(Topic ID: 24704)

Data East speaker noise - ideas for a cure

By roc-noc

11 years ago


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#379 6 years ago

Nah, that doesn't reduce noise. It bumps the output voltage up by a hair.

4 months later
#425 5 years ago
Quoted from wayout440:

Been there, done that. My big screen is a Samsung I got for free (well, for helping someone install the replacement they bought.) If you run into one with picture problems like horizontal lines, you'll be opening it up so you can cover these ribbon cable bonds across the top edge of the TV with foam weatherstripping tape to put some pressure on them.

Curious -- which Samsung and what did the horizontal lines look like?

1 year later
#523 4 years ago

Actually, minor correction - Data East calls out 0.018F caps, not 0.18F caps. When you see 0.18F such as on the Secret Service schematics - that's a typo. Remember that they pretty much plagiarized Williams for awhile including power supplies. Bulk capacitance of that value in the 1980s would have been horribly expensive and impractical.

The noise you are seeing on the input is directly related to the switching supply.
Looks like the replacement board has included the capacitance for the switch mode power supply. But is there really no bulk capacitance? That seems very strange.

The input wave (yellow) appears to be a considerably higher frequency than the standard 60Hz/120Hz(FWB) hum and is actually the switching frequency of the regulator. You would have needed to change your time base on the scope to a much longer period to see the 120Hz hum. Your trace looks exactly like the "Input ripple voltage plot" posted in the Texas Instruments "Input and Output Capacitor Selection Application Report" for no (or insufficient) external capacitance. Their plot had a saw tooth like yours but had a 200mV p-p which they considered high.

What a few of the board makers use in their design is the application notes for their specific voltage regulators. The app notes specify that the regulator requires x amount of low-impedance input capacitance so that is what the board makers use. What the board maker totally disregards is that the regulator manufacturer is basing the design on a DC power source that has *already been sufficiently filtered* at the input - i.e. has sufficient bulk capacitance. A single, 1000uF cap for both bulk filtering and voltage regulator ramp up is insufficient. All that gobletygoop you see on that yellow scope trace would have been seen at the sound board as well (minus the minor inductance filtering of the cabinet wiring).

When I was selling pinball boards - I used 4x input capacitors. Two paralleled large caps for bulk capacitance and two paralleled smaller, low-ESR caps for the voltage regulator high frequency ramp up. I would have expected to see the same sort of arrangement on the replacement board as well.

What you did is unclear by how its written. You took the 12V unregulated power, inserted a regulator to give you...12V regulated output? Or did you take the 5V output and use the boost function to take it up to 12V?

I was going to suggest using a pi filter at the input of the DE board. But DE already has them on their board and pi filters are typically for higher frequencies. Is there really only one pin assigned for the power ground connection? Also looks like DE has only one 1000uF filter on the input, that could have been better for lower freq filtering.
If they're relying on the now nonexistant 18000uF bulk cap for filtering - that one is now gone from the replacement board. Plus the fact that the cap was remotely located on the other board only exaggerated the problem. Better isolation between logic 12V and analog 12V is what DE needed. Would not be an easy task to fix. If I were designing a replacement power board - I would include two 12V bridge rectifier circuits on the board - one for logic power and one for analog power. And include real bulk capacitors and a heavy duty, shielded power connection between boards and all kinds of bells and whistles. Gee, aren't "If I could build these, I would do it better" always great!

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