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TAF Flasher Blowing F111 Fuse because Flashers 19a & 19b are Locked On on bootup


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Question

My TAF recently started blowing fuse F111 which based on the manual for Flashers....

 

As I start to look at this I thought I would post it here so I can potentially avoid screwing something up but also someone else might use it in the future...

 

I have fixed several flippers involving faulty diodes/transistors/predrivers but never had the pleasure of fixing flashers.... I dont think it would be much different but thought I would check in to see if there was anything I am missing or should know about before plugging in the soldering iron.

 

On replacing the fuse and observing the machine I noticed that 2 of the flashers (19a & 19b) on the machine engaged and locked on before any other lights or flashers when turning on the machine (just before the fuse would go)... (Was using the Resettable Thermal Fuse tool to save fuses... Link to Thread on Ressetable Thermal Fuse if interested)

 

 

To save anyone looking at this time I have collected the pics and screenshots from the manual to make commenting easy.

Click on the pics to enlarge

FlasherIssue1.thumb.jpg.02c943ecb92430f84db7101168a84bbd.jpg FlasherIssue2.thumb.jpg.f6007376d1d0564b8ff0041f2a9e80c5.jpg

 

Diagnosis steps done thus far:

  1. I have now unplugged the respective Flasher LED's (19a & 19b) and the fuse F111 stops blowing... result but now need to solve the issue why those flashers are causing the fuse to blow.
  2. I looked for a short to on the wires for the flashers (did not see anything)
  3. I have done a basic continuity check on the transistor Q38 (checked there is no short between any of the legs) but it seems to be fine... but based on previous experience a transistor can still be fried and does not fail basic continuity check.

 

 

I planned to replace the Q38 transistor but before I do that is there anything else I should check or do?

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Seems likely that your driver transistor is toast and permanently on. It's easy to find out whether that's the case (no need to unsolder anything). With the machine off, use your multi-meter set to Ohms and measure one of the the transistors that you know is good. Get a reading of all possible pairs of legs both with forward and reverse polarity of the leads. That gives you six readings. Write them down, and repeat with the suspect transistor. It should read very much the same. If it doesn't, you have found the culprit.

 

Seeing that the fault disappears when you remove the flasher LEDs, that indicates that there isn't a short in the wiring, so the transistor really is the only other thing left, unless there is a problem with your MPU that causes it to incorrectly turn on those flashers permanently. But, if the machine is playing fine otherwise, that would seem to be unlikely.

 

Michi.

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Your problem is most likely the driver transistor or its pre-driver transistor. The attached diagram shows the complete chain. You can measure this with power on and the flashers involved removed.

 

Measure the base of the 2N5401 it has the 47R and the 4K7 resistors attached to it - the left most leg) - using a meter set to DC volts and with the black lead on ground this point should read high = 12V I'm guessing, but certainly not 0~3 volts.

 

Next in the chain is the point marked "B" in that schematic which is the base of the output transistor. This should measure low or as close to ground as doesn't matter, certainly under 0.5V.

 

Next is point "C" which should read 50V (but more likely about 70V as this power rail isn't regulated and is unloaded).

 

Those three measurements will tell you exactly where your fault is without removing any boards or desoldering anything.

 

 

 

 

758794540_ScreenHunter_242Feb_0509_45.jpg.0055c5668c80bf4acd32397bb2a24c37.jpg

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Your problem is most likely the driver transistor or its pre-driver transistor. The attached diagram shows the complete chain. You can measure this with power on and the flashers involved removed.

 

Measure the base of the 2N5401 it has the 47R and the 4K7 resistors attached to it - the left most leg) - using a meter set to DC volts and with the black lead on ground this point should read high = 12V I'm guessing, but certainly not 0~3 volts.

 

Next in the chain is the point marked "B" in that schematic which is the base of the output transistor. This should measure low or as close to ground as doesn't matter, certainly under 0.5V.

 

Next is point "C" which should read 50V (but more likely about 70V as this power rail isn't regulated and is unloaded).

 

Those three measurements will tell you exactly where your fault is without removing any boards or desoldering anything.

 

 

 

 

[ATTACH=CONFIG]105194[/ATTACH]

 

Erm.. Long day was it Homepin? :rolleyes You posted & quoted voltages from the solenoid, not the flasher circuit diagram.

 

Point A = No more than Vcc (approx +5v)

Point C = Between 30-34VDC (& only if the flasher bulb is in circuit)

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Erm.. Long day was it Homepin? :rolleyes You posted & quoted voltages from the solenoid, not the flasher circuit diagram.

 

Point A = No more than Vcc (approx +5v)

Point C = Between 30-34VDC (& only if the flasher bulb is in circuit)

 

The flashers are driven the same way from the same direct circuits

 

Flashers are not part of the lamp matrix.

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Flasher power is a nominal 20vdc but could get slightly higher

Power is at the bulb socket all the time waiting for the transistor to ground to light the bulb(like a coil)

Without studying the diagram isn't F111 on the AC side of the bridge?

I would be checking the bridge out

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The flashers are driven the same way from the same direct circuits

Yes, they are physically & electronically the same circuits & they both switch to ground.

BUT, the flashers are supplied power from the +20V rail, not the +50V rail.

 

Flashers are not part of the lamp matrix.

I never said they were part of the lamp matrix. Just that it wasn't part of the solenoid circuit.

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Flasher power is a nominal 20vdc but could get slightly higher

Power is at the bulb socket all the time waiting for the transistor to ground to light the bulb(like a coil)

Without studying the diagram isn't F111 on the AC side of the bridge?

I would be checking the bridge out

Yes, TP7 is marked 20V, but under a no-load condition you will measure within the range that I quoted.

This no-load condition boils the guts within C11, slowly degrading it's capacitance,

placing more & more load on BR4 when the flashers are active & eventually killing it.

Though, I wouldn't go chasing BR4 just because F111 is blowing. The fuse is likely to be blowing because the flash lamps,

instead of being pulsed, are locked on & presenting enough of a load to do so.

Replacing the 2N5401 & TIP102 transistors in the offending circuits will most likely be enough cure JasonJoe's problem.

TOO MANY people go chasing bad caps & bridges when the problem is usually much, much simpler.

Not saying that it's impossible, Williams did f### up this particular circuit because they used a 25V cap instead of a 35V

for C11 (Which they corrected in the WPC95 series), but not probable in this instance. ;)

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Yes, TP7 is marked 20V, but under a no-load condition you will measure within the range that I quoted.

This no-load condition boils the guts within C11, slowly degrading it's capacitance,

placing more & more load on BR4 when the flashers are active & eventually killing it.

Though, I wouldn't go chasing BR4 just because F111 is blowing. The fuse is likely to be blowing because the flash lamps,

instead of being pulsed, are locked on & presenting enough of a load to do so.

Replacing the 2N5401 & TIP102 transistors in the offending circuits will most likely be enough cure JasonJoe's problem.

TOO MANY people go chasing bad caps & bridges when the problem is usually much, much simpler.

Not saying that it's impossible, Williams did f### up this particular circuit because they used a 25V cap instead of a 35V

for C11 (Which they corrected in the WPC95 series), but not probable in this instance. ;)

 

If only the transistor is locked on then wouldn't that just cause the lamp permanently on?

Why would a light permanently on blow a fuse?

You would expect this from a coil circuit where the coil gets hot and becomes a dead short but not with a bulb

There must be something else going on here

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If only the transistor is locked on then wouldn't that just cause the lamp permanently on?

Why would a light permanently on blow a fuse?

You would expect this from a coil circuit where the coil gets hot and becomes a dead short but not with a bulb

There must be something else going on here

Just because they are lamps doesn't mean they can't draw enough current to blow the fuse.

Remember, they are a 12V lamp running on a 20-plus volt circuit & as a result & will be drawing over twice their normal current.

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Yes, they are physically & electronically the same circuits & they both switch to ground.

BUT, the flashers are supplied power from the +20V rail, not the +50V rail.

 

 

I never said they were part of the lamp matrix. Just that it wasn't part of the solenoid circuit.

 

They are EXACTLY a part of the solenoid circuit apart from the supply voltage.

 

Anyone who is unable to determine this shouldn't be touching the machine as they are bound to make things worse. This was the first drawing I came across that showed (essentially) how the driver circuit works.

 

There are far too many people muddling about with electronics these days that really should be leaving things alone. I read a new thread or post almost daily where another perfectly repairable circuit board bites the dust - makes me shake my head.

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I followed the steps laid out by @Michi to test the transistors and below are the readings.

 

FlasherTransitorTesting_Ohm_Readings.JPG.5f4b0660da157b0c497796e9ebf4bb59.JPG

 

As originally thought and confirmed by the readings on the testing the Q38 transistor was fried....

 

I replaced Q38 TIP102 (whenerver I replace transistors I normally also replaced the pre-driver in this case Q37 (2N5401) but I only had 2N4401's so left the Q37) and the issue has been solved.

 

What I have learnt:

 

  • Solving flashers transistors issues are no different (possibly a little easier) to those driving coils...
  • Locked on Flashers rather than pulsing Flashers can blow fuses
  • I learnt and may seem obvious to others but thought I would share is when testings the transistors you need to test transistors which drive similar things, in my case Flashers because I tested all the transistors and the readings are constant for the transistors driving similar play field parts but get quit different readings for those driving other types of items like motors, coils or magnets... eg Q26 transistor (drives Thing Motor) & Q32+Q34 (drives the Magnets) should have very different readings to Q36, Q38, Q40 & Q42 which in the TAF drive the flashers.

 

Thanks @Michi @Homepin @sparkythepinmonkey @Boots

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Glad to hear that the mystery is finally solved! :)

 

I learnt and may seem obvious to others but thought I would share is when testings the transistors you need to test transistors which drive similar things, in my case Flashers because I tested all the transistors and the readings are constant for the transistors driving similar play field parts but get quit different readings for those driving other types of items like motors, coils or magnets... eg Q26 transistor (drives Thing Motor) & Q32+Q34 (drives the Magnets) should have very different readings to Q36, Q38, Q40 & Q42 which in the TAF drive the flashers.

Yes, when measuring in-circuit, you are measuring the load that is switched by that transistor in parallel to the E-C path, which changes the reading depending on polarity (with an LED), or resistance (with a motor or coil). But, as you found out, it works fine if you have another working circuit with the same load.

 

Cheers,

 

Michi.

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