followup: MVS: some sound

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V3rtigo

Mr. Big's Thug
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A few weeks ago, i started a thread about not having sound in my MVS. since then, i installed a cap kit and i have sound! well, kinda. i have sound at low volumes. anything above the level of a normal conversation, i get a horrific buzzing. or it'll have sound for 15 seconds, and then the buzzing starts. i swapped the old speakers in but same problem so i think i can rule out the speakers. i'm sure i'm not the greatest at soldering, but they look pretty darn clean.

so at this point, i'm stumped again. anyone have any ideas?

short in a wire somewhere? gremlins?
 
Xian Xi

Xian Xi

JammaNationX,
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post a pic of your soldering. I'm gonna guess that a cap is backwards or something got heat damage.
 
channelmaniac

channelmaniac

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I've seen a board that would oscillate in the audio amp section for some reason... Never could figure out why. I swapped electrolytic caps, foil caps, volume controls, amp chip, and checked all the resistors to no avail.

I found that if I pulled the foil cap from the output section on one channel the problem went away. Check the foil caps next to the 470uf caps you installed on the board.

RJ
 
V

V3rtigo

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i'll post pics later of my soldering.

RJ, does it matter which channel i pull? that wont kill the sound to that channel? if not, better if i pull it for both channels?
 
V

V3rtigo

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finally an update after a busy month.

so i pulled one of the two foil caps (the red ones) and the sound improved greatly. at a playable volume, there is no longer any buzzing. im still tempted to pull the 2nd foil cap but i'm holding off on that since the sound is now acceptable.

anyways, on the screen problems, no luck. i replaced the flyback, transistors, cap kit... no luck. whatever i did the fuse would blow. i can't figure it out so i guess it looks like i'll be looking for a new screen. so should i stay stock or is there a decent upgrade that wont break the bank?
 
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li_gangyi

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Have you also checked the posistor and degauss circuitry? Make sure that it's not failed shorted.

If the fuse is blowing instantly, no whine and stuff before it cuts off. Check these

Degauss Posistor - very likely
Horizontal output transistor
Power supply regulator if there is one
Power supply chopper (switchmode) transistor if there is one
Diode(s) in main bridge rectifier
Main filter capacitor(s)
 
Hewitson

Hewitson

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Good advice.

channelmaniac: What is the intended purpose of those caps? Can they both be removed?
 
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V3rtigo

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i guess ill go hunting for the posistor and pull it see it i can get some glow...
 
channelmaniac

channelmaniac

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Depending on the chassis, you may be able to get it fixed by sending it to Chad @ Arcadecup.com

Hewitson: They are bypass to ground. I'm not sure for what frequency though. For some reason they can induce an oscillation in the audio amplifier circuit. I've tried replacing them and replacing the audio amp to no avail. The other parts in the circuit tested properly. The only thing I can think of is a failure on the board itself but it's not a 4 layer board. Pulling one of the caps fixes the oscillation.
 
arfink

arfink

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As for the oscillating, could it be caused by a bad ground? Or some kind of intereference in the ground coming from on the Neo board? I had some really nasty audio problems which were being caused by a bad ground in a PSU.
 
channelmaniac

channelmaniac

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Possible, but unsure.

On the one I fixed here that had that problem it was showing up on my JAMMA testbed which has seen hundreds of boards. It was definitely a problem with the board and not the testbed.
 
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li_gangyi

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If the capacitor is part of what is know as a 'zobel' network, it's purpose (together with an inductor or resistor) is suppossed to remove high frequency oscillations... as to why it is causing it...
 
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V3rtigo

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i looked last night and couldnt find anything that remotely resembles a posistor. is it not a 3-pronged square on an mvs board? a pic would be great if anyone has one handy.
 
Hewitson

Hewitson

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Its on the chassis. :/

It is usually a square component about 10mmHx4mmWx4mmD.
 
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