Hello Pin-Gurus

I'm new to the forum.

My name is Andreas. I'm 49 years old and from Stuttgart / Germany.

After reading a while and learning many things about pins and their electronics I decided to subscribe to the forum.

Thanks for your valuable input!

A few weeks ago I startet my first pinball project.

I'm trying to bring a Power Play (MPU 2518-17) back to life. After it's many years of hibernation I'm trying to restore the pin. The MPU board had the expected problems with oxidation so i changed all headers and sockets with new ICs (U8, U9, U10, U11) after cleaning the board carefully.

It boots fine but there seems to be a bigger problem with the switch matrix. After I got several wrong (and differend) results on switch testing, I isolated the MPU from the playfield and connected only J4. Then I measured the Strobes and Inputs on J2 with a scope from PIA up to the header pins.

I got readings of very nice and crisp signals up to the anode of the Diodes. In case of ST0 the diode CR43. On the kathode side of the diodes the signals goes bad. I looks like the signal can't "discharge" fast enough. Or some other signal or potential is crosstalking and keeps the strobes on "High". Sometimes after powering up the signals on the strobes are good. But only for a few minutes the they stay high again.

I'm staring at the schematics for hours and disconect caps and resistors along the trace. But no difference.

Any ideas what can cause the signals to stay high?

ST0_error (resized).jpg

Thank you Quench for taking up the thread.

I will come up with some HighRes pics from the board and a detailed switch matrix table with listed switch numbers showing during the test as soon as I am home from work.

So much in advance:

When I'm doing a switch matrix test even without playfield the disaplay shows me switch "1" as closed after booting up. Somtimes it shows "3".
And sometimes "0" is blinking. When this is the case I do the I/O tests by wire jumping Rows and Colums from J2 and read the Numbers from the Display.
Nealy every new test shows a different result than the test before.
One time some switches are missing completely, next time Input-Row[#1] shows same numbers as Input-Row[#0] tested before.
I can not recognize a pattern to encircle the problem at one specific Strobe or Input. The only common is that the signals stay neraly at high level so there is no correct strobing...but only from the katheode side of the diodes (as in the printout).

Atari_Daze I sure will
I'm very grateful that so skilled people in this forum like Quench are pushing me into the right direction.
I already read many of his comments helping people to get their pins running. Especially in hopeless cases.
Quench : Thanks for taking care!

Find attached some pictures from the MPU and some results of the last few days from the switch matrix testing (without playfiled, jumper wired only)
Faulty readings are marked red.

And here the readings from strobes and returns to ground as you recommended.
First measuring from ST0 arranged me to change my meter 3 times! Didn't get any better...

ST0 --> GND 1 MOhm (!)
ST1 --> GND 49 kOhm
ST2 --> GND 43 kOhm
ST3 --> GND 43 kOhm
ST4 --> GND 50 kOhm

I0 --> GND 48 kOhm
I1 --> GND 48 kOhm
I2 --> GND 39 kOhm
I3 --> GND 25 kOhm
I4 --> GND 27 kOhm
I5 --> GND 36 kOhm
I6 --> GND 40 kOhm
I7 --> GND 43 kOhm

Measured with dip swiches in ON and OFF postion with same readings.

hires_front.jpg

hires_back.jpg

South_front.jpg

hires matrix.jpg

hires matrix_back.jpg

20190412_230652 (resized).jpg

20190412_230658 (resized).jpg

20190412_230706 (resized).jpg

20190412_230711 (resized).jpg

Gentleman, you all were so damn right...

@barakandl ; @G-P-E :

R132 was dead. As you predicted. I soldered it out a few days ago because it was in path with ST0 I'm having trouble with. I measured it with one leg out of the bord. It read OK. And resoldered it. May be it didn't survive my soldering skills...
Took it out today. Dead resistor!
Soldered in a new one.

@quench :
One shot one dead!
Days ago I replaced the header pins on J2 as recomended in every pinball tech channel concerning Bally MPU.
I took care on removing corrosion as good as i could. But I focused only on the backside of the MPU because there were the traces to the next components...
I didn't take care too much on the front side.

As you assumed...
Today I took out the header pins from J2 again.... after measuring the pins continuity with your predicted values of shorted header pins between them... chapeau man !!!
There was realy a big amount of....greasy dusty corrosive schmuh under it....
I overhauld the J2 section. And worked on both sides against corrosion. Could it be that my first attempt of renewing header pins worsed it? I used ab bit more flux to burn the oxides away...Are the residues of flux conductive???

New readings with new resistors:

ST0 --> GND 56 kOhm
ST1 --> GND 56 kOhm
ST2 --> GND 56 kOhm
ST3 --> GND 56 kOhm
ST4 --> GND 59 kOhm

I0 --> GND 56 kOhm
I1 --> GND 56 kOhm
I2 --> GND 56 kOhm
I3 --> GND 56 kOhm
I4 --> GND 57 kOhm
I5 --> GND 57 kOhm
I6 --> GND 56 kOhm
I7 --> GND 56 kOhm

Time was running out so could not reassemble the MPU into the backbox to test it...

For now...

I'm taking my hat off! Thank you!

unsoldered_1 (resized).jpg

unsoldered_2 (resized).jpg

overhaul (resized).jpg

I reinstalled the board today and expected a nice starting up.
The MPU was booting correct and lights went on...but no sound from the chimes and none of the other coiles were firing...
All diaplays show static zeros on all digits.
And the board doesn't change into diagnostic mode when the test switch pin is shorted to ground.
I only had J4 and J1 connected.

Then I ozilloscoped the new J2 headers with their Strobes and Inputs.
The signal quality is so much better now.
But I wonder why the signals on all Strobes are the same pattern. Is that normal?
I expected a different pattern on each strobe.
I have no idea what could have caused this problem.
Even with the faulty signals an semi shorted header pins on J2 the the game started up an got into reday mode.

new_signals (resized).jpg

@quench : The GI is going live and a bit of the feature lamps

The MPU is passing it's self-diagnostics and flashes 7 times on power up.
So I went for the signals on U11 and U10.
Pin 40 of U11 ist static at 5V. Pin 18 of U10 is static at 0V.
I overhauld header J4 because of the experiences on J2. But with minor success at this time.
Then i wanted to look for the signal from U12 and when I touched pin 7 with the probe the machine startet it's game ready tune and fired coiles and chimes...and scared the .... out of me!
You already pionted out U12. I put in a new one in today.

@bigal56 : The PIAs are in sockets. Maybe it's the perspective of the picture. The new sockets for the RAM are one the way and will be replaced.
It's a hobby project. I'm just getting in touch with pinballs and solid state electronics.

A new U12 is soldered in. Everthing is O.K. now.
MPU is booting fine. Coils are all triggered one after another, chimes are playing and the strobe signals look much better know.
Tomorrow i will report a new switch matrix test. I hope for better results...

Under the culprit... And strobe signals after changing U12

ne555out (resized).jpg

new strobes (resized).jpg

Today I tested the switch matrix again and it showed the right values in every Column/Row combination. Finally

So I startet to look for the next problem where I'm in need of a hint.

In one of five times when the pin starts it seems to be confused in wich sequence it has to fire the coils to play the ready tune and to reset targets etc...
Some coils are firing twice others not at all.
When I do a self testing of the coils it happens that the tophole saucer fires at "3" (right number ) and "7" (wrong number, chime 1000)....
Shorting the transistors on the SDB to ground fires only the right coil at one time.

Can it be that there is a problem with the decoder U2 on the SDB?