(Topic ID: 150315)

Anyone try MOSFETs in lamp columns on DE?

By johnwartjr

3 years ago

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  • 12 posts
  • 7 Pinsiders participating
  • Latest reply 3 years ago by flip78
  • Topic is favorited by 11 Pinsiders


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    #1 3 years ago

    I've added mosfets to the lamp columns in WMS boards from Sys 3-11 with zero problems.

    My process involves removing the current limiting resistors and installing zero ohm jumpers in their place, and then replacing the TIP42s with IRF9Z34 mosfets.

    The purpose of this - if you aren't familiar, is to reduce the heat on the board. In extreme cases, the 27 ohm resistors bake the board to the point where the solder crystalizes, the boards burn, and the resistors even fall off in rare cases.

    Never a problem on a WMS board, but I did my first Data East board last night, as I had one come in with a really baked lamp column section.

    Well, it didn't go so well. As soon as the lamp matrix fired up, a couple of the 6427s exploded.

    I double and triple checked my work. MOSFETS installed correctly. The board is identical to the WMS lamp matrix as used from Sys 3-11. But something has to be different - something I've missed perhaps.

    Worse case scenario, I can return to the original configuration, but this board has seen a lot of heat in that section, so I'd prefer to make the MOSFETs work.

    Any thoughts? I'm trying to get another DE MPU I have here to boot, so I can take some voltage measurements and compare them against a working sys11 board. If the 2n6427s are exploding, I have to think I'm getting voltage back fed somehow.

    My work is clean. I didn't find any solder bridges, or other mistakes.

    #2 3 years ago

    Interesting.......and the MOSFETS are still okay? Personally I find this mod a little bit tricky because applying a 18V Vgs to the mosfets is very close to their absolute maximum ratings of 20V. As an electronics designer I wouldn't do it like this. It is to bad the 2K2 pull up resistor is at the "wrong leg" of the 27 Ohm resistor, otherwise it would have been easy to add a 2nd resistor as a divider. Looking at the schematics I can't see why it go bad here, the only reason I see at the moment would be a failing MOSFET.

    1 week later
    #3 3 years ago

    It is a rainy Saturday evening here….. No need to go outside. And after critisizing the MOSFET modification a few times it is a nice evening to experiment with a MOSFET modification in a way I should do it. Victim is the Data East CPU board from my Time Machine.

    Problem with the modification as it is done by most people is the fact that the FET's are driven by a 18 Volts Source to Gate Voltage which is dangerously close to the absolute maximum rating of 20 Volts. This also counts for Williams driverboards. This is why I never performed the MOSFET modication like the way it is “advertised”. Typical, these MOSFETs are driven by about 10V Vgs (or -10V for P-Channel MOSFET's which we are using here).

    Below some schematics. On the left the result of the modification as the way it is done by most people. On the right the way I should do it. With 2 1K Ohm resistors which divides the 18V in 2. Now the MOSFET is driven with a 9V Vsg. This is no rocket science


    Unfortunately the original 2K2 pull up resistors are connected to the wrong side of the 27 Ohms base resistors by design. We have to improvise a little bit here.

    What we need for this modification? Well, 8 P Channel MOSFET's like IRF9530 (which I used) or IRF9Z34N. We also need 16 1K Ohm resistors, 0.25W.

    This is how I performed my modification:

    – Remove the TIP42 transistors, the 27 Ohm base resistors and 2k2 resistor network RA8

    – In stead of the 27 Ohm resistors, install 1K Ohm resistors

    – In stead of the TIP 42 transistors, install the P-Channel MOSFET's.

    – At the solder side, install eight 1K Ohm resistors between the gates from the mosfets and the 18V. The upper resistor I installed at the component side between the Common connection from the resistor network that was installed before and the Gate. See pictures below. I am sure there are other ways to add these extra resistors. This seemed to me a reasonable solution.



    A little bit excited i reinstalled my CPU and turned my game on......success!! No smoke or technical odeurs... Just a beautiful light show. Very satisfying


    #4 3 years ago

    Another remark about replacing the 27 Ohm resistors by 0 Ohm resistors or wires as most people do when doing the MOSFET modification. I think it is better to replace them by 1K - 1K5 resistors to avoid de-headed 2N6427 transistors or even more if one of the MOSFET's fails for some reason. Even if you use a MOSFET with a wider Vgs range or if you insist to perform the modification in the "traditional" way without a voltage divider.


    #5 3 years ago

    Nice work Marco. Mind if I add this to the PinWiki, with credit of course?
    Chris Hibler - CARGPB #31
    http://www.PinWiki.com - The new place for pinball repair info

    #6 3 years ago
    Quoted from ChrisHibler:

    Nice work Marco. Mind if I add this to the PinWiki, with credit of course?

    Go ahead Chris, no problem


    #7 3 years ago

    Rainy Saturday evenings can have their advantages.

    #9 3 years ago
    Quoted from johnwartjr:

    Nicely done! It does make sense!

    Maybe to you. But a mouth breather like me is completely lost. If it reduced heat and didn't blow anything up, we'll done!

    1 week later
    #11 3 years ago
    Quoted from MarAlb:

    No smoke or technical odeurs...

    Technical odor, I really like that term. You never want to experience them, but that is a great way to describe them.

    1 week later
    #12 3 years ago

    Good Job Marco !

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