(Topic ID: 258370)

(Solved) Two Bits MPU with flickering (strobing) displays

By JT-Pinball

1 year ago


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  • 24 posts
  • 5 Pinsiders participating
  • Latest reply 1 year ago by Quench
  • Topic is favorited by 6 Pinsiders

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#1 1 year ago

I am working on a Stern Hot Hand that has a Two Bits MPU in it. Problem is limited to MPU as I have swapped out the MPU and the problem goes away. Unfortunately I don't have another MPU laying around so... I have checked all the components around the 555 for the Display Interrupt and they all seem to test good. I have swapped out the 555 chip and changed the resistor at R21 to 33K. Nothing has changed... Suggestions? Thanks all.

#6 1 year ago
Quoted from Quench:

Any chance you can upload a video of the strobing?

I tried but the site wont let me post a mp4 video and thats all I can take it seems

#7 1 year ago

<blockquote cite="#5381952"
What frequency are you measuring out of pin 3 of the 555 timer?
Does your Two Bits board have a polyester capacitor at C16 (next to the 555) or a ceramic capacitor?
What was the original resistor value at R21?
.

Seems like the max hz I measure jumps to like 190
The cap is polyester
The 36k called for in the schematics.

#8 1 year ago

I took some voltage readings also off the pins and compared tp the schematics.
Pin 1 gnd
Pin 2 2.53v
Pin3 3.18v
Pin4 5v
Pin5 3.35v
Pin 6 2.54v
Pin7 2.53v
Pin 8 5v
They all look close except pin 3 shows 4.5v

Thanks Quench!

#9 1 year ago
Quoted from Quench:

My Two Bits MPU board came with a ceramic capacitor at C16. When I pulled it out it measured about 83nf and was causing my display refresh out of the 555 timer to run at around 520Hz (too high). I installed a polyester cap that measured about 102nf which brought the 555 timer output down to around 420Hz where it should have been. It made a big difference to certain timing aspects of my game.
That capacitor should be a polyester type (there's a reason Bally used them), ceramics are not temperature stable and will drift.
Having said that, the OP already has a polyester cap on his board - this thread is a continuation of a year old thread where he built the board:
https://pinside.com/pinball/forum/topic/two-bits-dash-35-build

Yes I built this board had this issure right after I finished it and life came by and said hello to me. Board has sat in a box fpr a year and had other projects. I guess I could have tried to revive the old thread.

#11 1 year ago
Quoted from RC_like_the_cola:

I've wanted to try to build one of these. Does anyone have a component list, so I can just order everything in one shot?

I had a parts list that was published from somewhere. I'll see if I can find it... Although I am sure some of these guys could just recite from memory

#14 1 year ago
Quoted from Quench:

If you're only measuring 190Hz out of the 555 then that's likely your problem. The display digits are not being refreshed often enough and your eyes are seeing it.
Are you measuring the 555 frequency with an oscilloscope or a meter with frequency counter?
Do you have another Bally/Stern board to measure and check that your frequency counter is giving you correct readings?
Or you can clarify by measuring the frequency at pin 4 of U14 (Zero Crossing detector) which should measure 120Hz.
The output frequency of the 555 is determined by R21, R22 and C16 (36k Ω, 4.7k Ω and 0.1uF respectively) and the 555 itself. So it should be one of those 4 components (unless there is an accidental solder bridge across pins of the 555).

I figured it out. The 4.7k was actually a 47k. Looks like I had consolidated a few resistors. I checked the resistance but the meter was in a scan mode and showed a decimal. I put the meter out of scan and recheck. Then when I grabbed the 4.7k resitors I saw I had a couple 47k mixed in my 4.7k bag....
Thank you Quench!!!!!

2 weeks later
#23 1 year ago

Thanks to all for the replys this is great information if someone had an issue moving a known good board between games

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