Snap On timing light MT1261A - Page 1
Forum section: electronics / repair

author: Mjolinor
author_rank: Frequent Contributor
timestamp: April 08, 2019, 10:07:59 pm
content: Trying to mend this thing. The symptoms are that it will not flash when connected to a vehicle but it does flash if I stick a wire through the inductive pickup and step input a couple of amps so bulb and Xenon circuit, micro, rev counter and all the workings seem to be OK.The inductive pickup has three wires, red, black and white. White is connected to ground and white > red and white > black give me differential signals from the inductive pickup so I am assuming that part is OK.That leaves me with this CRL8645M thing, a yellow blob and that yellow stuff is HARD. Diamond wheel jobby to get through it.There was an eight pin DIL lump under the yellow so I carefully ground it away and it was as I expected, an LM393N dual opamp.Once I got access to the opamp pins I was able to buzz out a few things and it seems that the two outputs have a resistance of only one ohm between each other. This does not (to me) seem very sensible.So now I either try to remove more yellow rock to gain access to the circuit or I remove the opamp and buzz again. Opamp suitably discorporate and joined the dust that is ever present in our polluted planet and the one ohm is still there so it is in the circuit under the yellow rock.I didn't have a 393, only a 358. Looking at the datasheets they don't seem that dissimilar so I stuck one in and now no flashing at all.About what I expected to happen.The question is really, is that one ohm reasonable under any circumstance? I cannot think of one situation where a small, dual opamp like this would only have one ohm between two output pins.The output from this yellow rock goes off directly into the micro pin with just a couple of capacitors to ground as it travels the 10 inches or so from handle to micro.

author: fzabkar
author_rank: Super Contributor
timestamp: April 09, 2019, 05:38:47 am
content: The LM393 is a dual comparator, not an op amp.Is there a patent reference on the unit? The last Snap-On timing light I worked on had a full description of its workings in a patent filing.Is this the relevant patent?http://patentimages.storage.googleapis.com/pdfs/US4713617.pdfhttp://www.freepatentsonline.com/4713617.pdfhttp://patentimages.storage.googleapis.com/pdfs/USD292893.pdfIt refers to an "inductive pickup IC56 … of a type manufactured by Centralab Inc, and which operates as a comparator which triggers at a specific voltage". I'm guessing that the "CRL" in the part number is Centralab.Here is a circuit diagram from a different patent:https://patentimages.storage.googleapis.com/b3/1f/03/aba25bb9b21cda/US5767681-drawings-page-6.pnghttp://patentimages.storage.googleapis.com/pdfs/US5767681.pdf

author: Mjolinor
author_rank: Frequent Contributor
timestamp: April 09, 2019, 08:09:24 am
content: Quote from: fzabkar on April 09, 2019, 05:38:47 amWell D'oh.Cheers for the links. Lots to peruse. Sadly this yellow thing is just shown as a box but having pointed out that it is a comparator it should not be too difficult given that it has a 5 volt supply only I will stick some 5 volt square waves up it and make sure all is good before I make something to replace the yellow brick.

author: fzabkar
author_rank: Super Contributor
timestamp: April 09, 2019, 09:54:33 am
content: The circuit diagram in the second patent shows what is probably inside the hybrid device. In fact the author of that patent claims it is a better design than Snap-On's. :-)

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 21, 2019, 02:39:02 pm
content: Quote from: Mjolinor on April 09, 2019, 08:09:24 amDoes reply work if I quote?

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 21, 2019, 02:42:28 pm
content: Quote from: Mjolinor on May 21, 2019, 02:39:02 pmI give up. I took all the yellow stuff off and wanted to post a picture for anyone else that wants to know but the forum won't let me.Interestingly 1 (output A) and 7 (output B) and connected directly together by a trace on the PCB. That makes no sense to me. Hmm, open collector outputs.

author: james_s
author_rank: Super Contributor
timestamp: May 21, 2019, 05:13:38 pm
content: There is a max size limit on photos so you may have to resize first.Tying outputs in parallel is a very common way of getting more current than a single output can provide. It is also done sometimes just to keep an unused element from floating and misbehaving.

author: floobydust
author_rank: Super Contributor
timestamp: May 21, 2019, 05:26:58 pm
content: I find the inductive pickups get dropped and the ferrite core cracks, or dirt gets in and the jaws don't fully close and leave an air gap.The timing light works but sensitivity is poor and things don't trigger.Also, most pickup coils are polarized so clamping them on backwards (spark plug end) and the light does not trigger well.I look at this stuff before blaming the electronics.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 22, 2019, 12:49:42 pm
content: Quote from: floobydust on May 21, 2019, 05:26:58 pmI did that before I even took the screws out. Ferrites are good and signal is good from the pickup.Try the picture again:

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 22, 2019, 12:58:59 pm
content: Pin 1 is top left. What do you reckon that thing between pins 5 and 6 is. Assuming all the resistors were laid with the copper on the ceramic (gotto love those tuning cuts on the resistors) then it can only be a capacitor in which case the resistance of zero ohms could be a problem.By the way, it is stil not working with a new comparator fitted.

author: floobydust
author_rank: Super Contributor
timestamp: May 23, 2019, 02:21:47 am
content: Sketched a schematic of the module, most of it makes sense... numbering it's pins from left to right 1,2,3,4 - 5,6.I'm not sure of the module pins- pin 3 traces I couldn't see and pin 6 the output doesn't seem to have a pin soldered to it.LM358 would not work because the two outputs (IC pin 1,7) are tied together and contention would happen.LM393 comparator is open-collector to OR the two comparator's output.A magnetic pulse just seems to just trigger either comparator, one for N and the other for S.edit: updated the schematic to avoid confusion.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 23, 2019, 05:44:54 am
content: This is what I got. I removed the shorted cap (?) between 5 and 6 and it blew the comparator. It started drawing 1 amp. Presumably it was oscillating. I can't get a sensible measure for the capacitor between 2 and 3. I got 300 pF at one point but that seems pretty stupid to me.Pins 6 and 5 stayed on the main PCB. There is no R6 and R1 is the same as R4, down to deck. Otherwise the circuit seems correct which is a feat considering I hadn't removed all the yellow rock it was embedded in.I will draw it in Kicad today sometime.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 24, 2019, 04:20:04 pm
content: OK. This bit is now working. I stuck a 150 nF between 5 and 6 and it is fine now.Sadly it still is not flashing and as this signal goes straight into the micro I don't think I can do any more, the micro must be faulty. I may replace it with an ESP but will probably just stick it on ebay.Thanks for the help.

author: jmelson
author_rank: Super Contributor
timestamp: May 24, 2019, 05:05:02 pm
content: Quote from: Mjolinor on April 08, 2019, 10:07:59 pmNo. Most likely, the spark voltage jumped to the sensor coil and blew the comparator. It may have damaged other parts, too.Jon

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 24, 2019, 06:23:30 pm
content: Quote from: jmelson on May 24, 2019, 05:05:02 pmOpen collector output and thin film tracks.All as it should be in fact.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 27, 2019, 11:41:40 am
content: This is what I get. Quite clever double circuit so it does not matter which way round the clamp goes. I am now quite sure that the micro input is stuffed. I removed it and measured impedance to ground on every pin and that pin is completely open circuit so looks like the bond wire got fried or something. I will use an ESP to replace it I think.

author: fzabkar
author_rank: Super Contributor
timestamp: May 28, 2019, 01:07:34 am
content: FWIW, the patent refers to a TM7042 microcontroller:https://datasheet.datasheetarchive.com/originals/scans/Scans-002/Scans-0046016.pdf

author: floobydust
author_rank: Super Contributor
timestamp: May 28, 2019, 02:47:12 am
content: Quote from: Mjolinor on April 08, 2019, 10:07:59 pmThere's nothing to protect the comparator's inputs to the pick-up coil... zapI find Snap-On electronics are always weird and proprietary, and sometimes weak designs.TMS7000?

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 28, 2019, 01:13:37 pm
content: Quote from: floobydust on May 28, 2019, 02:47:12 amI am with you on that. It seems pretty stupid.I have actually built a new one, the old one suffered from being run over with my chair after I dropped it and being ceramic it is not take that well. I stuck some series resistors and zeners on the inputs that will hopefully stop whatever happened from happening again.

author: james_s
author_rank: Super Contributor
timestamp: May 28, 2019, 11:13:35 pm
content: I'm betting it's similar to the really poorly designed electronics in a lot of clothes washers and dishwashers and such. Snap-on probably has some very good mechanical engineers but electronics are not really their thing. It wouldn't surprise me if they contracted that stuff out to some guy in his garage, or if they had a ME on staff that knew just enough EE to come up with something that worked.

author: floobydust
author_rank: Super Contributor
timestamp: May 29, 2019, 04:40:05 am
content: When you see a thick-film module like that, you know it's a good concept to make it small and cheap but very high risk.The NRE cost is very high and the design has to be rock solid because there are high minimum order quantities.I've seen engineers do a SMPS with custom thick film modules and after getting 20,000pcs in, a design bug is found. They had to be tossed in the garbage bin and management never again tried any product development, they were so pissed off.Snap On is expensive, but no protection at the IC inputs, so too late to fix... Who even needs a MCU in a timing light? I guess it could pre-flash for longer duration but that still makes a blur.I did likeSilicon Chip magazine's automotive LED timing light May 2005. There are schematics of it floating around the web, it's free to look at the Circuit Notebook or next url they want money, confusing.This site ripped off the schematichttp://www.learningelectronics.net/circuits/automotive-led-timing-light_18.htmlPre-amp, one-shot and LED array driver. No xenon tube or HV PSU.It would be nice to permanently mount one under the hood of a muscle car, LED at the harmonic balancer.

author: techman-001
author_rank: Frequent Contributor
timestamp: May 29, 2019, 07:49:25 am
content: Quote from: floobydust on May 21, 2019, 05:26:58 pmAgree 100%. I've fixed hundreds of these things, most of the time it's a cracked ferrite, otherwise a cracked pcb or dry solder joint, sometimes a dead xenon tube.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: May 29, 2019, 12:24:37 pm
content: I reckon the inputs could be protected easily externally. the + input is biased to be slightly above 0 volts. The - inputs are run from the pickup. A simple forward biased diode at the pickup and it would work just the same, 0.6 ish being enough to flip the comparator.It has a micro so that you can have the engine revving at max advance but retard the flash so the TDC mark still lines up so you can check the advance mechanism in case you only have the TDC mark on the flywheel. As it happens my old BMW bike that I use it for has max advance marked on the flywheel anyway so no need for that facility. It should however also display revs on the timing light which is very useful.

author: 87V10
author_rank: Newbie
timestamp: November 03, 2019, 08:35:13 pm
content: HOw do you fix the cracked Ferrite? Is there somewhere that you can buy just the ferrite block?

author: techman-001
author_rank: Frequent Contributor
timestamp: November 03, 2019, 09:08:58 pm
content: Quote from: 87V10 on November 03, 2019, 08:35:13 pmI just replaced them with new ferrites from our stores but I imagine one could superglue the ferrite with a small loss of signal.Easy enough to try.

author: 87V10
author_rank: Newbie
timestamp: December 13, 2019, 07:00:37 pm
content: What stores did you get the ferrite cores?

author: Simon Spiers
author_rank: Regular Contributor
timestamp: September 26, 2025, 02:39:08 pm
content: I have a similar problem on mine. One of my opamps is open on the input I think. Swapping the black and red wire around from the pickup at least alows the light to work, but the pickup will only work in one direction. I might make a new hybrid on vero board, see if it works correctly again.