(Topic ID: 196782)

Checking MPU blanking circuit on the bench.


By oldschoolbob

2 years ago



Topic Stats

  • 40 posts
  • 4 Pinsiders participating
  • Latest reply 6 months ago by Ballypalooza
  • Topic is favorited by 5 Pinsiders

You

Linked Games

  • Magic Stern Electronics, 1979

Topic Gallery

There have been 5 images uploaded to this topic. (View topic image gallery).

digit enable (resized).PNG
IMG_3702 a (resized).jpg
IMG_3699 a (resized).jpg
d58c148da81aec7265430b3a2e4cdc3ab2cab97f A (resized).jpg
Screenshot_20170830-223528 (resized).jpg

#1 2 years ago

I’m working on a Stern Magic for a friend. It boots up fine and the voltages are fine but I have no displays. I checked the first display and have 175 volts at TP2 and 5 volts at TP1. I checked PinWiki and it says it might be something in the blanking circuit.

I’m not good at working on a board in the game but I have a pretty good bench tester for 12 and 5 volts. Can I check the blanking circuit on the bench without having the displays connected?

Thanks

Bob

#2 2 years ago

The mpu will not fully boot up with out the 43v for zero crossing interrupt. The displays will never go active on the bench with only 12v and 5v.

Check for activity on the 4502 chip outputs p2, p5, p7 etc. If you don't have a logic probe, the schematic says 2.5vdc typical. It latches the displays and sometimes will completely fail in a manor you get no displays. The display blanking circuit could be failed. u19b, pin 4 should be active. Needs to be in attract mode or better yet display test tho to work.

If the feature lamps work as they should, you can pretty much assume the display latches (except the credit dsp) and binary count data at the PIA level is OK as they share the same PIA ports.

U11 does the digit enable. An MPU causing displays to be blank is usually a latch strobe issue (the 4502 died as mentioned above).

#3 2 years ago
Quoted from barakandl:

If the feature lamps work as they should, you can pretty much assume the display latches (except the credit dsp) and binary count data at the PIA level is OK as they share the same PIA ports.

Once it boots up it goes into attract mode and the feature lamps flash (at least some of them). Then I went into test and the feature lamps flashed as they should (at least some of them). (probably bad lights).

Where to look next?

Thanks

Bob

#4 2 years ago
Quoted from barakandl:

Check for activity on the 4502 chip outputs p2, p5, p7 etc. If you don't have a logic probe, the schematic says 2.5vdc typical. It latches the displays and sometimes will completely fail in a manor you get no displays. The display blanking circuit could be failed. u19b, pin 4 should be active. Needs to be in attract mode or better yet display test tho to work.
If the feature lamps work as they should, you can pretty much assume the display latches (except the credit dsp) and binary count data at the PIA level is OK as they share the same PIA ports.
U11 does the digit enable. An MPU causing displays to be blank is usually a latch strobe issue (the 4502 died as mentioned above).

#5 2 years ago

To be sure, I pulled out a display and put into another game (Meteor). It worked - one digit out and one segment out but it did light up.

Here's what I got on U20 (in attract mode)
pin 1 = 5.21
2 = .03
3=1.5
4=0
5=.03
6=1.5
7=.03
8=0
9= reboot ???
10=1.5
11=.12
12=4.81
13=5.04
14=.34
15=0
16=5.2

If I was supposed to have 2.5 V at p2, p5, and p7 it failed. But it was in attract mode and not in display test mode.

I have a logic probe but not too sure how to use it. If you need more info I guess I can figure it out.

Thanks

Bob

#6 2 years ago

hi bob,

i think that looks like u20 is OK. The display latch is at / near the speed of the display interrupt, so would be very low voltage reading.

u19a p4 is the display blanking. Would be slow frequency too and read low voltage, but not zero volts. You can look at that.

Good idea to use just one good confirmed display connected and go from there. Its possible one display with a bad decoder chip to blank out the rest even tho they have the resistors between each other.

Triple check the MPU J1 plug too. The display blanking goes to the inihibit pin on the displays BCD decoder chips i believe. It could float a way you get no displays with a connector issue.

#7 2 years ago

Tomorrow's projects:

I'll install one good display from my Meteor into the Magic. I'm guessing the best place is player 4 because it's first in the daisy chain. Then disconnect the other displays.

I'll check voltage at U19 pin 4.

Looks like display blanking is at J3 pin 10. Is there anyway I can check that it's working?

I might use the logic probe if you can give me a crash course - it has 2 clips (red and black). Black goes to ground. Does red go to 5 volts or 12 volts? Do I set it on TTL or CMOS? the rest I think I can figure out.

Thanks Andrew.

#8 2 years ago
Quoted from oldschoolbob:

Tomorrow's projects:
I'll install one good display from my Meteor into the Magic. I'm guessing the best place is player 4 because it's first in the daisy chain. Then disconnect the other displays.
I'll check voltage at U19 pin 4.
Looks like display blanking is at J3 pin 10. Is there anyway I can check that it's working?
I might use the logic probe if you can give me a crash course - it has 2 clips (red and black). Black goes to ground. Does red go to 5 volts or 12 volts? Do I set it on TTL or CMOS? the rest I think I can figure out.
Thanks Andrew.

black on ground, red on +5v. you can use your meteor game to compare against at mpu j1 connectors in display test. i think u20 is wired up the same in meteor too. 4000 series parts are cmos. 7400 plain is ttl. The PIA has CMOS and TTL ports. The pia ports labeled PA# are cmos, the PB ports are TTL.

Leaving the Elenco on TTL mode works in a lot of cases for the elenco probe as a usual failure mode you are looking for is shorted high, low , or floating.

https://pinside.com/pinball/forum/topic/terrybs-guide-to-logic-probes

#9 2 years ago

I've read terryb's guide several times in the past but only now it's beginning to make a little sense.

Once I got into the shop today I pulled out one display from the Meteor and put it in the Magic and disconnected the other displays. I got several little red glows in the bottom of the display (don't think I got a glow before). Also I got a couple of digits to flicker then they go off.

Then when I went into test I sometimes get a loud static noise from the speaker (like an old AM radio between stations). Hard to tell what test mode it was in because I don't have test numbers in the displays.

Does any of this help?

Bob

#10 2 years ago

Andrew, I'm really glad you're working with me on this - I'm really over my head but I got the best on the case.

I just checked U19 pin 4 and got 5.21 Volts when it's in display test mode.

Also I'm getting that awful sound from the speaker.

Bob

#11 2 years ago
Quoted from oldschoolbob:

Andrew, I'm really glad you're working with me on this - I'm really over my head but I got the best on the case.
I just checked U19 pin 4 and got 5.21 Volts when it's in display test mode.
Also I'm getting that awful sound from the speaker.
Bob

The display blanking is not working if u19 p4 is high constantly.

Check u14 p6 and p7. Check u19 p5.

U14 or u19 likely has a problem.

Screenshot_20170830-223528 (resized).jpg

#12 2 years ago

I know it seems to take me a while to get the info you need but my workshop is just below the bedroom and my wife goes to bed early.

I was going to check those points with my logic probe this afternoon but I got called away. Tomorrow I'll check those voltages and maybe check them with the probe also.

Would any of this effect the sound. The sound worked the other day in solenoid test but now I get that buzzing noise. Also the potentiometer seems to be backwards. - turn to the right lowers the volume.

Got some things to do tomorrow but I'll get those as soon as I can and report back.

Thank you so much.

Bob

#13 2 years ago

Checked U14 P6 = 4.84 V
U14 P7 = .35 V

U19 P5 = 4.18 V
U19 P4 = .37 V
U19 P6 = 4.84 V

Went back and used the logic probe (set on TTL):

U20 P2 = lo with pulse
same for P5, 7, 9, 11, 14

U14 P6 = hi with pulse
U14 P7 = ho and lo with pulse

U19 P5 = hi - no pulse
U19 P4 = hi / lo - with pulse
U19 P6 = lo with pulse

pretty sure I got this right. need anything re-checked let me know.

Bob

#14 2 years ago

A little more investigating - I checked J1 pin 10 and I get .8 V and low no pulse (but I get a buzzing noise on the probe). I also get the same thing at the display J1 - pin 11.

#15 2 years ago

By the way - all these checks were done with the game in display test mode - I think. - (one switch push after feature light test - second push)

#16 2 years ago

Those tests look OK to me. Make sure the mpu J1 connector is good.

You check signals at a display plug, just dont touch the logic probe to high voltage. you can check the +5v level stuff with the hv fuse pulled.

#17 2 years ago

Checked the connector at credit/ball count display (not connected) - player 4 was connected. All other displays are disconnected. Game was in display test mode.

Pin 1 = .38V - low - no pulse
Pin 4, 5, 6, 7 = .59V - high and low with pulse
Pin 8 = .36V - low - no pulse
Pin 9 = .57V - high and low - no pulse
Pin 15 = .13V - high and low - with pulse
Pin 16, 17, 18, 19 = 1.3 to 1.5V - high and low - with pulse
Pin 20 = 5.25 V

If anything looks off let me know and I can re-check

I gotta get serious about fixing this. It's beginning to really bug me now.

Thanks for your help.

Bob

#18 2 years ago

I went back and re-checked the voltages on U20, U14, and U19 and made a map from your schematic. Maybe this will help you see what I have.

Strange thing when I checked U19 p 4, 5, 6 I got .37, 4.18, and 5.21 after I checked with a probe I got different voltages = 5.21, 0, 5.21

does any of this make sense?

I'm really lost!!!

Bob

d58c148da81aec7265430b3a2e4cdc3ab2cab97f A (resized).jpg

#19 2 years ago

check R89

#20 2 years ago

After dinner I checked U19 again and got the same results as before:

P4 = .37
P5 = 4.18
P6 = 5.22

then checked with logic probe

P4 = high - no pulse
P5 = low - no pulse
P6 = high - no pulse

then I checked voltage again:

P4 = 5.21 (it was .37)
P5 = 0 (it was 4.18
P6 = 5.22 (same)

This don't seem right. What is causing P4 and P5 to change?

Would R89 cause this?

Would a bad CR6 cause this?

Could U19 be bad?

Thanks

Bob

#21 2 years ago

is u14 P6 stuck always high? It should be pulsing at the inverse of p7. 5.22vdc sounds like flat line high of +5v supply voltage. Check the 4000 series on cmos setting with probe

#22 2 years ago

U14 P6 = 4.84 V - probe = H and pulse
P7 = .35 V - probe = L and L and pulse

Bob

#23 2 years ago

sorry
edit:

P7 = .35 V - probe = H and L and pulse

#24 2 years ago

checked U19 again:

checked with cmos setting

p4 = .37 V - High and Low with pulse
p5 = 4.19 V - Low and no pulse
p6 = 4.89 V - High and pulse

checked it a second time and got same results.

Bob

#25 2 years ago

That looks OK assuming we can ignore the other intermittent results that had U19 P4 stuck high.

I'd check everything at a display now. With your logic probe or DMM check at a display connector. YOu can have it pulled off of the display. You can remove the high voltage fuse so you don't get zapped (185v ouch) or damage the logic probe (hv is at p1, +185v with dmm).

The key things to look at the display female connector plug.

Blanking P10
Latch strobe p15 (slow pulse your dmm was 0.04v i think)
BCD data pin 16-19 pulsing.
Digit enables p4-9

If you find something missing use the wiring diagram page, the MPU page, and the display page in manual to figure out where everything connects to. They are usually labeled well. Can use a bally manual here too.

#26 2 years ago

checked the credit display connector (un-plugged) - Display 4 plugged in - all other displays un-plugged. Probe on CMOS. Pulled fuse for high voltage. Game in display test mode. Used display 4 for ground and 5 volts for probe.

P1 = .35 v - low - no pulse
P4 = .59 v - low - pulse
P5 = same
P6 = same
P7 = same
P8 = .32 v - low -no pulse
P9 = .56 v - low - pulse
P10 = .08 High - no pulse - but it was buzzing
P15 = .13 v - high and low - with pulse - the high LED is about half lit.
P16 = 1.3 v - high and low - with pulse - the high LED is fully lit.
P17 = same
P18 = same
P19 = same
P20 = 5.2 v

I sure hope this means something to you - I don't know what it means.

Bob

PS - some of my first results may have been incorrect - not sure I was testing the correct IC. I may have been testing U18 instead of U19. (I said I'm not good at testing in the game) I now have labels on the IC's.

#27 2 years ago

when you say buzzing, does it make a different sound than any other probe spot? Is it similar to the noise you would get from touching the logic probe to your finger tip (while not being earth grounded).

im thinking mpu u19 is bad. the display connector p10 blanking should be both the high and low led and the pulse led flashing. it would be making a repeated beep noise, not buzz.

The buzzing type noise you hear when you touch the probe to your finger is what blown gate can sound like on the logic probe. I think its high impedance / open circuit type of fail.

#28 2 years ago

Something must be wrong with me - I get no noise from the probe touching my finger.

I suck at videos but does this help?

#29 2 years ago

The first probe is at P10 - then I'm going down - P9, P8,
I recorded it a second time - same way 10, 9, 8.
My camera will only record for 10 seconds.
If this don't help maybe my wife can make a better recording with her phone.

Let me know.

Bob

#30 2 years ago
Quoted from oldschoolbob:

The first probe is at P10 - then I'm going down - P9, P8,
I recorded it a second time - same way 10, 9, 8.
My camera will only record for 10 seconds.
If this don't help maybe my wife can make a better recording with her phone.
Let me know.
Bob

The sound the probe makes on p10 is not right. It should be making beep beep beep with both the probe green and red light on solid and the yellow pulse light blinking.

If the probe makes the same noise at U19 P4, I think its time to replace MPU U19

#31 2 years ago

checked display P10 again = .05V - high with buzz - no pulse
U19 P4 .38 V - H & L with pulse

U19 P5 = 4.19 V - H - no pulse
U19 P6 = 4.86 V - H - with pulse

U19 P4 and Display P10 don't seem to match. The only thing between is C60 and R89. What should I get at those components?

Bob

#32 2 years ago

I just checked R89 (both sides) - I got H & L with pulse (also at U19 P4). Then I stuck probe into window of the connector at P10 and got High - buzz and no pulse.

Something on the connector? Bad solder joint? The header and crimp connectors are new.

Should I pull the board and check?

Bob

#33 2 years ago

open circuit somewhere. follow the route and track to see where it drops out at.

#34 2 years ago

Tomorrow I'll pull off J1 and see what I get on pin 10 - if not the same as R89 I'll pull the board and look.

Will it work with J1 removed?

I think we're getting close (at least I hope so).

Bob

IMG_3699 a (resized).jpg

IMG_3702 a (resized).jpg

#35 2 years ago

Today I checked U19 P4 and got same results as yesterday = .37 V - H & L with pulse. Got same at R89. At J1 P10 I got .05 V - High - no pulse and buzzing. This was in display test mode.

Then I disconnected J1 and I thought I went into display test mode but I got 5.21 V at U19 - P4, and R89 and at J1 - P10. But I'm not sure I was in display test mode.

I turned it off then back on and left it in attract mode. I tested U19 P4 and got .37 V - H & L with pulse. Got the same at R89 and J1 P10. (connector not connected)

Next I guess I'll replace the crimp connector at J1 - P10.

Thanks

Bob

#36 2 years ago

I removed the crimp connector at MPU - J1 P10. It looked a little flat but I might have done that removing it. So I replaced it. I checked continuity from the new connector to the display - buzzed OK. Then before I plugged in the connector I checked U19 - p4 = .34 V. Then checked header pin 10 - 5.21 V - not good! Went back to U19 - p4 and got 5.21 V. I may have touched something wrong. I turned it off then back on - U19 P4 = .34 V - Header pin 10 = .34 V. Then checked with logic probe = H & L with pulse at both U19 and j1 pin 10. Then I plugged in the J1 connector. I got H & L with pulse at U19, J1 P10 and display J1 P10.

Then I turned it off and re-installed the high voltage fuse. Turned it back on and I got the display working. (one digit missing). Turned it off and connected the other displays - they all work but second digit missing on each one. I haven't changed the display headers or connectors yet. (that's next).

Andrew, I want to thank you for all your time and work you've put into this - I would have never done it without all your help.

And to think all this for one mashed crimp connector.

Thanks Andrew.

#37 2 years ago
Quoted from oldschoolbob:

I removed the crimp connector at MPU - J1 P10. It looked a little flat but I might have done that removing it. So I replaced it. I checked continuity from the new connector to the display - buzzed OK. Then before I plugged in the connector I checked U19 - p4 = .34 V. Then checked header pin 10 - 5.21 V - not good! Went back to U19 - p4 and got 5.21 V. I may have touched something wrong. I turned it off then back on - U19 P4 = .34 V - Header pin 10 = .34 V. Then checked with logic probe = H & L with pulse at both U19 and j1 pin 10. Then I plugged in the J1 connector. I got H & L with pulse at U19, J1 P10 and display J1 P10.
Then I turned it off and re-installed the high voltage fuse. Turned it back on and I got the display working. (one digit missing). Turned it off and connected the other displays - they all work but second digit missing on each one. I haven't changed the display headers or connectors yet. (that's next).
Andrew, I want to thank you for all your time and work you've put into this - I would have never done it without all your help.
And to think all this for one mashed crimp connector.
Thanks Andrew.

Connectors connectors connectors...CONNECTORS!

You missing digit is probably.... CONNECTORS! i would repin all of MPU j1. it is a pain. Takes an hour or so.... but totally worth it in the long run.

Happy to help. I love making pinball WORK is FUN. More games saved from the graveyard the better.

#38 2 years ago

Not totally out of the woods yet. But hopefully we're in the tall brush. I checked MPU J1 - p1 to p6 (pin 5 is the one for 10K) and got the same for all windows - I think low - no pulse. Then I checked the same wires on the display connector and got the same. I didn't check voltage (too tired) Then I realized I was still on CMOS and I think I should be on TTL. I'll check again tomorrow.

Going to bed now.

Thanks

This thing needs rescued - it's old and dirty but the playfield and plastics are in great shape.

digit enable (resized).PNG

#39 2 years ago
Quoted from barakandl:

Connectors connectors connectors...CONNECTORS!

One of the first things I do on a board is replace the header pins. Then I replace the crimp connectors. This eliminates a lot of problems down the road. So I knew the headers and connectors were new. But after finding the flattened connector in the blanking circuit, I put my meter down, put down my logic probe and went straight to the source. I pulled out connector 5 and found it too was flat. I replaced it and now all digits are working. Some missing segments, some missing digits and player 1 shows all zeros. But I suspect all that is problems on the displays and I haven't touched then yet.

Not sure how I got flattened connectors - I guess I need to be more careful when I install them.

Anain, thanks Andrew. You've taught me a lot over the past few years.

And to anyone following this thread, remember - connectors connectors connectors.

Bob

2 years later
#40 6 months ago

Hey everyone, I’m having very similar problems with my MPU and the displays. But with different readings and not much experience, I’m lost. Any help is appreciated. This board is for a EM friend that is helping another friend out. You know how it goes!
Bally -35 jumpered for 2716 EPROMs for a Power Play
Fully Boots on test bench and machine with no displays. Has the glow on the bottom of displays. Known good displays.
Flipped U10 and U11 no change.
Changed out U14 no change.
Repinned J1 on MPU. No change.
Followed post #5 but with different readings as follows... like old school Bob, I just received a logic probe but not sure how to use it yet. Voltages taken on U20. I think pin 1 should be higher?
Pin 1- 1.44
2- .04
3- 1.38
4- .01
5- .04
6- 1.38
7- .05
8- .02
9- .05
10- 1.40
11- .14
12- 4.54
13- 4.81
14- .40
15- .02
16- 4.99
Thanks for any help.

Promoted items from the Pinside Marketplace
From: $ 155.00
From: $ 18.00
Apparel - Men
Pinside Shop
From: $ 45.00
Displays
PinballSolutions.eu
$ 6.00
Cabinet - Other
Siegecraft Electronics
$ 49.99
Electronics
PinballElectronics.com
$ 9.00
Cabinet Parts
Third Coast Pinball
$ 119.95
Boards
Allteksystems
From: $ 15.00
From: $ 14.00
Electronics
Third Coast Pinball
Trade
Sale Pending!
Milford, DE
$ 149.95
Boards
Allteksystems
$ 27.95
Playfield - Toys/Add-ons
ULEKstore
$ 69.99
Playfield - Toys/Add-ons
Lighted Pinball Mods

Hey there! Got a moment?

Great to see you're enjoying Pinside! Did you know Pinside is able to run thanks to donations from our visitors? Please donate to Pinside, support the site and get anext to your username to show for it! Donate to Pinside