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Old 16 October 2018, 13:49   #1
Cv365
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Audio distorted on early CDTV units

I have tested at least 6 different CDTV units over the years that I have owned them and as far as I recall all of them seem to be suffering from distorted audio from Paula (the CD-DA audio sounds fine). Paula’s audio sounds like the gain is turned up too high, so the sound becomes overdriven / distorted. There’s a very noticeable difference between Paula’s output level and the CD-DA level (Paula is way louder). It’s not noticeable on every piece of audio, but one sure fire way to test it is to load up Lotus Turbo Challenge II and listen to the intro music.

For some reason I was always under the impression that this was due to the age of the CDTV's and that is was maybe due to bad caps. Some discussion I found while Googling also pointed to the possibility of the PSU being the culprit. However, I did find it odd that it seemed to occur on nearly every unit I’ve come across over the years.

However, while I was checking out Almathera’s DEMO CD recently I stumbled upon a README that contained this paragraph:

Quote:
On early CDTV models 100% volume may be distorted. Set volume to between
70% and 80% and turn up your amplifier to compensate. This is due to a
hardware fault in early CDTV units.
Holy crap! I never read about that before and I also couldn’t find anything about it online. It certainly would explain a lot! Does anyone know if this truely was an issue with early CDTV models? And, even better, how it can be fixed? It’s an annoying problem, especially if you have CD-DA and Paula audio mixed together.
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Old 16 October 2018, 14:11   #2
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I have no idea about the problem, but it would be easy to check for clipping with a scope. You'd likely just need to change a couple of resistors to correct it.
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Old 18 October 2018, 21:55   #3
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Quote:
Originally Posted by Hewitson View Post
I have no idea about the problem, but it would be easy to check for clipping with a scope. You'd likely just need to change a couple of resistors to correct it.
Thx for the tip! I might try that out on one of my CDTVs.
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Old 18 October 2018, 23:01   #4
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Is this another set of coupling caps set the wrong way around?

Not that I have any idea if that'd cause distortion of that kind.

John
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Old 30 October 2018, 00:46   #5
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I once compared the audio circuitry from A500 and CDTV to the schematics and noticed that there were couple of resistors swapped between the units. Swapping them so the values match those on the schematics lowered the Paula audio on the CDTV and boosted it on the A500.

So I currently have two machines with similar audio levels without clipping/distortion.
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Old 11 November 2018, 15:54   #6
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Thx for the tip!! Do you happen to recall what resistor(s) that/those might have been? Yeah, I know... you probably don't remember anymore, but thought I'd ask just the same

I opened up two CDTVs this afternoon and compared every resistor value from the audio circuit to those on the schematics from the CDTV Service Manual, but the resistors on the board all seem to match up with those in the schematics. Damn, I was really hoping to find a mismatch that could explain the distorted Paula audio..

I looked at every resistor that's in the audio section on page 4 of the schematics from the CDTV Service Manual. There are various versions of the CDTV Service Manual floating around on the internet, but they all seem to have the same revision of schematics inside and the resistor values correspond to those on the CDTV mainboard.

Sorry for the late reply btw, somehow I missed the notification of a new post to this thread.
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Old 21 January 2019, 09:18   #7
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Quick update. After re-reading this thread I decided to check the voltages on my CDTV. The original poster of said thread suffered similar audio issues too mine and discovered that the voltages being supplied by his PSU were much higher than they should be. I was replacing the noisy case fan of my CDTV this weekend and while I was in there I thought I might just as well check the voltages. I measured 12.04V on the connector going to the CPU fan and 4.98V across Paula, which are perfectly fine values compared to the PSU ratings (5V, 12V and -12V). So, I guess I can rule out problems due to too high a voltage... No visual issues noticeable with the capacitors on the PSU board either (I know... they could still be bad).

My next step will be to swap out the whole PSU with a new one and see if that makes any difference (I ordered an ATX adapter and a PicoPSU that I will be using to replace the original CDTV PSU). Might take a while until I find the time again, but will update here when I do.
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