Black Hole sound issue

playpin35

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My black hole has an issue now where there is a loud constant hum through the speakers. The sound board caps have been changed and it still persists. Once in a while it will fade out and you can hear the normal sounds, but most of the time not. During the hum, the game sounds seem somewhat distorted. Any ideas?
 
In addition to the connector suggestions splattergatz made, replace the QDs at the speaker. You might pull the driver out and have a look at it too.

If none of that reveals any problem... you might be looking at the amp IC on the sound board... and they aren't easy to find.
 
In addition to the connector suggestions splattergatz made, replace the QDs at the speaker. You might pull the driver out and have a look at it too.

If none of that reveals any problem... you might be looking at the amp IC on the sound board... and they aren't easy to find.

Oh, and you might also look at your Sound/Speech Power Supply (A7) board. Particularly the C3 & C2 filter caps on it. Check your DC outputs for AC content. That board generates + & - 12VDC as well as +30VDC for the sound board. The 30VDC, in particular, is used by the audio power amp.

If all else fails, the amp IC is an LM379S. I think arcadechips.com sells them...
 
OK, after checking the sound/speech power supply output I have found my -12 output only reads -3.75. Checking further I get -13 volts on one side of R2 and only the -3.75 volts on the other side. All other voltages check ok. R2 measures out at 100 ohms as per manual, but I suppose that doesn't mean anything? Changing R2 should fix the problem?
 
OK, after checking the sound/speech power supply output I have found my -12 output only reads -3.75. Checking further I get -13 volts on one side of R2 and only the -3.75 volts on the other side. All other voltages check ok. R2 measures out at 100 ohms as per manual, but I suppose that doesn't mean anything? Changing R2 should fix the problem?

If R2 measures about 100ohms, it's fine.
Sounds to me more like a bad zener diode (CR2, 1N4742A).
 
Power supply has an output of -12.3 volts. When the sound board is connected it drops to
-3.7 volts. What on the sound board uses the -12v that could be pulling the voltage down?
I have U21 socketed, no change by swapping in another one. Most likely the LM379S amp?
 
The LM379 actually doesn't use the -12V (or the +12V), it runs off of the +30V.

The only things I see, looking quickly at the schematic, that use -12V are the two LM741s (one at U21, another at U22) and the DACs (U19 & U20).

A close look may reveal more... but that's all I see right now.
 
Not sure if the forum will re-size the graphic to make it un-usable, but attached is the schematic for the analog section of the sound/speech board.

I'm pretty sure a "1408A" can be substituted with a DAC0808 (but perhaps someone with more definitve knowledge can chime in).

Not sure of an easy way to determine which of those guys is pulling down (or, uh, up) your -12V line. Maybe just check to see if one if noticably warmer than the others?
 

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I replaced the LM741 at U22 with no change. For some reason tonight this issue would come and go more frequently. I did notice the cabinet volume pot was getting warm?
 
How warm is the pot getting? Like "ouch", or just barely noticable?
The way it's wired (between the amp an speaker) it's always disappating a Watt or so. I'll play mine some this evening and see how warm it feels...

So I kinda assumed, once we found out that something on the sound board is pulling the -12V down (or up), that it made the most sense to address that first. The implicit hope was that fixing whatever caused that would also fix the humming.

So: you've now replaced both LM741s, and something is still pulling the -12V to -4V when the sound board is plugged in. All that right?

If so, then it seems to me that one of the following must be true:
1) one or both of the DACs (U19 & U20) are bad and dragging the -12V line.
2) something else, that I missed on the schematic, which uses -12V is bad and pulling it down.
3) there's a short somewhere on the PCB that's pulling the -12V down (up).

I think you mentioned recapping the sound board. Which caps, specifically, did you replace? Have you double/triple checked your work for proper polarity of those caps?

Also, you might try putting a DMM in resistance (ohms) mode on the -12V & GND line of the sound board. If there's a dead short (or nearly so), you've got something to look for.
 
How warm is the pot getting? Like "ouch", or just barely noticable?
The way it's wired (between the amp an speaker) it's always disappating a Watt or so. I'll play mine some this evening and see how warm it feels...

So I kinda assumed, once we found out that something on the sound board is pulling the -12V down (or up), that it made the most sense to address that first. The implicit hope was that fixing whatever caused that would also fix the humming.

So: you've now replaced both LM741s, and something is still pulling the -12V to -4V when the sound board is plugged in. All that right?

If so, then it seems to me that one of the following must be true:
1) one or both of the DACs (U19 & U20) are bad and dragging the -12V line.
2) something else, that I missed on the schematic, which uses -12V is bad and pulling it down.
3) there's a short somewhere on the PCB that's pulling the -12V down (up).

I think you mentioned recapping the sound board. Which caps, specifically, did you replace? Have you double/triple checked your work for proper polarity of those caps?

Also, you might try putting a DMM in resistance (ohms) mode on the -12V & GND line of the sound board. If there's a dead short (or nearly so), you've got something to look for.

The sound pot is warm, not hot. Both LM741s replaced. Caps replaced were the 47 uf radials and yes, polarity is confirmed correct.
I will do some more checking later tonight.
 
2) something else on the schematic, which uses -12V is bad and pulling it down.
3) there's a short somewhere on the PCB that's pulling the -12V down

you would think if it was a short, a component would get real hot and even burn up and hopefully blow a fuse. it's just pulling the -12v down to 4 or whatever and not to 0 and burning

anything else feel hot on the board ?
any caps feel round on the top instead of flat ?
 
FWIW, I played some BH tonight. I couldn't feel any warmth whatsoever on the volume pot.

I also checked my -12 (with everything hooked up) it measured about -11.6VDC.
 
you would think if it was a short, a component would get real hot and even burn up and hopefully blow a fuse. it's just pulling the -12v down to 4 or whatever and not to 0 and burning

anything else feel hot on the board ?
any caps feel round on the top instead of flat ?

Good points.

I suppose that type of zener regulator can't supply very much current before the output voltage begins to drop.

If you shorted the -12V line to GND, it'd drop about 17V across the 100ohm resistor, and the current would be 170mA... so there'd be almost 3W being disappated by the 100ohm, 1/2W, resistor. Not sure how long a 1/2W resistor can handle 3W before charring...
 
The sound pot is warm, not hot. Both LM741s replaced. Caps replaced were the 47 uf radials and yes, polarity is confirmed correct.
I will do some more checking later tonight.

OK, I assume you're talking about the 4 that filter the incoming power, C38-C41. Just looking at those, I noticed that the cap for the -12V (C41) is supposed to be installed opposite of the other three (negative lead away from the edge connector). No offense intended, just pointing it out as it is different from the others and an easy mistake to make.

Regarding your volume pot getting warm... one possible explanation I can think of is that there's DC flowing there. There shouldn't be any; the output coupling cap, C36, is supposed to keep DC out of the signal while passing the AC thru. You might consider replacing C36 (470µF 35V), or at least measuring to see if there's a DC voltage drop across your 100ohm volume pot (between the two outer lugs).
 
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