SST - flashers on permanently.

[Jetsetron]

I haven’t used my SST for months, just changed the batteries on the motherboard and turned it on for first time in a while.

Four of the flashers seem to be stuck on, tho the game plays fine. When I try cycling them in the flasher menu, they still stay constantly on. Any ideas what could’ve happened? Many thanks.

 

[Jetsetron]

It’s actually now blown one of the flashers, which melted the plastic cover!

 

[Spandangler]

Transistor will be locked on I expect. Don’t keep it on with flashers locked, they get bloody hot and like you’ve found out, they melt stuff!!

 

[Jetsetron]

Thank you, what’s the best course of action? I think I’ve located them on the board for the flashers. Is it home serviceable?

 

[Garry Sp8]

First thing I would do is (power off) disconnect and reconnect all the ribbon cables. I’ve had flashers stuck on which are switched via the little 8 driver board. Disconnect and reconnect the cables on that too.
I’ve had a flasher stuck on on TZ without realising and it started to burn the plastic above - found out because of the horrible smell!
Also had a stuck flasher on Roadshow - again fed via the 8 driver board.

 

[Jetsetron]

Thanks, I guess you mean unplug/plug back in the J plugs and ribbons? Which is the little 8 driver board? I have the IO power board, CPU sound board, and Display Power Supply in the head.

 

[Garry Sp8]

Sorry - my bad! I thought it was STTNG you were talking about. Different game.

 

[Garry Sp8]

Are the flashers on the same output? As said - could be a locked on transistor.

 

[Jetsetron]

Are the flashers on the same output? As said - could be a locked on transistor.

All seem to be off this J plug. I unplug it they all go off.

 

[Spandangler]

You’ll have to find a manual and find out the associated Driver driver transistor. Check with a meter for a short. It’s definitely doable. I can do it and I’m rubbish. Parts readily available from all good book shops!!!

 

[Garry Sp8]

According to the manual - 4 yellow flashers are switched via transistor Q26 so could be that transistor has failed and locked on. Alternatively one of the others that serves multiple flashers - Q25 to Q31.
Do you have a multimeter to test them?
Alternatively you could take the flash lamps out and live with those 4 not working until you can get it fixed.

 

[Jetsetron]

Thanks guys, I do have a multimeter. I see each transistor has 3 legs, any advice on the multimeter settings and which leg to test?

Many thanks.

 

[Garry Sp8]

With power off. Put meter on diode test. Put negative lead on centre tab or leg, then put positive lead on outer tabs and you should get a reading between 0.4 and 0.7 V dc. If you get a bleep or zero reading then it’s shorted.
Above assumes it’s a TIP 102/122 type transistor.

 

[Garry Sp8]

 

[Jetsetron]

Thanks Garry, very helpful! They are TIP122’s. I ran the test as you suggested. Getting readings of 500’s for the upper leg/centre, and 600’s for the lower leg/centre. Apart from Q26. The upper/centre reading voltage plummeted and the multimeter beeped.

I guess that means Q26 diode is faulty? Is it a case of remove the circuit board, and replace it? I’ve changed a few circuit board pins before.

Thanks again for the help!!

 

[Garry Sp8]

Yes that transistor will need to be replaced. Also test the other components in the same circuit to ensure they are in spec - resistor and diode.
I can send you a replacement transistor if you don’t have any .

 

[Jetsetron]

Sorry to be a pain, how do you test the resistor/diode? I see the diode is next door to the transistor.

Do any TIP122’s work? I’ve seen them cheap on eBay. Otherwise your off to send one is very kind, thank you!

 

[Garry Sp8]

Same test on diode as transistor with neg lead on banded end of diode - should get no reading with leads reversed.
For resistor - meter set on ohms with leads at each end of resistor - should get reading near to resistor value.
Yes any TIP122 should work but can't say whether cheap ones any better or worse than others. Always good to have a few spares.

 

[Jetsetron]

Cheers I’ll test those parts and I guess I’ll be getting the circuit board off. Had a bit of a read and apparently the TIP122’s are redundant, and the TIP102 is the modern replacement, does that sound correct?

Thanks for all the advice!

 

[Garry Sp8]

Yes that’s right.

 

[Jetsetron]

If I’m correct this is the flasher circuits. Tested D208-D215, and R25-R32. All give the same readings as their neighbours. The resistors next door to the transistors, R208-R215, give no readings (any of them) on the multimeter. Not being very electrics savvy, I guess that’s normal if they’re all the same?

R26 and D209 are on the Q26 circuit and gave correct readings.

 

[Garry Sp8]

Should be OK just changing the transistor then .

 

[Jetsetron]

Really at the end of my tether. Fitted the new TIP102, and the flashers worked perfectly for 5 minutes, then a neighbouring flasher circuit transistor locked on.

Is this just utter bad luck, or related to the install of the new transistor, or could there be a deeper fault?

Help!!

 

[Jetsetron]

Now Q26, 28, 25 and 27 are all giving bad readings. The 4 on the right

 

[Garry Sp8]

Oh dear you're really having bad luck with this.
Have you checked the voltage onto the flashers - there should be 20V DC onto each (orange wire).
Looks like all those transistors are controlled from the U4 LS74157 chip - maybe that is causing the problem. I wouldn't know how to test it - a bit outside my comfort zone now - sorry.

 

[Jetsetron]

Oh dear you're really having bad luck with this.
Have you checked the voltage onto the flashers - there should be 20V DC onto each (orange wire).
Looks like all those transistors are controlled from the U4 LS74157 chip - maybe that is causing the problem. I wouldn't know how to test it - a bit outside my comfort zone now - sorry.

Thanks Garry, you have been very helpful. I have spoken to a repairer who has also mentioned the U4 chip. I’m testing each individual lamp (under instruction) and I’m sending the board off to be repaired. I had a go, and I learned something!

 

[Moonraker]

It certainly suggests U4 but have you tried re-seating both ends of the ribbon cable (half a dozen times each end) that connects the MPU to the driver board? I would try that first before sending the board away. It does mean you have to replace each blown transistor first... and it may well be too late for U4 anyway but I think it's worth a try.

 

[Jetsetron]

I’ll give that a go to, it’s been disconnected a few times, but only on the IO board end. Is that cable capable of causing that much trouble?

 

[Spandangler]

Ribbon cables are the route of all evil. They can cause the strangest of things.
I once had a leaky radiator in my lounge.
Reseated the ribbon cables on my wh20 and it fixed the leak.
Miracles they are

 

[Moonraker]

The ribbon cable sends the signals from the MPU to U4 telling it when to and when not to operate...