(Topic ID: 236572)

STERN MPU-100 - Solid led on booting


By Theonlylilo

3 months ago



Topic Stats

  • 14 posts
  • 6 Pinsiders participating
  • Latest reply 87 days ago by Quench
  • Topic is favorited by 1 Pinsider

You

Linked Games

Topic Gallery

There have been 12 images uploaded to this topic. (View topic image gallery).

04 (resized).jpg
03 (resized).jpg
02 (resized).jpg
01 (resized).jpg
MPU_Sockets 004a.jpg
MPU100_ita.jpg
05 (resized).jpg
04 (resized).jpg
03 (resized).jpg
02 (resized).jpg
01 (resized).jpg
Cattura (resized).JPG

#1 3 months ago

Hi everybody.
I’m new on repairing Bally/Stern SS boards, and now I’m trying to resurrect a Stern Lectronamo.
The cpu is a MPU100, I’ve read some guide on the web but I still have the solid LED on.
I rebuilt all the reset section because some acid damage was visible, but this was not the problem, I mean… I’ve stable +5V on pin 40 of CPU U9, so reset seems to be ok.
New sockets for U6/U7/U8 and with multimeter the continuity from pad to pad is ensured, also where old sockets are already installed (U9/U10/U11).
From the guide and following pinwiki, at least the first flicker is needed and, in order to have it, the only necessary components are U9, U11 and U6. Perfect… I still have the solid LED on, +5V on pin 40 and grounding pin 40 to 39 nothing happens. Ok, the voltage drops to +0V but the led doesn’t flick. At the moment, the only measure not quite right is about pin 5 of cpu… VMA signal must pulsing, but in my case is high +5V.
I read that three are the ICs involved… U14/U15/U16 but in most cases U15 is the culprit. So I took form another board the U15, replaced in my MPU100 and.. tadaaaaa... nothing. Always solid LED on.
Now… it’s becoming a personal case. I don’t never give up! Never. And I want to find the issue without changing all components…
Anyone has suggestions for me?
Any help would be appreciated.
Thanks
Ciao

#2 3 months ago

How is the clock signal at CPU pins 3 and 36/37? If you don't have a logic probe, check with multimeter and you should see about 2.5 volts. Replace U16 if the clock is stuck high or low.

#3 3 months ago

The voltage here is around +2.6/+2.7V, so the clock itselfs is ok...
Also the CPU outputs from pin 9 to 22 an 24 to 33 are pulsing.
Pin 5 is not pulsing but stuck high +5V...

#4 3 months ago

You sure your chips are good?

It could be that something else is stuck forcing vma high maybe. Trying bending that leg out so it's not in the socket, see if it still stays high

#5 3 months ago

a lot of times a stuck VMA is a symptom of a locked up board, not the cause.

U19 mixes the VMA with the external memory. U19 could lock up the board if j5 ext. mem. pin is shorted to a neighbor. I suppose u19 p6 could be shorted to Vcc. U18 could lock up the board if the gate that inverts A9 (/A9)is blown. I think the same could be said for other gates in U17/U18 (/A12). Those should be pulsing at least some when the board comes out of reset. So can be checked with a logic probe.

Hate to say it, but when i would fix these original boards so many locked up MPUs would magically start working after I installed all new IC sockets and jumpered for the fewest ROMs possible to use (mpu100 can use a single 2732 u2 so you dont have to mess with u6 socket). So don't rule out a bad IC socket. I would only try replacing chip sockets if you are well practice at working on CPU boards... a bunch of ripped out through holes make the problem worse.

Only having the CPU chip installed and then probing the other empty chip sockets with a logic probe is a quick way to look for open circuits.

#6 89 days ago

I'm on ther board now.
Recap:
- Reset section has new components.
- All the sockets have been changed except U10 and U1 (not used in my case).
- Pin 40 his high +5V.
- If I short pin 40 to 39, Voltage drops but nothing happens to the LED.
- There's not a problem with LED itself because pin 18 of U11 is stuck low. The board doesn't pass the boot.
- Pin 3, 36, 37 are pulsing (+2.6V). So the clock is ok.
- Pin 2 (HALT) is high +5V.
- Pin 5 VMA is not pulsing. I have +4.14V.
- Next step, following the guides, is to check U14 4049 inverter. Also from the manual I must have the input gate pin 14 high (~+4,5V). Also pin 10 and 5. In my case these are low... Pin 9 for example must be low, but mine is high. Could be this the cause, or a symptom of a faulty board?
- Next step...U15 and U16: readings are ok compared to the manual.
- Next step U19 (4011). Pin 4 for example must be ~+0.4V... I have +5.4V... I cannot understand if these are fault ICs or "normal" behaviour due to the blocked board...

Cattura (resized).JPG
#7 89 days ago
Quoted from Theonlylilo:

Also from the manual I must have the input gate pin 14 high (~+4,5V). Also pin 10 and 5. In my case these are low... Pin 9 for example must be low, but mine is high. Could be this the cause, or a symptom of a faulty board?

Nope, this is the zero crossing detector circuit and has no effect on the CPU accessing the ROM at U6 to boot.

Quoted from Theonlylilo:

Next step U19 (4011). Pin 4 for example must be ~+0.4V... I have +5.4V... I cannot understand if these are fault ICs or "normal" behaviour due to the blocked board...

This is a symptom, not a cause of the board not booting.

Have you carefully checked your solder work around the IC sockets to see if there's any accidental shorts between pins/traces?

Can you post some high resolution clear pictures front and back of the board?

#8 88 days ago

Sorry for the delay but I was busy at work...
Here the pictures. I hope are well for you.
Thanks
Ciao

01 (resized).jpg02 (resized).jpg03 (resized).jpg04 (resized).jpg05 (resized).jpg
#9 88 days ago

Hmm, the lighting is not well enough to see clear fine detail in the pictures. Any chance you can take the photos outside during the day (but not in direct sunlight) and possibly higher resolution?

I can't quite tell but is there a short circuit circled below?
MPU100_ita.jpg

Where possible, check the IC sockets you've installed using a light from behind the board to see if you have any short circuits front side of the board between the pins and traces between them under the sockets.

MPU_Sockets 004a.jpg

#10 88 days ago

I can't tell from the picture, are you using 2516, 2716, 2732, or 2532 roms?

#11 88 days ago

good point about the roms. they look like MBM2716 eproms which would take cutting and jumping on the MPU to use.

http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Bally_AS-2518-17_and_Stern_MPU-100_Jumper_Info

#12 88 days ago

Looks like it's got at least some of the mods for 2716s. I can see the leg bent off on U18

#13 87 days ago

Hi! Here new pics. Let me know if they are ok or not!
The board has already been modified to accept both 2716 in u2 & u6, The modification consists of:
- Cut traces to pin 21 of u2 & u6, and connect pin 21 to pin 24
- Under u18, join pin 4 & 5 and bend leg 4 of IC 4049.
In addition, ther's not a short between C30 leg and neighboring path.
Forgive me for the DIY R140 , but it's difficult here to find a 20K resistor.

01 (resized).jpg02 (resized).jpg03 (resized).jpg04 (resized).jpg

#14 87 days ago
Quoted from Theonlylilo:

Hi! Here new pics. Let me know if they are ok or not!

Thanks, these pictures are better.

The only thing I can notice is it looks like you have bad corrosion under the J5 pin header that could be causing resistive shorts. Carefully inspect it and if it is corrosion, remove the J5 pin header to clean it.

BTW since it looks like there is corrosion at the top of the board, what condition is the CPU socket in?

Quoted from Theonlylilo:

Forgive me for the DIY R140 , but it's difficult here to find a 20K resistor.

Understood - we didn't question it

Promoted items from the Pinside Marketplace
From: $ 140.00
From: $ 155.00
$ 14.50
Electronics
PinballElectronics.com
From: $ 15.00
From: $ 45.00
Displays
PinballSolutions.eu
From: $ 149.95
$ 19.95
$ 399.95
From: $ 155.00
$ 3.00
Playfield - Decals
Doc's Pinball Shop

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