I'm new to the forums and have a general understanding of some electronics but I'm very green.

I'm trying to repair my pinball machine CPU and am having a problem with a type of "Reset circuit" as its described.

Essentially on boot this reset circuit is to hold the voltage at -12v until +5v can be regulated or smoothed and then it should go to +5v. The problem I'm seeing is that there never is -12v only +5v and I think that is what is keeping it from booting. I also could be fishing in the wrong pond with the Reset circuit but it seems logical.

I belong to another forum for pinball users and have posted there (http://rgparchive.com/rgpforum/showthread.php?p=1667029#post1667029) but I think this problem is really fit for the AAC forum. The link above has things that I have tried and replaced, etc. I'm could use some suggestions or ideas?

From a great link on repairing System 1 games (http://www.pinrepair.com/sys1/index.htm)

[SIZE=-1]Dead CPU: Next Steps.
The CPU board is completely dead, with no score display activity after 5 seconds of power-on. Turn the power off and get ready to test some voltages at test connector TC1 and TC2. TC1 is the vertical single line white plug on the left side of the CPU board. TC2 is the vertical white plug in the dead center of the CPU board. On both TC1 and TC2 pin1 is the top most pin. [/SIZE]

[SIZE=-1]Reset Circuit: The reset circuit holds the CPU chip low for a set period of time until the +5 volts can stablize. If the CPU chip is never told to go "high" from the reset circuit, the board will never start to boot. [/SIZE] [SIZE=-1]Measure TC1 pin 14 and power the CPU on. It should show immediately at power on -12 volts. This will rapidly changie into +5 volts after about half a second. This is the RESET signal. Another place to check the Reset is at chip Z2 pins 7,9. (both should go high to 5 volts after about one second of power-on). If the reset is not working and does not change to +5 volts, it is best to replace the Q5 and Q6 (MPS-A70) in the reset circuitry. If the reset is still not going from -12 to +5 volts, change chip Z2 (4528 CMOS). Still not working, check or replace caps C31 and C32 (.1 mfd, and these do sometimes fail). Note that the "Reset" button on the CPU board has nothing to do with this Reset signal (it is only used to reset bookkeeping values). [/SIZE]

I've also attached a couple of pics of the CPU and the circuit in question.

Any help would be appreciated.

 

If Z2 pin 6 is +5v, then you will get -12v to TC1 pin 14.
If Z2 pin 6 is lower than about 4v, then you will get +5v to TC1 pin 14.
I have no clue what Z2 is. However, pin 6 may be an open-collector output. If you try to pull it up to 5v, you will likely do damage to the device.

You might try going between R161 and the base of Q6, connect one end of a resistor of ~5k, and connect the other end of the resistor to +5v. You should then get -12v out of TC1 pin 14.

 

Z2 is a One-shot. A dual monostable multivibrator.

hgmjr

 

Sgtwookie, thanks for the reply.

Z2 Dividers and Multipliers (http://allcomponents.ru/re/scl4528be.htm)

You might try going between R161 and the base of Q6, connect one end of a resistor of ~5k, and connect the other end of the resistor to +5v. You should then get -12v out of TC1 pin 14.

I'll try your suggestion and post back. I assume you mean to test on the base side of the transistor Q6?

Edit: Z2 pin6 is +.18v

 

I had a 4.6k and gave that a try. Applying 5v to r161 and testing voltage at r161 gibes .88v?

I would have thought it should be 5v?

Edit: Sorry wrong side of the resistor...

 

Ignoring my previous post, sorry.

Applying 5v to r161 does indeed get negative voltage on tc1 pin14 but it never goes positive...?

 

OK. I had you use a ~5k resistor just in case you connected it to someplace wrong, it would make it less likely that you would burn something up.

So, you know that Q6 works when it gets the proper signal on its' base, and you also know that Z2 pin 6 is "stuck" low.

You need to find out why it's stuck low.

If C31, an 0.1uF cap, were open (between Z2 pins 1 and 2) or had a bad connection, that might cause the symptoms you're seeing.

[eta]
Z2 could be bad as well.
In the meantime, to see if you can get the thing to boot manually, try powering it down, and connecting your 4.6k resistor from R161/Q6 base via a switch to +5v, and close the switch. Then turn on the power, wait a sec, and then open the switch. If the board doesn't boot then, you have more problems.

 

As a quick test I simply unsoldered the jumper from R161, held it on the resistor and powered up the board then gave it second and removed it, the board seems to boot! (very exciting...!)

So, I follow you on the circuit being stuck low. I've replaced z2 already and the cap at c31, I'm not sure if the cap is of the correct type? I got it from ratshack (.1 uf polyester film)???

 

The 0.1uF polyester film cap should be OK.
I don't know why that pin 6 is staying low.
Check to see if Z2's pins are the voltages they should be.

You should see pin 2 rising up in about a second when you first power on.

 

If C23 were shorted, it would kill the battery pretty quickly - it would also keep Vdd pin 16 low, which would cause the output on pin 6 to be low.

Do you get 5v on the emitter of Q5?
How about the collector of Q5, or pin 4 of Z2? (they should be connected together).

 

I'll replace C23 this evening when I get home it appears fine but just in case..

VSS from C16 is +5.08v appx.

Z2 Pin2 after start is +2.88v
Z2 Pin4 after start is +4.24v

Q5 B= 3.54v C=4.31v E=4.31v (curious why the voltage is so low?)


Can I ask a question about Q6 and the test wire?

The emitter had -12v and the collector +5v how did you determine that +5v would trigger the transistor? or is the +5v supposed to hold it off like the demonstration? (http://www.allaboutcircuits.com/vol_3/chpt_4/1.html)


Thanks for the help.

 

I'll replace C23 this evening when I get home it appears fine but just in case..

VSS from C16 is +5.08v appx.

Z2 Pin2 after start is +2.88v
Z2 Pin4 after start is +4.24v

Q5 B= 3.54v C=4.31v E=4.31v

Well, that all looks about what I'd expect.
Note that Q5 collector is connected to Z2 pin 4, so they should be the same voltage. You measured Z2 pin 4 at 4.23v and the collector at 4.31v.

(curious why the voltage is so low?)

+5v is on the anode of CR37, which supplies current to the emitter of Q5 and the high side of R156.
R156 and CR38 cause current flow through CR37 (which has a forward voltage of ~0.7v; 5v-0.7v=4.3v) when the breakdown voltage of CR38 (3.5v) is achieved (signifying that +5v is available), causing Q5 to turn on due to Vbe exceeding -0.7v, thus supplying ~4.3v to Z2 pin 4.

Can I ask a question about Q6 and the test wire?

I think you got me there.

The emitter had -12v and the collector +5v how did you determine that +5v would trigger the transistor? or is the +5v supposed to hold it off like the demonstration? (http://www.allaboutcircuits.com/vol_3/chpt_4/1.html)

The collector of Q6 is connected to -12v via R162, a 6.8k pull-down resistor.
The emitter of Q6 is connected to +5v directly.
If the Vbe is <= -0.5v (closer to zero) then the transistor Q6 will be considered in cutoff, meaning only a very small leakage current measuring in micro-amperes will flow into the emitter. The 6.8k pull-down resistor will be able to pull the collector down to ~-12v.

Once the Vbe of Q6 reaches about -0.65v, the transistor will conduct, and the Vce (collector-emitter voltage) will be low, causing the collector to read nearly +5v.

I'm thinking that Z2 is damaged, or perhaps you have some solder splashes somewhere.
Excessive heat or static electricity will destroy CMOS IC's (and many other types as well).

 

You measured Z2 pin 4 at 4.23v and the collector at 4.31v.

I'll check this again, I see your point since they are tied together.

I also have a working CPU board (not mine). Would it make sense to check Z2 Pin4 and Pin6. If Pin4 and Pin6 are the same voltages as the non-working board then Z2 would be bad for sure?

 

Since you have a working board, install the working board and read the voltages on all of Z2's pins that are in use, relative to ground. Then test the other board.
1 - C1 (one side of C31)
2 - C1/R1 (junction of C31/R159)
3 - R1 (to VAB)
4 - +TR1 (about 4.3v)
5 - -TR1 (to VAB)
6 - Q1 (junction of R160/R161,
8 - GND
16 - VDD (to VAB)

 

Since you have a working board, install the working board and read the voltages on all of Z2's pins that are in use, relative to ground. Then test the other board.
1 - C1 (one side of C31)
2 - C1/R1 (junction of C31/R159)
3 - R1 (to VAB)
4 - +TR1 (about 4.3v)
5 - -TR1 (to VAB)
6 - Q1 (junction of R160/R161,
8 - GND
16 - VDD (to VAB)

Z2 measurements.

Working board:
1 - C1 (one side of C31) - .02
2 - C1/R1 (junction of C31/R159) - 3.50
3 - R1 (to VAB) - 4.45
4 - +TR1 (about 4.3v) - 4.29
5 - -TR1 (to VAB) - 4.46
6 - Q1 (junction of R160/R161) - .18
8 - GND
16 - VDD (to VAB) - 4.46

Not working board:
1 - C1 (one side of C31) - .02
2 - C1/R1 (junction of C31/R159) - 2.88
3 - R1 (to VAB) - 3.67
4 - +TR1 (about 4.3v) - 4.29
5 - -TR1 (to VAB) - 3.67
6 - Q1 (junction of R160/R161) - .18
8 - GND
16 - VDD (to VAB) - 4.47

EDIT: After looking at the schematic Pin3, Pin5 and Pin 16 are tied together at VAB strange Pin16 is correct. Pin2 also goes to VAB, they all have in common VAB... coincidence?

 

OK, the differences you noted are all a result of a different VAB (battery voltage) between the two boards.

This could be caused by:
1) CR26 is open (this is the most likely cause, particularly if one or more battery cells had become shorted and the battery has been replaced already.) Your +5v is 5.08v, VAB is 3.67v, so that means a 1.41v drop across CR26. I would expect VAB to be at minimum 4.38v when the battery is charged and CR26 is good.

2) The battery on the non-functioning board is discharged lower than the one on the functioning board, or has a shorted cell. This is also a very likely cause.

3) C23 has an excessive leakage rate (partially shorted).

4) Something else on the board connected to VAB is loading down the battery charge circuit.

VAB is used on Z2 (pins 3,5,16, the right of R159, and top of R160), on a 4081 AND gate above TC1, Z22 pin 22, and perhaps other places that are not on this schematic or that I missed.

 

The battery backup is mainly to retain some book keeping functions and in both cases the battery is non-existent at the moment I should have stated this earlier and it usually contains 3 - 1.5v AA batteries, I think that can eliminate option 2).

Of all the components that I have replaced, CR26 and C23 were not some of them, I will do so tonight and retest.

Thoughts on Z2, would it appear to be functioning correctly in your opinion?

 

I haven't really gone through the datasheet for Z2 much; just got an overview. I'm not going to say it's OK, because the output still is not what is expected - but that could be due to the voltages it's getting not being in the correct range. You are welcome to go through the datasheet.

Check to make certain that the battery holder or battery leads aren't shorted.

They wouldn't be standard alkaline batteries, they would be 1.2v NiCD's; 3 x 1.2v = 3.6v.

SOMETHING is loading down VAB on the bad board. There should not be a 1.4v drop across CR26.

 

I went ahead and replaced C23, CR26 and CR37.

Something has changed, now when I power off the board I get a negative voltage at TC1 Pin14 that drains back to 0.

The only part not replaced in the diagram is the zener diode CR38.

4081?? don't see it...

I'll check the voltages at Z2 again and post back.

VB,

 

I went ahead and replaced C23, CR26 and CR37.

Something has changed, now when I power off the board I get a negative voltage at TC1 Pin14 that drains back to 0.

That's kinda what I would expect when +5v falls off.

The only part not replaced in the diagram is the Zener diode CR38.

It didn't need replacing.

4081?? don't see it...

SCL4081BE. Above TC1. Above "STATIC RAM MEMORY". Above and to the left of R163. To the right of Z22 pin 17.

I'll check the voltages at Z2 again and post back.

Definitely re-check those voltages. How else would you know if they match the good board?

Do you get the -12v when starting up yet?