Mine does not do this, nor did any F2K I have played "in the wild". Something's not quite right..

Mine doesn't normally doesn't do this but at times it has made a squelchy burp as you have described. then the normal LED-tones

today it powered on normal, then needed to be "rebooted", switching off for a few moments then back on, it made that noise.

It also does have that hum that changes when cycling back & forth to "high score to date"

Usually, the first ping is loud and the rest of the pings a few decibels lower.

I fixed one up for a friend and ours did the same thing. Never could figure out why... we recapped everything and couldn't figure it out. I've heard others say theirs doesn't do this so I'm not sure what the issue is.

I have noticed this weirdness when using a Alltek and other aftermarket MPU's. With an actual stern MPU-200 mine doesn't make the sound at power up.

Many solid state games of this era have the "hum" sound changing between HSTD and current score in attract mode. I think it's feedback from the high voltage displays coming through the ground circuit. Some of the higher amperage rectifiers used on the repro boards may be adding to this. I notice less noise when using low volt Pinscore or Xpin displays.

Mine makes that noise sometimes. I haven't bothered checking into it.

On powering up my Star Gazer the first beep is at max volume and all the rest are normal. That's the only classic Stern that behaves this way. I've never swapped sound boards to see of the issue travels or stays with the game. Everything plays and sounds normal; so I never bothered to investigate further.

I think it is caused by timing of the caps charging up on the 12v supply. High current as the caps charge up will make problems on the 12v and the amp can let out a nice belch (ground at different potential / voltage drop??). Fresh high quality caps for 12v filtering on driver/sound board and ensuring the sound board has a good +12v and earth ground would probably help. The SB-300 floats up off the backbox ground. It has one ground return with two wires on a single .156" crimp. One wire goes to the SDB, other to the rectifier board. There can be no/little resistance. There is extra ground pins on the connector if you want to spread things out while repinning the board.

Another thing to consider about aftermarket MPUs is the clock. The SB-300 sound effects will be changed by small differences in the clock speed. The original MPU-200 usually ran round 0.8mhz, but the clock circuit used 5%-20% tolerance parts, so it can vary board to board. The nvram.weebly MPU runs at 0.895mhz (3.58 / 4) and based on the schematics the Alltek runs at 1mhz. I would imagine both replacement MPUs will control the sound board a bit differently than the stock MPU. It is most noticed in the high freqeuncy sounds (nine ball makes a bunch, but big game, not as noticable). Unless you had another game to compare against, most people would not notice it.

Even using a CM601, which i think is a soviet 6800 clone, in place of the mc6800 on the stock mpu can make the sb-300 sound different. Minor timing differences i guess.

My stock Stern Wild Fyre does this.

Thought I had read somewhere that this type of problem was caused by ground issues.

The mpu in my F2K is a Twobits that I got about 8 years ago, not sure what the clock speed is but sounds normal. So changing the caps on the sound board would help with the belch?? I remember a earlier thread about pin 34 not being necessary but I can't find it. Can you refresh my memory. I was thinking the belch had something to do with it.

p34 is the IRQ. It does not need to be connected. The 6840 on the sound board does not interrupt the cpu. Low ESR caps on the sound board and driver board, good connectors and solid ground should help the power on belch.

Perhaps even a blocking diode added to the SB300 on the 12v input would help, but I havent tried that. Bally sound boards usually have one.

Mine does this. Alltek in there if that matters.

All Stern pinballs from that time period did it new out of the box.

The game has been redone. All new connectors and boards minus the sound board, It works well so I didn't do anything to it. There's no belch with the original stern mpu. The original doesn't boot reliably for some reason, I have a power it on and off a few times and it will boot. Once running it's fine. I could go through the ground and make sure it's good. Although, I replaced the backbox foil, every connector in the machine and ran new cab braid. Maybe I should try grounding the speaker. Never seen a classic stern with the speaker basket grounded but Ballys always are, wonder why.

I wouldn't think the MPU would have anything to do with the power on belch unless the /reset signal is doing something I am overlooking. The ground return that goes through the ribbon cable is only for the digital side of the sound board. THe analogue ground for the SB300 is separate and returns through the driver board and rectifier board.

Grounding the speaker is just safety thing id imagine.

I honestly wouldn't worry about it too much as long as there is no hum after power up.

I think the belch is coming from the speech board, at least that's what the blurp coming out of it sounds like.

Actually the game is very quiet after booting, even when switching from HSTD and current score. The transformer hums pretty loud but there's nothing we can do about that.

Not the speech card as the games with no speech do it.

The power on belch is from high current as the caps charge. Ground potential of the amp gets off and until the caps are charged the amp belches out. I am making replamcenet SB300 sound boards. My first layout had poor ground return issues for the amp. I tried a few ground changes and I could make the power on belch continuous, never stopping, with a really poor looping connection to ground to the amplifier.

So why power belch only with after market MPU?

I look forward to seeing your new SB-300

Ground return / longer reset delay? I wouldn't think the mpu would have much to do with it.

My SB300 replacement will be ready early next week. Panasonic electrolytic caps are the only part I am still waiting on.

20171014_234936 (resized).jpg

Will it still need the jumper for games with speech? Looks great!

It is a faithful recreation of the latest, revision D, schematic. I have the track routed that mixes in the speech, so no jumper needed.

2oz copper, thick traces, and paid careful attention to how things are grounded. All new parts except the shift register, attenuator and programmable timer (used tested good pulls, no longer made). Sounds great, no hum in any of my SB300 games. If I power cycle numerous times I can just barely get it to let out a power on belch, not loud power on belch at all tho like some of the original SB300 boards i have with old caps.

Sounds great. Any idea of selling price yet?