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Drop Target Transistor Issues - Data East Tales From The Crypt

Andy B

Andy B

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1 10 Years
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Oct 6, 2014
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Location
Hull. England.
Working on a Tales From The Crypt for a mate after he told me he could smell burning and I told him to switch it off!

The TIP122 (Q43) for the drop target bank had literally melted/exploded so I replaced that and the 2N4401 pre-driver (Q35) and the coil for good measure. Both transistors were testing good in circuit but when I took the board back and installed in machine the coil would not work in game or in diagnostics. It did work when I shorted the tab on the TIP102 I replaced the TIP122 with to the grounding strap. I brought the board back and have replaced the resistor R78, the one before the pre-driver transistor.

I didn't want to take the board back and test again without trying to get some reassurance that it would first (his arcade in is in the city centre and it's a 20 min drive each way and £2.50 in parking each time I go :) ).

Now I admit that I am lost as far as what I should be testing pre-transistor on a board to diagnose a coil fault so I tried basic stuff like using a multimeter to test continuity backwards from the other transistors and pre-driver transistors in that same group and compared to what I have with the transistor and pre-driver transistor for the coil that's not working and what I found was that:

  • The other 2N4401s in that group have a reading of 0.545 from the middle lead to the resistor array at RA10 whereas I get a buzz tone from the middle lead of the 2N4401 to the faulty coil to that resistor array.
  • I get a reading of 0.090 across each of the resistors in that group of resistors for the pre-drivers whereas I get a reading of 0.587 across the replacement resistor I put in. I am sure I have installed a correct replacement resistor as though the replacement resistor has 5 bands and the original 4, the colours are Blue/Grey/Black/Black/Brown on the 5 band and Blue/Grey/Brown/Gold on the 4 band which is 680 ohms for both right?

I have absolutely no idea what this means and am wondering if anyone can help with further diagnostics please?

Coincidentally (or maybe not) when I was testing the drop target bank coil in diagnostics I also found that the ball launch coil wasn't working and the coil sleeve on that one had melted along with the plastic end of the ball launch plunger so that one has gone also (new one ordered) and is driven by the transistor which is right next to the melted transistor I had for the drop target bank coil. Though the transistor for the ball launch tests good.

Thanks in anticipation of any help.

Andy
 
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I'd be testing the SN7408 chip that drives the coil circuits, 4J

If you have a logic probe, check pin 12 input and pin 13 output. They should follow each other, ie if input low, output should be low, high = high. The input should go high to activate the coil.
 
AlanJ said:
I'd be testing the SN7408 chip that drives the coil circuits, 4J

If you have a logic probe, check pin 12 input and pin 13 output. They should follow each other, ie if input low, output should be low, high = high. The input should go high to activate the coil.
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Appreciate that. I don’t have a logic probe but think it’s time to invest.
 
Andy B said:
Thanks.

Ordered this £15 one from Amazon:

https://www.amazon.co.uk/dp/B006IRIP8I/ref=pe_27063361_485629781_TE_item
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Did you order these at the same time:

Amazon.com: 4pcs. Motorola MC74F08N Quad 2- Input AND Gates FAST PDIP14 7408 74S08 74LS08 74F08 74AS08 74HC08 74HCT08 74BCT08 74ALS08 74C08 : Everything Else

Amazon.com: 4pcs. Motorola MC74F08N Quad 2- Input AND Gates FAST PDIP14 7408 74S08 74LS08 74F08 74AS08 74HC08 74HCT08 74BCT08 74ALS08 74C08 : Everything Else
www.amazon.com
 
Homepin said:
Did you order these at the same time:

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I did not. I’m totally new to this logic stuff. Comfortable with removing and soldering components. Understand and am comfortable with transistors but anything prior to that in the circuit I’m on a learning curve so don’t want to be ordering or swapping out anything until I know what I’m ordering 😊

Would these be suitable replacements (based purely on the fact that they have 7408 in the title!):

https://www.amazon.co.uk/5PCS-5pcs-...hild=1&keywords=7408+ic&qid=1626426542&sr=8-4



Thanks
 
Andy B said:
I did not. I’m totally new to this logic stuff. Comfortable with removing and soldering components. Understand and am comfortable with transistors but anything prior to that in the circuit I’m on a learning curve so don’t want to be ordering or swapping out anything until I know what I’m ordering 😊

Would these be suitable replacements (based purely on the fact that they have 7408 in the title!):

https://www.amazon.co.uk/5PCS-5pcs-...hild=1&keywords=7408+ic&qid=1626426542&sr=8-4



Thanks
Click to expand...
Looking at the schematic, yes, they would be perfect. Most other chips in the circuit have "LS" in the middle (IE 74LS244) but not the 7408's. Buy 14 pin sockets at the same time, you should always fit a socket when replacing an IC to aid fault finding. Sockets are dirt cheap.
 
Homepin said:
Looking at the schematic, yes, they would be perfect. Most other chips in the circuit have "LS" in the middle (IE 74LS244) but not the 7408's. Buy 14 pin sockets at the same time, you should always fit a socket when replacing an IC to aid fault finding. Sockets are dirt cheap.
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Thanks I most certainly will.
 
Those 7408 (and the 7406) chips seem to fail quite a lot. when the fail they can trash their input which often leads you to think the next part of the circuit up the line is faulty. so once you’ve got the logic probe, if the input correctly switches from low to high but the output doesn’t follow, then it’s the 7408 at fault.

but, if the input is always low (or always high) then just snip the one leg to the chip that is the input. that then allows you to figure if it’s that chip of the one before it which is the problem. the 7408 is driven by a LS273 chip (5H) so that could be the faulty one. It will be one or the other of those. by separating the two chips you can test the 273 chip output (pin 5) to see if it’s working correctly without it being affected by a potentially duff 7408.
 
AlanJ said:
Those 7408 (and the 7406) chips seem to fail quite a lot. when the fail they can trash their input which often leads you to think the next part of the circuit up the line is faulty. so once you’ve got the logic probe, if the input correctly switches from low to high but the output doesn’t follow, then it’s the 7408 at fault.

but, if the input is always low (or always high) then just snip the one leg to the chip that is the input. that then allows you to figure if it’s that chip of the one before it which is the problem. the 7408 is driven by a LS273 chip (5H) so that could be the faulty one. It will be one or the other of those. by separating the two chips you can test the 273 chip output (pin 5) to see if it’s working correctly without it being affected by a potentially duff 7408.
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Thanks for the explanation. Looks like I'm gonna have some fun with this logic probe when it arrives.
 
You probably already know this but when replacing a chip, you have already condemned the part so don't make any attempt to save or salvage it. The PCB is what is most important.

Remove the suspect chip by cutting off its legs and that way it's easy to remove the pins one at a time. I use a combination of de-solder wick and a puff bulb to clear the holes - many swear by de-soldering stations and I have owned them all from cheapos to Pace and Hakko's but braid and the bulb work best for me.
 
Homepin said:
You probably already know this but when replacing a chip, you have already condemned the part so don't make any attempt to save or salvage it. The PCB is what is most important.

Remove the suspect chip by cutting off its legs and that way it's easy to remove the pins one at a time. I use a combination of de-solder wick and a puff bulb to clear the holes - many swear by de-soldering stations and I have owned them all from cheapos to Pace and Hakko's but braid and the bulb work best for me.
Click to expand...
Great advice, thanks.

No, wasn’t planning to try and salvage the parts. If they’re found to be duff I’ll be removing and adding some sockets before replacing them.

Almost embarrassed to say I’d never heard of solder wick until you mentioned it. I’ve always just used an iron and a solder sucker in the past. Watched a few YouTube videos and looks like wick might help do a tidier job.

Thanks again.
 
AlanJ said:
Ive never had any luck using wick, the manual solder sucker seems to suit me best.
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I think it all comes down to individual experience and experiences. I've owned some of the (supposedly) best vacuum de-solder stations and they just didn't do it for me.

I cut pins and extract each one using tweezers and a standard iron. Then using a puff bulb I BLOW the solder in the hole through rather than trying to suck it out. This works WAY better. Then wick cleans up any splashes or leftover small bits.

I don't try and use wick to actually desolder parts - actually I almost never try to desolder a complete part, that risks board damage.

Solder suckers are very hit and miss. Some work OK for a while then they just don't, no matter how much you clean them out. I gave up on them as well.
 
Homepin said:
I think it all comes down to individual experience and experiences. I've owned some of the (supposedly) best vacuum de-solder stations and they just didn't do it for me.

I cut pins and extract each one using tweezers and a standard iron. Then using a puff bulb I BLOW the solder in the hole through rather than trying to suck it out. This works WAY better. Then wick cleans up any splashes or leftover small bits.

I don't try and use wick to actually desolder parts - actually I almost never try to desolder a complete part, that risks board damage.

Solder suckers are very hit and miss. Some work OK for a while then they just don't, no matter how much you clean them out. I gave up on them as well.
Click to expand...
I’m certainly going to give the wick a go - I’m no marksman when it comes to using a sucker - number of times I’ve tried to apply the sucker tip to the melted solder and fire the plunger and missed!

My biggest issue is holding the board while trying to suck the solder and pull the part. I’ve tried pcb holders but they’re either not wide enough (gap between the grippers) or not heavy enough to hold the board without toppling over.
 
Andy B said:
I’m certainly going to give the wick a go - I’m no marksman when it comes to using a sucker - number of times I’ve tried to apply the sucker tip to the melted solder and fire the plunger and missed!

My biggest issue is holding the board while trying to suck the solder and pull the part. I’ve tried pcb holders but they’re either not wide enough (gap between the grippers) or not heavy enough to hold the board without toppling over.
Click to expand...
The trick with wick is to (sometimes) add a little fresh solder to it and that starts the 'wicking'. It's all fudging really to find the method that works best for you.
 
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