for giving me a head start!
 

MisterBill2

Joined Jan 23, 2018
18,909
Probably a 1000 ohm or 5000 ohm pot can serve as an adequate voltage divider to do initial testing. Any powr supply without current limiting can certainly do damage if something is not quite right.
 

Thread Starter

cadman777

Joined Dec 23, 2022
39
This is how I'm going to test b/c I have a BK FG & Scope.
Now that I have a test for caps under load, the next thing is testing diodes and transistors the same way...one component at a time.
But the end of this, I'm going to have a 1000 dollar timing light (in labor that is)!
 

shortbus

Joined Sep 30, 2009
10,046
And you may even find out it was the zenon flash tube that failed. Even though someone said they don't fail in this thread, it doesn't match what the flash tube makers have to say about them. They can and do fail, mostly due to age.
 
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