Big Game
Stern Electronics, 1980
Catacomb
Stern Electronics, 1981
Flight 2000
Stern Electronics, 1980You're currently viewing posts by Pinsider fytr.
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For what it is worth, my Star Gazer has the same issue. Constant background hum just slightly louder than I'd like to live with.
I also replaced the main large caps on the sound board and it made no difference (as well as on the solenoid driver board).
I'll try the re-pinning the ground connectors when I get a chance. What is the coin door lockout solenoid issue?
Quoted from rcbrown316:thank you so much for this direction. I will get to this in a few days hopefully.
Ditto, been crazy busy but will be looking at this soon.
Here are my readings from Star Gazer:
All mpu ground connections were via TP3/grd on my Alltek mpu board.
- TP6 on the SB-300 and ground on the MPU: 0.08vdc. 0.003 vac
- TP6 on the SB-300 and ground on the chassis somewhere (the braid is fine.): 0.085vdc. 0.024vac
- Now, disconnect J3, and measure DC and AC volts from pin 8 of J3 to ground on the MPU, and the chassis somewhere: 0.1vdc. 0.3vac cab braid, -0.009vdc 0.004vac mpu ground
... clip the black lead from your meter to TP1 on the SB-300, and you can leave it there
while you take the next set:
- Measure DC & AC volts at TP3 (+12V): 9.99vdc. 0.003vac
- Measure DC & AC volts at TP2 (+10V): 14.67vdc 0.07vac
- Measure DC & AC volts at TP4 (+5V): 4.89 vdc. 0.006 vac
What do these indicate? 
Quoted from Majdi:Sorry for the delay: rough work travel week.
I assume you swapped TP2 and 3 around in that post.
In which case: TP2 seems a bit high. It's listed on the SB-300 schematic as 11.9VDC.
Can you check at TP3 on your power driver board as well?
And do you know if the power transformer is set for high or low line voltage (is the line
input to the transformer between pins 1 and 9 or 1 and 5?
My reading for TP2 is correct, but you're right that it is the 12v testpoint, not the 10v one. 
On the SDB TP3 I read 14.85vdc, board writing says it should be 13VDC.
My transformer is set to 115v, yellow wire to lug 9.
IMG_1315 (resized).JPGIMG_1314 (resized).JPG
Quoted from Majdi:Do you actually need it at 115V? That might cause unregulated supplies to float up a bit higher than expected.
What's the line voltage coming into the machine? (Obviously, be careful checking this one
That's just how it was setup when I got it and I've never given it any thought until now.
AC at the plug reads 128v. 
I guess I should try switching the transformer over to 120v.
Quoted from Majdi:It doesn't surprise me. 99% of the pins I've looked at are set for 115V. Most don't need to be these days (my own line voltage at home is consistently 121-122V.)
Your 128V line voltage and 115 tapped winding probably explain the 15V where it says 13.
If nothing else bringing down the unregulated lines will take some heat out of the backbox (linear regulators used to derive the regulated busses from them have to dissipate the excess as heat, so switching to the 120V tap should cool things down a bit.)
Might also be worth seeing if your hum improves after. Less stress all around is never bad.
Yeah it's easy enough to do, worth trying for sure. I'll report back once I have a chance to switch it over to 120v.
Okay, switched it over to 120V at the transformer. No noticeable change in the background hum level, unfortunately.
I redid the level tests on the sound board as before, only changes were:
- Measure DC & AC volts at TP2 (+12V/13V): Was 14.67vdc now 14.03vdc
- Measure DC & AC volts at TP4 (+5V): Was 4.89 vdc Now 4.84vdc
So very slight drop in those two values.
I also thought the noise might be caused by a new switching-type 5v power adapter I used on my power board, so tried the game with the power board out of my Flash Gordon with the analog style adapter, no change in the background hum.
Finally, I noticed that the hum only starts once the MPU board has booted up. When in attract mode, the sound kind of changes with the light pattern. I tried running the game with everything unplugged from the MPU board except J4, so no actual controlled lamps are switching, etc, same background hum. Note that my game is using an Alltek MPU board as well.
At this point I'm wondering if a high-pass filter/cap at the right spot might cut most of the noise, hopefully while preserving the normal range of game sounds and quality? Ironically, my Medusa which seems a bit muffled now has virtually no detectable background hum.
I'm working on getting the original Star Gazer MPU board working again (https://pinside.com/pinball/forum/topic/help-me-fix-this-stern-mpu-200-board).
Once I achieve that, it will replace the Alltek board, which actually belongs in my Flash Gordon.
So at some point in hopefully the not too distant future I can try that.
Quoted from barakandl:Built my first SB-300 replacement board and have been messing with it all day. Found that the 60cycle hum can be a bit tedious to get rid of. I thought my analogue grounding at the bottom of the board would be plenty sufficient but its not on the first proto. Running a wire to the ground of C7 and C8 and soldering it right to the analogue ground the header pins (j2-p2&3)clears up a hit of hum. Soldering a wire from the ground connection of the tda2003 amp right the analogue ground header pin cleaned up the rest of the hum. I am going to have to go back and fix the analogue ground pour at the bottom of the PCB.
Another note. If the V10 circuit is out of spec (low) the sound will be too loud and you get what i think is wave clipping on higher tone notes (power on tones sound like crap). I did not have a 10v zener on hand yet, so i used two combined two zeners to make a single 9.8v zener. That 0.2v was enough to cause a substantial sound quality problem. I stole a 10v zener off another board (it was goin 10.2v) and it made the sound volume go down and be much better sounding. So good to take a peak at the V10 circuit (TP3). Failing electro caps will drag the voltage down too. V10 is used for the MC3340 circuit.
Pretending that I understood most of what you are saying, would any of this likely be applicable to the Alltek board as well?
Quoted from barakandl:buzzzzzz
Have something to try that may help games with buzzing sound boards. I noticed the ground (and +12v) connector pin are double wire crimped together. One wire goes the SDU for ground return, the other back to transformer for return (same with +12v). There is two PCB headers for the ground connection (and +12v) so lets tie those all together for a strong connection.Take the two ground wires and separate them apart. Use a DMM to check for almost no resistance between each ground wire and the earth ground(should be same nearly same resistance as shorting your DMM leads together).
Use two new crimp contacts and put one wire at p2, the other at p3. A couple inches back, cut both ground wires apart. Use a wire nut or neatly solder and heat shrink tube all four wires together. You can do the same for the +12v connection. Install new headers at J2.
The transformer and SDU J3 connectors must be good. SB-300 1000uF and 470uF cap must be good as well as the sb-300 V10 circuit needs to be bang on 10v.
Interesting. I'll give this a try on my Star Gazer when I get a chance. Thanks!
I never got a chance to try barakandl suggestions above, but I did replace the Alltek MPU with the original Star Gazer MPU-200 board that I finally got working again (battery damage) and the loud background hum is gone.
Something about the Alltek board I guess. Maybe because it only used that one grounding screw on the bottom left corner...
Strangely, I moved that same Alltek board into a recently acquired Big Game while I fix its MPU board (battery damage, always battery damage) and I don’t have the same hum as I did with Star Gazer. Big Game has leds in the backglass and GI, Star Gazer doesn’t, I wonder if that is a factor?
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Crown Point, IN