(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.

#2 1 year ago

Any chance you can upload a video of the strobing? - will help give us a better idea of the cause.
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?

The Two Bits MPU board is basically an identical replica of the Bally factory board - no difference in how they work.

#3 1 year ago

The 2 differences I have found with the two bits MPU vs an original MPU (& have read that other people have the similar issues with) was the two bits boards were shipped with a 4049B at U14 when it should be a 4049UB - which will quite often cause feature lamp strobing & for some reason they are slower to tally up the playfield score & the bonus countdown.

#4 1 year ago
Quoted from Quench:

Does your Two Bits board have a polyester capacitor at C16 (next to the 555) or a ceramic capacitor?

I noticed on my two bits MPU that it was shipped with a ceramic capacitor at C16.

I removed the ceramic C16 & replaced with a polyester cap - now the MPU runs perfectly & no longer running slow.

Certainly worth changing the C16 cap to polyester if the op's is a ceramic & see if it makes a difference.

#5 1 year ago
Quoted from Joydivision:

I noticed on my two bits MPU that it was shipped with a ceramic capacitor at C16.

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

#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.

#10 1 year ago

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?

#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

#12 1 year ago
Quoted from JT-Pinball:

Seems like the max hz I measure jumps to like 190

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).

#13 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 was wondering this too as I was thinking of buying a bare board & assembling one, then I saw an ebay seller selling the bare boards had a component list attached.

https://www.bobsokol.com/ebay/dash35/DASH35BOM.pdf

#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!!!!!

#15 1 year ago
Quoted from Joydivision:

I was wondering this too as I was thinking of buying a bare board & assembling one, then I saw an ebay seller selling the bare boards had a component list attached.
https://www.bobsokol.com/ebay/dash35/DASH35BOM.pdf

Sweet. I'm not seeing wattage of every resistor. Only a few and caps don't seem to show which type. Just don't want to order the wrong stuff.

#16 1 year ago
Quoted from RC_like_the_cola:

Sweet. I'm not seeing wattage of every resistor. Only a few and caps don't seem to show which type. Just don't want to order the wrong stuff.

The two 2.0k resistors at R16 and R113 should be 1/2 watt rating
The 82 ohm resistor at R11 should be 3 watts rating.
All other resistors are 1/4 watt.

The 0.1uF capacitor at C16 must be a polyester film type.
All other capacitors are ceramic (go for multilayer ceramic caps where you can).
The two 820pF capacitors at C1 and C2 should be at least 100 volts (especially C1).
Use 100 volt caps everywhere else if you can get them, though 50 volt caps will work.

Get beefy inductors for L1 and L2 if you can:
https://www.greatplainselectronics.com/proddetail.asp?prod=LM2-R22K
https://www.mouser.com/ProductDetail/871-B78108E1221M009

#17 1 year ago

BTW, resistor R21 needs to be:
24k ohms for 7 digit display games (display interrupt generator runs around 430Hz)
36k ohms for 6 digit display games (display interrupt generator runs around 320Hz)

27k ohms for Stern MPU-200 games (display interrupt generator runs around 400Hz)

7 digit display games require a higher display interrupt generator frequency to reduce the effects of minor flickering/strobing on the displays which some people notice more than others.

Bally forgot to update the MPU board schematics til circa '84 when they redrew them.

#18 1 year ago
Quoted from Quench:

7 digit display games require a higher display interrupt generator frequency to reduce the effects of minor flickering/strobing on the displays which some people notice more than others.

That's good to know - all my 7 digit Bally games have the same quite noticeable flicker compared to no flickering on all the 6 digit games.

I wondered about the cause of the flickering on the 7 digit games before & sent Tom Callahan from the Repairconnection an email about it about 20 years ago. He replied suggesting to change the R22 resistor from a 4.7K to a 1K to increase the display interrupt generator from 300Hz to 350Hz.

I forgot & never go around to trying it.

#19 1 year ago

The amount of flicker on the displays varies game to game for whatever reason. Like Eight Ball Deluxe does not do it nearly as much as Mr and Mrs pac and Black Pyramid. I could understand why Mr/S pac does it more with the extra display.

I don't think the resistor change happened until really late in the -35 era and well past the first 7 digit game. I'm not 100% but looking through a stack of MPUs I don't think you find the 24K resistor until the Bally MPUs started coming out with the Midway Part numbers on it. Seems like the PCB manufacturer changed around this time too.

The faster display interrupt for sure helps cut back on the 7 digit display flicker but also keep in mind that if the CPU works on updating the displays more it can slow down other things. Like in Bally chime game the fast the display interrupt runs the slower the chime bells ring out at.

#20 1 year ago

In addition to the above, Stern fitted a 27k ohm resistor at R21 on their MPU-200 boards.

Quoted from barakandl:

I don't think the resistor change happened until really late in the -35 era and well past the first 7 digit game.

Skateball was the first Bally game with 7 digit displays. The first two pictures below are from two Skateball MPU boards I have with R21 changed at the factory to 24k ohms (presumably using existing MPU boards in stock).
The third picture is from my Medusa, the fourth picture is from a Flash Gordon (sorry for the low quality picture), fifth picture of a board from an unknown source, sixth and seventh board pictures are from Centaurs.
There's enough detail to see R21 is Red-Yellow-Orange-Gold (24k ohms). 36k resistors have blue bands and none of the below pictures have blue in the R21 resistor color codes.

Note the dates bottom right on the MPU boards.

I currently have a MPU board in my Medusa with a 36k resistor at R21 and aspects of game timing are definitely not right.

Skateball MPU board 1
Skateball MPU board 2
Medusa MPU board
Flash Gordon MPU board
Unknown game MPU board
Centaur MPU board 1
Centaur MPU board 2

#21 1 year ago

Here is one of the later MPUs as can be seen dated ‘84. This has a factory 24k resistor at R21 & trying this board in a Flash Gordon their is no flickering with the displays, No flickering with the two bits mpu either.

FA70044B-2B91-4A8D-9CDD-FE93F3A2786A (resized).jpeg
#22 1 year ago

All the original MPUs left in my stash are so corroded the bottom right area silk screen with the date is missing. There was numerous pcbs with 7 digit game ROMs installed with a 36k resistor but hard to say that they where not donor MPUs where ROMs got moved around and it is actually an older made PCB.

24k resistor and a 10k trimpot wired like rheostat in series with each other can give you adjustment between about 320hz and 420hz at each end of the trim pot. Thats how the NVRW replacement MPU is wired up.

Some games like Mr/Mrs pac you can enter a wierd timing setup because of when the display interrupt triggers. At around 360hz It makes the GI blinking triac oscillate and the GI gets flickery. Adjusting more towards 400hz+ clears that issue.

#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

2 weeks later
#24 1 year ago

Added more MPU board pictures to post #20 above. All boards with date codes from Skateball onwards have the 24k resistor at R21.

https://pinside.com/pinball/forum/topic/two-bits-mpu-with-flickering-strobing-displays-#post-5408417

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