1200XL has audible "buzz" on TV. What might be the source?

[ACML]

I have and have had several 1200XLs, but this last one is exhibiting a strange audio issue.  This one has a "buzz" that can be heard over the speaker and gets louder with volume, so it's coming across the audio signal.  For a brief second I thought it's because it has no RF shield, but I have tested boards without the RF shield, much less the case and have never come across this.  I am using both the 8-bit Classics and Hercules monitor cable and get the same "buzz".  I had modified this unit and had it running for days just resting on an electrostatic bag with no issue.  Once I assembled it in the case, the sound started.  All my assembled 1200XLs have the RF shield still on, but this one is the only one that does not.  Anyone know why an assembled 1200XL without an RF shield might have this audio "buzz"?  Again, when the board was just laying on my desk, no buzz.  First time I've actually encountered this working on Atari 8-bits?  

[Paul Westphal]

It’s just one of those weird things I have come across on the 8 bits and one Mega ST I have worked on.

I always leave the top off. 

[Rybags]

I suppose you already know but that's also sign of an overvolting PS.

Maybe without the shielding it's picking up mains hum from somewhere.

[ACML]

19 minutes ago, Rybags said:

I suppose you already know but that's also sign of an overvolting PS.

Maybe without the shielding it's picking up mains hum from somewhere.

The same PS was used during testing just prior to assembly and no noise.  I've used that same PS on several machines and none exhibited this issue. 

[ACML]

1) Tried different PS and same noise.

2) Tried another 1200XL, same PS, quiet like it should.

3) Disassembled unit to bare mobo, sound now present.

 

So, assembly is not a contributing factor.  Something happened to unit to change behavior.  What is the tall red potentiometer looking thing.  I assume its for video/audio adjustments?

 

 

[Paul Westphal]

The color pot is usually green or just bare metal. 

Edited February 1, 2023 by Paul Westphal

[ACML]

[_The Doctor__]

First things first, anti static bags can be slightly conductive and have been known to mess up stuff creating all kinds of paths that should not or could not exist otherwise when a PCB sits directly on them, sometimes things can fail spectacularly other times damage is more subtle.

 

Next up, some 1200XL's have been noted that the shield does in fact complete a needed ground, do a search about cartridges or hums and the 1200XL and you will see it mentioned in at least one thread on AA but might be more.

 

I think @bob1200xl talks about this at some point as well

Edited February 1, 2023 by _The Doctor__

[+x=usr(1536)]

10 hours ago, ACML said:

So, assembly is not a contributing factor.  Something happened to unit to change behavior.  What is the tall red potentiometer looking thing.  I assume its for video/audio adjustments?

If you're referring to the component at L11, that's an adjustable inductor, not a potentiometer.  It can have an effect on audio, but adjustment may not fix the symptom you're describing; that sounds like a possible grounding issue.

 

Should you decide to adjust it, do so very gently.

[sup8pdct]

I would start by checking the filter capacitors around the voltage regulators on the motherboard.

 

James

[+bob1200xl]

What have you done to the m/b?

 

That red inductor only affects the RF output. I cut them out when I do some mods.

 

Bad idea to lay a powered m/b on antii-static anything.

 

Did you remove the modulator? You need to add ground from the modulator to the PCB common.

 

Yes, check the filter caps on the power traces. Can you scope them?

 

Bob

 

[ACML]

3 hours ago, bob1200xl said:

What have you done to the m/b?

 

That red inductor only affects the RF output. I cut them out when I do some mods.

 

Bad idea to lay a powered m/b on antii-static anything.

 

Did you remove the modulator? You need to add ground from the modulator to the PCB common.

 

Yes, check the filter caps on the power traces. Can you scope them?

 

Bob

 

1) I converted to 28 pin OS and have the 600/800XL and Omniview XE operating systems burned on a pair of 27128 EPROMS.

2) I use the Channel 3/4 selector switch to select which OS to boot using Pin 26 (hi/lo) on the 27128s.

3) I installed the ClearPic2002 and the video is awesome.

4) I replaced the 74LS14 with a 74AC14 and replace C7 with 15 uF capacitor to remedy the cold start issue.

5) R63 jumper.

I have done all five multiple times on previous projects.  Very reliable.

Modulator is still there.

 

I have always rested the mobo on a thick pink electrostatic bag instead of resting in directly on a hard surface.  I assumed it was not conductive.  Something different this time WRT the electrostatic bag.  I always use the same think light pink bag for my projects, except this time.  This time, the mobo came with its own electrostatic bag so I decided to use it.  This was not a 1980's era bag, but a recent bag with a black line pattern printed on the outside (made me think of Eddie Van Halen's guitar stripe scheme).  Anyway, what was different this time when I did initial Ops checks (USAF for operational check), I noticed the screen background color changed from blue to green.  Touched the monitor cable and the color changed to red?  at first, I thought the monitor plug was wonky.  Then I pushed on the VLSI chips and the hues of the colors changed???  OK, now I realized there is something going on here that I don't understand.  About five seconds later, it hits me, "what's different"?  The bag!  My trusty 5 mil thick 1980 bag never gave me a problem.  This new one certainly did.  Got rid of the new bag.  Now this is all before any mods.  I'm still just checking out the reliability of this board.  

 

All during this checkout, I did not notice the audio buz, but the volume may have been set really low at that time (not sure).  Completed the R63, 74LS14 and C7 swap, Ops checked fine.  Completed the 28 pin OS mod, Ops checked good.  Moved on to the ClearPic2002 mod, Ops checked good.  Assembled computer in case, ran some programs.  Showed my 19 year old son who is taking a Python class how I had to do the same program in BASIC, just 40 years earlier.  No noticeable buz, but again, not sure if volume was set low at that point.  Hooked up the SIO2SD and Ultimate Cart the next day and noticed the buz on the audio.  I was loading games, so I probably had the volume up at that point.  The READY prompt in BASIC now had the buz???  

 

All graphics and sound look and sound fine. I swapped 9VAC power supplies, same buz.  Could I had damaged something when I powered the board up on the bag?  Right now, the machine checks out Code 1, all except the buzz that is proportional to the volume setting.

 

Which caps should I look at?  I do not have a scope.  All I have is a Radio Shack Multimeter and some Chinese capacitance meter.

 

 

 

Edited February 3, 2023 by ACML

[ACML]

UPDATE:  I removed the RF modulator, added a ground line from the RF pad to the GND pin where the modulator was installed (see photos).  I also removed L11 since ClearPic2002 rendered the RF inoperative.  Still have same buz/hum, but what I now have noticed was that the audible level gets distinctly louder when there is a cartridge installed.  Regardless if it's a plastic or OEM case with metal fuji plate.  I thought I read somewhere on AA there is a grounding issue with 1200XL and the cartridge slot, but I can't find it.  Anyone remember what the fix is for this?

 

 

[+bob1200xl]

Wire from pin 8 of the MMU (or pin 1 of the CPU) to pin B of the cart connector. Not an audio issue, though.

 

The filter capacitor is the large, round cylinder to the right of the rectifier bridge.

 

Any way you can play the buzz for us?

 

Bob

 

 

[ACML]

54 minutes ago, bob1200xl said:

Wire from pin 8 of the MMU (or pin 1 of the CPU) to pin B of the cart connector. Not an audio issue, though.

 

The filter capacitor is the large, round cylinder to the right of the rectifier bridge.

 

Any way you can play the buzz for us?

 

Bob

 

 

I've attached the audio.  In reality it is louder and more distinct than this recording picks up.  Best to turn up the volume to hear it better.

 

Edited February 4, 2023 by ACML

[TGB1718]

Must admit, that's not quite the noise I was expecting, couldn't hear anything before cart, but with cart

it's very bad, doesn't sound like the usual "hum" from a grounding issue, sounds more like there's an

instability somewhere causing a small oscillation you are hearing as the buzz.

 

This might be one of those things you need a scope to trace the source. 

[ACML]

6 hours ago, TGB1718 said:

This might be one of those things you need a scope to trace the source. 

Unfortunately, I do not have a scope.  I haven't used an Oscilloscope since is was an intern while going to college (1980's).

 

Anyone have any ideas on what would cause this kind of audio noise?

  

[phaeron]

Ran the audio through a spectrogram and it looks like the hum is 60Hz with the most audible harmonics at 360Hz and 480Hz, so maybe it is ground related?

[_The Doctor__]

Grounding, filter caps, maybe 4050.  I'd scrutinize the Power Supply section.

[ACML]

Which are "Filter caps"?  Why would the presence of a cartridge amplify the audio.  It's like a gain.

 

I am familiar with FFT Power Spectrum Density (PSD), but it's usually with accelerometers.  Why is there no harmonic at 180?  Does that mean 120hz is more prominent than 60hz? 

 

120hz, 240hz, 360 and 480hz are strong.

 

 

Edited February 4, 2023 by ACML

[+bob1200xl]

It sounds like a hi-impedance input that is open/disconnected. Inserting a cart seems to add more antenna. I would try probing all around the audio circuits with my finger to see if I can localize the source. (keep the other fingers in your pocket - don't ground them) If you find a sensitive net, you can then hold one finger on ground and probe the suspect wiring with your other finger. See if it absorbs some of the unwanted signal.

 

Power supply hum is normally a trapezoid, open circuits are usually a sine wave.

 

Does the audio work otherwise? 

 

Bob

 

[ACML]

24 minutes ago, bob1200xl said:

Does the audio work otherwise? 

 

Bob

 

Yes, the audio works fine, just the annoying hum in background.

[ACML]

35 minutes ago, bob1200xl said:

I would try probing all around the audio circuits with my finger to see if I can localize the source. (keep the other fingers in your pocket - don't ground them) If you find a sensitive net, you can then hold one finger on ground and probe the suspect wiring with your other finger. See if it absorbs some of the unwanted signal.

 

Bob

 

Bob,

   I touched pretty much all the components and "no joy".  I could not find any component that changed the sound of the hum.

 

[ACML]

So I tried putting on the full RF shielding hoping that would provide a cure for the mystery ground issue, but to no avail.  The hum/buzz is unchanged even with the RF shielding.  Not sure where to go from here.  I'll troubleshoot a bit longer, but if that does not provide a solution, I may just harvest the VLSI chips and a few components and concede defeat.  This would be the first 1200XL board that I could not make work to my satisfaction. I sure would not pass it on to someone else with this annoying feature.

 

Edited February 20, 2023 by ACML

[_The Doctor__]

The pictures were too blurry for my eyes to check when at full screen or zoom, I still saw scratches etc. but really can't tell if they are superficial or not. The RF modulator was removed and there are places where the PCB show signs of excessive heat from someone soldering about. Tape over wires do not let us see those portions nor the modifications. a quick rundown on what the 'fixes' are would be in order.

Better pictures, and 'fix' run down would be good.