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(Topic ID: 274576)

System 9 Power Supply board issue - Comet


By yaksplat

82 days ago



Topic Stats

  • 20 posts
  • 3 Pinsiders participating
  • Latest reply 45 days ago by yaksplat
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#1 82 days ago

I have two power supplies for Comet and I have a set of xpin displays in the machine. One of the power supplies won't even light up the displays, and the other works fine. I checked all of the test ports on the board and got the following:

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Nothing seems too far off... is the 5V too low? What should I be ordering up?
Circuit:
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#2 82 days ago

Replace capacitors C7, C8, and C12.

#3 82 days ago

Awesome. Thank you.

Now, my mechanical engineer brain wants to understand why. I'm still a rookie at electronics. If it's not too much trouble, could you explain how you know?

#4 82 days ago

After repairing literally hundreds of pinball machines over the years, I have found those 3 capacitors failing lots of times causing power supply issues.

#5 81 days ago
Quoted from KenLayton:

After repairing literally hundreds of pinball machines over the years, I have found those 3 capacitors failing lots of times causing power supply issues.

I figured that's what you were going to say

#6 81 days ago
Quoted from KenLayton:

After repairing literally hundreds of pinball machines over the years, I have found those 3 capacitors failing lots of times causing power supply issues.

Stupid question. When it says MFD in the manual, is that micro farad, as in µF?

Didn't they have a µ back then?

#7 81 days ago

Yes - micro farads.
Was a common but poor practice to use MFD refer to micro Farads instead of the more common "u" or "µ".
This occasionally throws in confusion as an upper case M is often used as Meg.
And sometimes you will see mfd as micro farad... even though m is often used for milli.

1 week later
#8 69 days ago

So, replacing C7, C8, and C12 didn't do it. 5V at TP1 is still at 4.92VDC and not high enough to power the Xpin displays.

What's next?
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#9 69 days ago

The 5V is a bit low but not too far off.
R7 and R8 are 1% resistors that set your output voltage.
To get 5V - these resistors should be R7 = 2.15K, R8 = 4.99K.

Vout = (Vref)(4.99K / (2.15K + 4.99K)) = (7.15)(0.698) = 4.99V
You can bump the voltage by setting R8 a bit higher. For example - a 5.99K (non-standard value) would result in 5.26V.
I would think 5.11K (standard 1%) would be good as it would set power at about 5.03V.

But -- to see why yours is so low:
Is transistor Q5 original or has it been replaced? If replaced - what is new part number and manufacturer?
What voltage do you measure at pins 5 and 6 of the regulator?
What values do you have installed at R7 and R8? And do they test good?

#10 69 days ago

R7 is 2.136k
R8 is 4.77k

4.93v on pin 5
7.07v on pin 6

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#11 69 days ago

So, based on my readings, a 5.2k ohm resistor would do it?

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#12 69 days ago

Your 2N6057 is an original. These are pretty tough - don't mess with him yet.

7.15 is the typical reference voltage for these but 723 regulators are pretty sloppy with their reference range and can range from 6.8 to 7.5.
You set the desired voltage at pin 5. The regulator monitors pin 4 and raises the output until pins 4 and 5 match each other (in theory).

Using your values: Vout = (7.07)(4770 / (2136 + 4770)) = 4.88V. You see 4.92 so that works out pretty close.
Your R8 is too low at 4770, lowest should be 4940 ohms. If you measured in circuit, this can mess with measurements.
But, yes - you can go up in resistance. 5.23K is the nearest standard value for a 1% resistor.
With your existing regulator and value of R7 then you should get:
Vout = (7.07)(5230 / (2136 + 5230)) = 5.02V
And note that the reference voltage can vary with room temperature as well as having a rather ingenious built in frustration detector.

#13 69 days ago

And - good that you replaced C7 and C8 as Ken mentioned. If you didn't replace them now and they were originals - you would have probably been replacing them in the near future. Replacing them now gives you a stable operating voltage for the 723 regulator.

#14 69 days ago

Ah, this is great, I appreciate the help. So I don't have a 1% 5.2K ohm resistor laying around. Is there anything wrong with going through the 5.2k ones that I have, measuring them, and then back calculating to check if the result is acceptable and then using that one if it is?

#15 69 days ago

If you can find a close carbon film or metal film resistor then no problem. Avoid really old carbon composition resistors here.

#16 68 days ago
Quoted from G-P-E:

If you can find a close carbon film or metal film resistor then no problem. Avoid really old carbon composition resistors here.

I think i'm going to swap out the 723. I can't get a combination of resistors in R7 and R8 that i have on hand. I'm ordering new 4.99k and 2.15k 1% resistors anyways, so a new 723 is only $0.90. Anything else that i should check and possibly replace since I'm placing an order?

#17 68 days ago
Quoted from yaksplat:

Anything else that I should check and possibly replace since I'm placing an order?

Yes, capacitor C10 the 18,000 uf @ 20 volt axial capacitor. It's your main +5 volt filter capacitor.

1 week later
#18 58 days ago

I'm still having some issues with this power supply. I changed out the 723 and the two resistors, along with the various caps and here's my readings

R7 is 2.145k
R8 is 4.99k

5.07v on pin 5 of the new 723
7.27v on pin 6 of the new 723

My 5V line is 5.08V. It appears that the xpin displays do not like a voltage that high as they don't even turn on, just like they wouldn't at 4.92V but everything else works fine. They run fine on my other power supply that's 5.01V, so I know that they're good.

1 week later
#19 45 days ago

I ended up changing R7 to a 2.2k resistor. 5V line is now 5.03-5.04. I'll call it a win.

#20 45 days ago

Well, it should have been a win. Still had issues running the xpin displays. Repinned connectors and still issues. Long story short, there was a cracked solder joint in the power to the display board. I reflowed the solder joints and the displays came on. Then i noticed that there was no sound. Where there's one cracked solder joint, there may be more.

Reflowed the remainder of the headers and the sound worked again. Most likely the power wasn't making it to that portion of the board.

Now I'm done.

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