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Author Topic: A4000 sound is... buzzy. Ideas?  (Read 1098 times)

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Offline TrixterTopic starter

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A4000 sound is... buzzy. Ideas?
« on: July 14, 2004, 04:57:57 AM »
In the course of trying to run some demos on an A4000/'060(USA, but internal jumper is set to PAL) donated to me by a friend, I noticed that the sound output is significantly more "buzzy" -- like it's overcranked/overmodulated or something.  I have a PAL A500 sitting right next to it that has just beautiful sound output (and quieter than I remember it, the noise floor is a pleasant -62dB) that I switch the cables back and forth between, so I know it's not the cables/speakers/etc.

What could cause this?  The machine overall seems like it's working properly -- it can run demanding demos like Little Nell/TBL and Lapsuus/Maturefurk at an acceptable rate so I am assuming the '060 is working... Is this a known issue or problem?  Advice?
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Offline TjLaZer

Re: A4000 sound is... buzzy. Ideas?
« Reply #1 on: July 14, 2004, 08:19:09 AM »
Hey can you send those demos for me to try on my 4000 060 66Mhz setup?  
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Going Bananas over AMIGAs since 1987...

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Offline TrixterTopic starter

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Re: A4000 sound is... buzzy. Ideas?
« Reply #2 on: July 14, 2004, 05:34:03 PM »
Just grab them from http://www.back2roots.org/ .  It's not just the newer ones like Lapsuus/Maturefurk and Little Nell/TBL, but also older demo music played through regular old Paula, like Love/VD+Fairlight.

Any suggestions on how to troubleshoot and/or fix this?  Or should I just buy a new A4000 motherboard?

(Speaking of which, where CAN you find new A4000s?  A quick search on ebay.com only shows one or two...)
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Offline patrik

Re: A4000 sound is... buzzy. Ideas?
« Reply #3 on: July 14, 2004, 07:18:22 PM »
@Trixter:

Check if the two surface mounted 22uF 16V electrolytic capacitors C433 or C443 has leaked (is it clean around them?). They are often the cause of problem with the A4000 sound, even if they havent leaked they might just have dried out and is the cause of these strange sound-effects.


/Patrik
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Offline Hyperspeed

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Re: A4000 sound is... buzzy. Ideas?
« Reply #4 on: July 16, 2004, 01:19:49 AM »
The sound on my A1200 desktop isn't too crisp either.

If I turn the volume up to max on my external speakers I can often
hear the CPU when it's working on something.

Like an audible CPU monitor...

;-)

Do you think a ferrite core on the RCA leads would help or is it more
to do with capacitor age/quality?

I've also heard that the serial port is somehow linked to the audio
out phono jacks for MIDI reasons, is there a -12v rail that is
susceptible to fault causing audio trouble?
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Offline TrixterTopic starter

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Re: A4000 sound is... buzzy. Ideas?
« Reply #5 on: July 20, 2004, 07:17:18 PM »
Thanks very much, I will investigate this and reply back.  

One last question:  Are C433 or C443 labeled easily on the motherboard?  Or are they on the CPU card?  I ask because at first glance I didn't see them, and I have an 68060 accelerator board not the original cpu board.
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Offline TrixterTopic starter

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Re: A4000 sound is... buzzy. Ideas?
« Reply #6 on: July 28, 2004, 07:36:34 AM »
We just got finished with this modification; the surgery was a success, but unfortunately it sounds exactly the same.  The only other thing we can check now is the OpAmp, and we're going to make sure that there really is 12VDC going to it before we attempt to replace the chip.

Thanks for the suggestion, even if it didn't help.
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Offline TrixterTopic starter

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Re: A4000 sound is... buzzy. Ideas?
« Reply #7 on: July 29, 2004, 04:32:38 PM »
Quote

Check if the two surface mounted 22uF 16V electrolytic capacitors C433 or C443 has leaked (is it clean around them?). They are often the cause of problem with the A4000 sound, even if they havent leaked they might just have dried out and is the cause of these strange sound-effects.


This wasn't it, as previously noted.  The new theory is that the voltages going to the LM880 op-amp chip are screwed up.  Here's a quote from an engineering friend of mine:

Quote

If the excursion of the waveform is asymmetric -- negative at lower amplitude and roundeder than positive -- then you may very well have a voltage problem. Most likely the op-amp runs on +12 and -12. With attenuated negative excursion (like my big words?), it seems to me that your -12 supply is not getting to the chip. This would be supported by the following experimental data: a sine wave of high amplitude shows up fine in the positive half of its cycle but is attenuated in the negative half. The waveform, though distorted thus, is otherwise perfectly smooth.


My question to the hard-core amiga hardware people here: If the voltage is messed up BEFORE it gets to the LM880, what's my next option?  What components should I start looking at to replace?
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Offline SilvrDrgn

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Re: A4000 sound is... buzzy. Ideas?
« Reply #8 on: July 29, 2004, 06:34:26 PM »
Quote

Trixter wrote:
My question to the hard-core amiga hardware people here: If the voltage is messed up BEFORE it gets to the LM880, what's my next option?  What components should I start looking at to replace?

Check the main motherboard power connector.  My A4000 sound problems were caused by one of the ground pins getting pushed out of the connector and not making contact with the motherboard.  I believe the power connector on the A4000 is six pins.  You have +5, +12 and -12 volt supplies.  Each supply has it's own ground pin (I think?) and the ground pin for the -12 volt on my connector got pushed out the back side of the connector coming from the power supply.  When I pushed that back in, everything came back to normal for audio from my machine.
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