Desperate help needed re IRFZ34N mosfet / tip 42

[alay1]

Hi all, I need some help please. I'm refurbishing my pinball machine, a pet project from a long time ago. I have a problem with the lamp circuit.
A quick explanation about what makes it ticks. 18V dc is supplied to the lamp matrix. To make a 6V lamp work, the PIA would switch the required matrix lines to ground 3 times a second.
A TIP42 transistor (see attached schematic - Q63, 65...) was used to drive the lamps, this would create so much heat that the associated 2.2K resistor would often burn out or unsolder from the board (see the picture in the schematic). A suggestion to fix this was to use a IRFZ34N mosfet instead. I had a look and this seemed a good idea so I purchased 8 of them (not cheap) and swapped out the TIP42's.
Here's the problem. When all of the lamps are driven at the same time, everything works fine. The lamps are nice and bright. When driven individually, they are really dim, as if there is only about 2V's going thru the circuit instead of 6.
I don't know what is causing this. All the voltage points check out ok. As I say, when everything is driven in game mode or diagnostics, all ok. When driven individually or by a row during the game, the lights are really dim as though the mosfet is not supplying enough current, ergo resistance issue?

Can anyone please help as this is driving me up the wall and I just can't work it out.

Thanks all

 

[kchriste]

The real question you should ask is why the transistors/resistors are burning up in the first place.
The basic problem is that you are trying to replace a PNP transistor with a N channel FET. You need a P channel FET instead. That said, it's not impossible to still use the IRFZ34Ns, but we need a better view of that schematic of yours because it is too small to see enough details. I assume it is the TOP row of transistors you have replaced? Also post a schematic of how you adapted the IRFZ34Ns in place of the TIP42s.

 

[alay1]

Attached is a pdf of the circuit, this should be clearer.
Sadly the design of the board by Williams at the time (1970's) showed it's true colours when the circuit was put under heavy load. There is not much one can do about it now except maybe this mod (or use cement resistors and keep an eye on everything).
Looking at the top of the design on the right hand side and reading downwards, it is Q63, 65, 67, 69, 71, 73, 75, 77 that was replaced with the FET's.
I haven't made any mods to the circuit, just a straight switch. I was told, and believed, that this would work in this application. Maybe we have got this horribly wrong somewhere although it does "almost" work as I said before.

thanks

 

[mvs sarma]

Attached is a pdf of the circuit, this should be clearer.
Sadly the design of the board by Williams at the time (1970's) showed it's true colours when the circuit was put under heavy load. There is not much one can do about it now except maybe this mod (or use cement resistors and keep an eye on everything).
Looking at the top of the design on the right hand side and reading downwards, it is Q63, 65, 67, 69, 71, 73, 75, 77 that was replaced with the FET's.
I haven't made any mods to the circuit, just a straight switch. I was told, and believed, that this would work in this application. Maybe we have got this horribly wrong somewhere although it does "almost" work as I said before.

thanks

Hi, while i am still trying to analyze, i have basic doubts

1. you were trying to replace a PNP transistor with N-channel MOSFET. How far this would be appropriate-- we have to see.

2. Can you put the part of schematic of lamp matrix also as another PDF please.

 

[alay1]

Attached is a diagram of the matrix. Sorry about the quality but I had to reduce the scan dps because the database kept failing on the upload of a 900kb file. 2J5 is the lamp strobe, 2J7 is the lamp row.
If it helps, here's a quick explanation of what the circuit is doing.
Lamps are connected as 8x8 matrix. PIA B-side outputs +18V to matrix columns via 7408 gate and two transistors. PIA A-side pulls matrix rows to ground via 7406, two transistors and a thyristor. Current in lamp row is measured over 0.4 ohm resistor and when it rises enough, the thyristor is triggered and row turns off. This prevents light bulbs burning out even in case the CPU suddenly stops.

 

[alay1]

This scan of the matrix is a little clearer.

 

[mvs sarma]

Hi, Fine, I expected thyristers as the word strobe had no meaning otherwise.

how abt the schematic way hopw you wired the IRFZ34N Please?
did you got thro the url
http://www.pinrepair.com/wpc/index.htm where 1990 versions are described - how to service etc-- whether this will guide you?

i shall study and come back

 

[alay1]

You are looking at a later version of pinball machine. The circuit is from machines dated 1970 - 1984, the document you need to look at is:
http://www.pinrepair.com/sys37/index3.htm#lamp

This is where I first read of the mod, you will find this at the end of the section on fixing lights. To quote:
"Yet another option to fix the burnt resistors is an idea documented by C.Eddy. He replaces the eight TIP42 lamp matrix transistors (Q63, Q65, Q67, Q69, Q71, Q73, Q75, Q77) with IRF9z34N mosfets. The MOS-FETs are installed oriented just like the TIP42 transistors. And the mosfets only need a tiny amount of current to drive them (compared to the TIP42 transistor), hence the large power resistors at R149-R156 never get hot. Because of this there is no need to replace the large resistors (the old burnt ones can be left installed, unless they are open). Heck if the TIP42s are replaced with Irf9z34n mosfets the power resistors R149-156 can even be replaced with jumper wires or zero ohm resistors."

I discussed this with some friends, we looked at the circuit and thought it would work. It does work, but not quite.
I have done no adaptions to the circuit to use the FET, I've just installed them as per the same orientation as the original TIP42's. At this point my electrical knowledge is running into road block. I'm just starting on this stuff so pardon me if I have missed anything basic. I was guided by the info in the document and advice (well intentioned) from friends who have more knowledge than me.

Thanks for looking and spending some time helping me. Whatever comes of this I'll feed back to marvin3m so others can know what to do.

 

[alay1]

Oh No........

I've just gone back and checked the packaging. I've been supplied IRFZ34N's instead of IFR9Z34N's. Curses on RS Components and my fault for not double checking. Between them and Mouser, this has been a disaster.
So now I really need some help. Can a N channel Mosfet be adapted or should I just put it down to experience (expensive ouch) and get the right part?

 

[mvs sarma]

Oh No........

I've just gone back and checked the packaging. I've been supplied IRFZ34N's instead of IFR9Z34N's. Curses on RS Components and my fault for not double checking. Between them and Mouser, this has been a disaster.
So now I really need some help. Can a N channel Mosfet be adapted or should I just put it down to experience (expensive ouch) and get the right part?

Hi, hold patience-- no self cursing-- let us do something coolly

 

[mvs sarma]

Oh No........

I've just gone back and checked the packaging. I've been supplied IRFZ34N's instead of IFR9Z34N's. Curses on RS Components and my fault for not double checking. Between them and Mouser, this has been a disaster.
So now I really need some help. Can a N channel Mosfet be adapted or should I just put it down to experience (expensive ouch) and get the right part?

Hi try to assemble the attached MOD for the strobe only on a proto board and adopt to strobe1* and see

let us check.

 

[on1aag]

I couldn't think of any.

Hi Alay1,

You can't replace the TIP42's with the IRFZ34N's and expect to get
satisfying results. You will always loose the Vgs voltage of the fet's
and your bulbs will be far too dim. You gonna have to order the
IRF9Z34N's to get it right, there's no other way.
After reading all posts of this thread I'm gonna have to download naked
women for the rest of the evening to restore my mental health.

on1aag.

 

[mvs sarma]

Hi Alay1,

You can't replace the TIP42's with the IRFZ34N's and expect to get
satisfying results. You will always loose the Vgs voltage of the fet's
and your bulbs will be far too dim. You gonna have to order the
IRF9Z34N's to get it right, there's no other way.
After reading all posts of this thread I'm gonna have to download naked
women for the rest of the evening to restore my mental health.

on1aag.

no reason to become so mental about my mod sir

 

[alay1]

Thank you for your help Sarma. I'll look at this next week when I get off work.

 

[mvs sarma]

Thank you for your help Sarma. I'll look at this next week when I get off work.

Hi, i feel no uregent need to again prchase-- we can make tese items work--
i have downloaded the old type schematics from the suggested site. i feel we can overcome

 

[mvs sarma]

Thank you for your help Sarma. I'll look at this next week when I get off work.

Hi, there is a mod again i shall post only mod of upper part seperately. until then dont wire it sir