(Topic ID: 205325)

FQP17P06 MOSFET - Williams Column Driver Replacement

By Platypus

6 years ago

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Latest reply 6 years ago by CactusJack
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I am thinking of replacing the lamp column driver TIP42's on my 1986 System 11 Space Station. The associated 27K ceramic resistors show considerable charring on the board. I understand that these mosfets reduce the heat on the resistors which sounds like a worth while improvement.

After doing a bit of research I am uncertain of what to do about the resistors (R82-R89).
John Wart suggests replacing with 0 ohm resistors or wire jumpers.
barakandl recommends replacing with 1/4w resistors in a low ohm range. Like 22-270 ohm to maybe absorb a blow if some oh shit thing happens
Chris Hibler has a more complicated fix to reduce the voltage in the mosfet by using two resistors (one on the back of the board) See pinwiki
Others just leave the ceramic resistors

I respect the opinions of all of these pros which puts me at a quandary.

Are there any other opinions or relevant suggestions out there?

The way the mosfet works the resistor is not that important. I still think you want a resistor there to limit current if something bad happens. Last board i did was for my Dr Dude and i used 1K 2W resistors. 2W just because they fit better. Looking at the gate pins with the scope the voltage swings between 0 and 18v typically in a square shape. The concern is the gate voltage compared to the source is close to the part's limit of 25v. I have had never had a problem with any williams games and done hundreds of of the earlier driver boards with leaving in 27ohm resistors or replacing it with a similar value.

There has been people with issues of the 2n6427 pre drivers shorting out when used in data east games that have a similar/ same circuit as wms system 11. I suspect the 18v runs higher in those games to the or there is some other kind of factor at work. These games are the ones the resistor divide circuit is probably needed on.

Also you have to be very careful with transistors. Transistors fakes are usually are similar part with worse ratings. Transistors seem to be one of the most commonly faked item, so be careful about buying them. I wonder if some of the reported issues was caused by bad parts.

Quoted from barakandl:
The way the mosfet works the resistor is not that important. I still think you want a resistor there to limit current if something bad happens. Last board i did was for my Dr Dude and i used 1K 2W resistors. 2W just because they fit better. Looking at the gate pins with the scope the voltage swings between 0 and 18v typically in a square shape. The concern is the gate voltage compared to the source is close to the part's limit of 25v. I have had never had a problem with any williams games and done hundreds of of the earlier driver boards with leaving in 27ohm resistors or replacing it with a similar value.

So the value of the resistor really isn't that important? the mosfets are not that hard to replace if one does fail

Quoted from barakandl:
There have been people with issues of the 2n6427 pre drivers shorting out when used in data east games that have a similar/ same circuit as wms system 11. I suspect the 18v runs higher in those games to the or there is some other kind of factor at work. These games are the ones the resistor divide circuit is probably needed on.

That's good to know.

Quoted from barakandl:
Also you have to be very careful with transistors. Transistors fakes are usually are similar part with worse ratings. Transistors seem to be one of the most commonly faked item, so be careful about buying them. I wonder if some of the reported issues was caused by bad parts.

Thanks for the heads up. That's why I buy most of my parts from you or from Ed at GPE. Thanks for your assistance.

It would be nice to get into a DE game that kills the fqp17p06 and pre drivers when using them as the lamp column drivers to try and see what is actually happening.

Negative voltage when the power supply collapses some how?? I wonder if a diode in series would help.

Is the +18v supply in a DE a lot higher than in WMS games?

I am not looking at the schematic but concerning reducing the gate voltage.

One way to do this is to use a zenor diode, but not in the traditional way with a resistor and the zenor limiting the voltage across itself. Instead you use the diode in series with the voltage source. You select a diode with a value of voltage that you wish to get rid of with the cathode pointing to the positive voltage source.

So, if you are at 18VDC and want to fall below the safer 15v gate rating, you can use a 3.9v zenor. The zenor begins to conduct when the voltage rises above 3.9.

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