a1200 audio sound problem

Hi guys, new on here so go easy.

I have an a1200 which recently developed crackling screechy audio at very low volume levels, I need to turn TV way up to hear anything.

I have had look around motherboard in particular the LF347 op-amp chip. All voltages seem ok except Vcc on PIN 4 is at -7v instead of +12v.

PSU is giving out +12v and -12v ok so can't be that and I'm getting +12v both sides of resistor R301. I noticed theres a FET directly connected to PIN 4 when tracing back the way on the LF347.

Could the FET be the problem?

TIA

[davideo]

It sounds like your capacitors have gone.

Have a look at this thread http://eab.abime.net/showthread.php?...ighlight=a4000 although it's talking about my 4000D the same happens with the 1200 and CD32

Dave G

Sorry forgot to mention I have replaced all 22uF capacitors and still same problem.

Don't see that causing the -7v at PIN 4 though?

[TheCorfiot]

@ITGuy

Have a look at this thread

http://eab.abime.net/showthread.php?t=41860

Post #22 by my friend Zetr0 has the Audio Power supply schematic if that's any help

Not sure if the FET may be causing the issue though ?

TC

[Loedown]

Greetings,
Pin 4 certainly shouldn't be negative anything, it's Vcc. The clipping is most likely occuring and I had this fault in an A500, that the amplifier cannot run full swing, I had lost -12V on the supply.

Go looking at your rails, or check to see if anything is dragging the rails down, I would suggest not only replacing the caps you did, but replace everything.

Paul

@ TheCorfiot

Yeah that was the schematic I was referring to when checking voltages and also at

http://www.ianstedman.co.uk/Amiga/schematics/schematics.html

I get 12v both sides of R301 but must go through something else before getting to PIN 4 as theres no continuity between R301 and PIN4.

I've checked the voltages on the back of the power connector on the motherboard and getting +12v -12v and +5v there ok when PSU is plugged in and Amiga switched on.

I'm stumped

[Loedown]

Pull the op amp out and see if the rail at that point miraculously jumps up to +12, if so, op amp is loading it down. Or even just desolder the pin and check what the pad reads.

Paul

[Loedown]

Just having a look at the schematic, I don't own a 1200, R301 is providing the +12V to pin 4, so check either side of that for starters. From looking at it, I would suggest the IC has gone belly up. I've also had an exceptionaly rare case where a ceramic cap went short, but that would load the +12 V rail down and the cap would get hot / catch fire. You also wouldn't read -7V on Vcc. The FET is only being used to switch the filter on / off, so even if it's gone dead short source / drain, it would make no difference, except when you wanted to use the filter.

Paul

Thanks Loedown, I'll have a look at the op-amp chip and report back when I get the chance.


Regards

Nice one Loedown!

I lifted the leg of pin4 on the LF347 op-amp chip and now getting a reading of around +11v from the FET output going to pin4 pad.

Looks like the op-amp was pulling the voltage down and is well and truly gubbed. Just need a replacement chip now.


Thanks again everyone