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Ely

Problem with Atari 800XL having quiet audio

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Hey there,

 

I recently picked up a really nice looking PAL 800XL on Ebay in the UK. This thing looks mint but it was sold with an issue with the audio volume being low. I did some research before buying (on here) and it looked like the likely candidate could be a dodgy LM358N audio amp chip.

 

So I took the risk and bought the machine and ordered some new LM358N chips I then took the old one out and replaced with a brand new one. And the result, the audio volume is no better, well maybe it's a little better. It's also slightly louder through the RF modulator and the video port but nothing drastic.

 

I've reseated the POKEY just in case that made a difference, but it didn't.

 

So has anyone got any ideas what else I could look at to the problem?

 

Thanks in advance for any help.

 

Ely

 

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I had the same problem with an NTSC 600xl. I eventually realized that the LM358N wasn't getting a tight enough connection in the socket. The audio would boost if I pressed on the chip. As soon as I took my finger off, the audio volume would drop. I eventually fixed it with a little solder.

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Thanks for the info, I am thinking maybe I've not soldered the new chip in that well. No socket on this machine though, all the memory, cpu and custom chips are but not the 358.

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Thanks for the reply Paul, I'll take a look at my board later today.

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Over on the left of the Mobo, there is a small 6 pin chip usually labeled RCA.. I had luck with this one on a 800xl sound fix...

You should use both hands and recount the pins on that chip labeled RCA, (aka) CA358

Edited by rdea6
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Ah, if the RCA is the LM 358 (8 pins) I've already swapped that chip.

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Hey there,

 

I recently picked up a really nice looking PAL 800XL on Ebay in the UK. This thing looks mint but it was sold with an issue with the audio volume being low. I did some research before buying (on here) and it looked like the likely candidate could be a dodgy LM358N audio amp chip.

 

So I took the risk and bought the machine and ordered some new LM358N chips I then took the old one out and replaced with a brand new one. And the result, the audio volume is no better, well maybe it's a little better. It's also slightly louder through the RF modulator and the video port but nothing drastic.

 

I've reseated the POKEY just in case that made a difference, but it didn't.

 

So has anyone got any ideas what else I could look at to the problem?

 

Thanks in advance for any help.

 

Ely

 

10 SOUND 0,100,10,15:GOTO 10

 

check for pulses qt pin 15 of GTIA while pressing a key. if pulses are missing, check GTIA.

If pulse are present check some capacitors (C23 and R5 on NTSC 130XE).

check RF modulator.

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Thanks for the reply. Sorry, but what do you mean by "check for pulses qt pin 15"?

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Thanks for the reply. Sorry, but what do you mean by "check for pulses qt pin 15"?

You can check for pulses at pin 15 with a logic probe. A logic probe simply does pulse beeps, no response is low,

, constant tone is high reading. So you can get high, low or pulse. The alternative is to check the pin with an oscilloscope.

In which case you would see a either a sine wave or other pattern, or a constant high line, or a low line.

basic info needed, besides the probe, pin 1 is at the bottom left with the chip oriented longways left to right and

not upside down as you can tell by the writing on the chip.

The pins go from bottom left is pin 1, then over to the right side and around to the same end on the top side,

then to the left to the last pin.

There's frequently a notch on the pin 1 side.

Of course the board has to have power to it.

Be careful, it is easy to cause a short by touching two points at once.

Edited by russg

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Thanks. It's bit tricky to do that without either a logic probe or oscilloscope. My Dad has one of the latter so I might take a trip over to see him for some help.

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Thanks. It's bit tricky to do that without either a logic probe or oscilloscope. My Dad has one of the latter so I might take a trip over to see him for some help.

logic probe...

 

http://www.electronicsurplus.com/philips-pr31-logic-pulser-probe

 

hook the black clip to a ground, any bare metal on the board.

 

hook the red clip to a power point. lots of places. I forget how to find one.

I think it is called a 'vcc' point.

 

then just touch a pin on a chip to read it.

Edited by russg

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A bit more information on this. This Atari 800XLs keyboard is currently not working. When plugged in it just sends full stops continuously, so clearly a membrane issue which I need to look into.

 

What's interesting though is that the volume of the keypress sound is at the correct volume, could this help diagnose this problem at all?

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It may not actually be the keyboard at fault, if the full stop key isn't pressed down or stuck down then its probably chip related, with this info I reckon one of these hardware guys will know the problem...

 

Best of luck

 

Paul.

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A bit more information on this. This Atari 800XLs keyboard is currently not working. When plugged in it just sends full stops continuously, so clearly a membrane issue which I need to look into.

 

What's interesting though is that the volume of the keypress sound is at the correct volume, could this help diagnose this problem at all?

 

 

It may not actually be the keyboard at fault, if the full stop key isn't pressed down or stuck down then its probably chip related, with this info I reckon one of these hardware guys will know the problem...

 

Best of luck

 

Paul.

 

Let's see ... POKEY stands for "POtentiometer and KEYboard" and also handles system sounds. So if POKEY is bad, it could well affect both aspects of the system.

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See, told you the clever types would come along :)

 

And yes DrV, I really should have thought of that one straight away...DOH!

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Hmm, I guess it could just be the POKEY with a problem.

 

When I got the machine the volume issue was present, but the keyboard issue wasn't until I took the machine apart, ever since then plugging the keyboard in I get the problem (BTW how rubbish is the connector for the keyboard!!).

 

EDIT: Just ordered a new POKEY for £11.50 off of eBay so let's see what happens when I replace mine with that one.

Edited by Ely

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I can confirm that the replacement POKEY I order made zero difference to the low volume or the keyboard issues. Which is a shame :(

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So, for anyone who's interested, this issue is now resolved. Turns out the C20 resistor had gone "open-circuit", thus breaking the signal path between the 2 amplifier parts of the LM358. My Dad found this, he's much better at electronics than I am, and retired to has the time to investigate :) Thanks for all the suggestions though.

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So, for anyone who's interested, this issue is now resolved. Turns out the C20 resistor had gone "open-circuit", thus breaking the signal path between the 2 amplifier parts of the LM358. My Dad found this, he's much better at electronics than I am, and retired to has the time to investigate :) Thanks for all the suggestions though.

 

...

 

Deleted,

 

Edit: I did not notice that C20 was an electrolytic capacitor. It become damaged by time/use.

 

 

 

madi

Edited by Madi
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Yeah sorry, i mistyped. It was a capacitor not a resistor that was the problem.

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Nice fix. I was following this thread and was thinking it was an issue with one of the caps in the audio/amp section but russg had already suggested it.

 

Quiet / humming audio is almost always a bad capacitor.

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Just to add my 2 cents worth, after my U1MB upgrade installation on my Atari 800XL, I was experiencing very muffled sound at low volume (even with display volume maxed). Key clicks seemed normal but games and demos sounded horrible.

 

After reading this and other forum threads, I replaced C19, C20 and C22 and the problem seems to be cured.

 

These capacitors are all 4.7µf 35V NP (no polarity) electrolytic capacitors. As I was unable to purchase caps rated at 35v , I purchased 4.7µf, 50v NP capacitors and they seem to work fine.

 

Here's some videos (fast forward towards end of first video if you want to get to the good stuff):

 

Before: https://youtu.be/JrnVlW4U7QE

 

After: https://youtu.be/P9zdhvcVFkg

 

Once again, saved by all the great knowledge on Atariage!! Thanks guys.

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