Defender hum on startup

AMG

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I just de-switchered my Defender, using a robotron/joust PS. PS was rebuilt with a Bob Roberts kit; and additionally the 18000uf cap, the 7905 regulator, and the 2n6057 darlington were replaced. Other than the 12v unreg being slightly low, the power looks good and the machine works. But now there's a hum through the speaker on powerup that lasts until the first sound is played (only a second). It didn't do this with the switcher. I swapped the sound board with one that was recently recapped, and it's the same. I removed the cable from the rom board to the sound board, and still no change. Is that hum normal on a Williams machine with a linear PS?
 
Mine does the same. I want to day that it hums at various levels depending on the screen its cycling through. Want to say the HS screen is the worse. When playing a game it's, more or less, quiet.
 
check your grounds
make sure everybody is grounded
and clean

I ran some extra grounds, including one directly from the PS to the sound board, no change. One thing I did notice though, if it's runing, then shut off, then started within about 10 seconds, the hum is reduced or eliminated.

It may be more appropriate to ask if this is normal for Joust or Robotron, as they have the same PS and sound board as my Defender does now.
 
I ran some extra grounds, including one directly from the PS to the sound board, no change. One thing I did notice though, if it's runing, then shut off, then started within about 10 seconds, the hum is reduced or eliminated.

It may be more appropriate to ask if this is normal for Joust or Robotron, as they have the same PS and sound board as my Defender does now.

Once again, I'll back you up on this. The loud HUMMMMM only happens on a "cold" start. Maybe it's the HV unit charging the CRT?
 
Once again, I'll back you up on this. The loud HUMMMMM only happens on a "cold" start. Maybe it's the HV unit charging the CRT?

You sure you're not just hearing the degaussing coil? Or, perhaps, the degaussing coil is interfering with the audio circuits somehow? Easy enough to check - disconnect the degaussing coil and fire the game up cold.

-Ian
 
You sure you're not just hearing the degaussing coil? Or, perhaps, the degaussing coil is interfering with the audio circuits somehow? Easy enough to check - disconnect the degaussing coil and fire the game up cold.

-Ian

I had that same thought earlier today and gave it a shot - no change. Next test will be to disconnect the 120v to the monitor.
 
Once again, I'll back you up on this. The loud HUMMMMM only happens on a "cold" start. Maybe it's the HV unit charging the CRT?

I wonder if we noticed this only because the switchers *didn't* make the noise, and for everyone else it's normal. The sound does come from the sound board through the speaker, I unplugged the speaker connection and the noise went away.
 
Mine does the same. I want to day that it hums at various levels depending on the screen its cycling through. Want to say the HS screen is the worse. When playing a game it's, more or less, quiet.

I think this is a different issue, as mine is pretty much silent after that first second or so.
 
My Defender has always done it from cold, and there are waaay more grounds in there than I need, and its running on its original rebuilt linear

Andy.

Good to know, but I'm not sure how similar the PS is since your mains run 230v 50Hz. Do you know if they just use a different transformer with the same PS?
 
I disconnected the 120v from the monitor and the hum was still there, but it seemed shorter. Is there anyone running a linear PS in a Defender, Joust or Robotron that *doesn't* have a hum on a cold start?
 
maybe you have an issue related with the power from the wall into the linear supply where it get chopped up into useable voltages for the game to run

you might want to investigate what other people have in the UK in terms of wiring configurations or capacitors there and/or other possible what not there in the wiring going in
 
Im gonna say its probably the marquee lamp ballast charging, btw mine does the same thing
 
Im gonna say its probably the marquee lamp ballast charging, btw mine does the same thing

I tried removing both the bulb and starter, I think having the starter out prevents the ballast from charging, but I'm not sure. It hummed with both of them removed. And I'm in the US running on 120v, it was Andy that was in the UK.
 
All...

Since this only happens with the linear supplies and not the switchers it probably has something to do with the +12 "building up" as it powers up. Most switchers are at stated output voltage as soon as they are powered up. We all know from the CMOS errors that WMS games can have with switchers that linear supplies (at least the ones in the WMS games) dont output all voltages exactly as they should be upon power up.

I'd be willing to bet its the TDA2002 (the amplifier IC on the sound board) not knowing how to deal with +7 to +10 volts for that split second.

BTW, the hum is normal...

Matt
 
There are definitely some that are worse than others, though. I had a Joust that had a noticible hum. I tried everything suggested here, monitor off, fluorescent unplugged. verified grounds (the sound & PS boards were mounted on the same steel plate) and it still hummed. Swapped PS boards. No difference. Tried a differerent speaker, no hum when it was plugged in outside the box, hummed when swapped in for old speaker. Finally rebuild the sound card, replaced every capacitor on it. Still hummed. Rebuild an old fried sound card. No hum.

Put the card that hummed in a different cabinet (Robotron) and it worked fine. Sometimes they just don't know the words.

ken
 
BTW, the hum is normal...

That's all I really needed to hear. :)

All...

Since this only happens with the linear supplies and not the switchers it probably has something to do with the +12 "building up" as it powers up. Most switchers are at stated output voltage as soon as they are powered up. We all know from the CMOS errors that WMS games can have with switchers that linear supplies (at least the ones in the WMS games) dont output all voltages exactly as they should be upon power up.

I'd be willing to bet its the TDA2002 (the amplifier IC on the sound board) not knowing how to deal with +7 to +10 volts for that split second.

Matt

That makes a lot of sense, and the 7805 voltage reg may play into this as well since it needs around 8v before it'll produce a nice steady 5v. It could be that the amp is running before the logic chips are powered up.
 
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