CDTV : Sound Problem (One channel saturated)

[gibs]

Sorry, I have removed this little thingy (Q4) and I could not put it back. (need to order a new one)

Should I test without it ?

[gibs]

Ok I tried without Q4. The problem is still here.

[Shadowfire]

I would say at this point, that the problem is most likely the +15/-15V coming out of your power supply. Do you have access to some benchtop power supplies? You can jumper a +12/-12 from benchtop supplies to the connector on the mainboard (replacing the output on the CDTV PSU) to check this.

Even though the opamp is rated up to +16V, the extra +20% amplitude could be clipped by the later audio stages.

[Toni Wilen]

I have two CDTVs and both have nearly identical PS output voltages, +5v, -12.2v, +12.2v, another have identical +5v but -12.1v and +12.1v. (I am not going to test sound problems using higher voltages, sorry )

I'll do quick power supply electrolytic capacitor ESR (internal resistance which increases due to drying out) check next week, voltages are ok but perhaps there is one or two capacitors that have higher than normal ESR and hopefully it is common problem..

[gibs]

Toni,
It's a luck that I noticed that values when you asked me to check the -5V.
Because I had tested the Voltage earlier and they were correct (+12V -12V 5V)

This afternoon I have noticed those high voltage.
Later I did another measure and the voltage were good, but the sound problem was here.

Now, I just did another measure and I have high voltage (more than 15V) again.

Yesterday, I had a look at the PSU's capacitors, there were no blow up, no enlargement...

Therefore this PSU should have a problem, because when it starts from cold, I get high voltage, then with the time, the voltage decrease to good values.

Shadowfire, I think I'm going to use an ATX from a PC for the test.

Pics of my PSU:
http://dl.dropbox.com/u/1566495/CDTV/P1020376.JPG
http://dl.dropbox.com/u/1566495/CDTV/P1020377.JPG

I don't see any suspect here.

Edit: Ok guys, I'm going to fix the PSU, then I'll send you a PM to say that I have updated the thread.
Again, thank you very much. Cheers

[Toni Wilen]

Originally Posted by gibs

Yesterday, I had a look at the PSU's capacitors, there were no blow up, no enlargement...

Dried out electrolytic capacitors rarely look any different, unfortunately.

I did quick ESR check on my CDTV power supply but everything looked fine. "Unfortunately" my power supply is not damaged enough yet

Therefore this PSU should have a problem, because when it starts from cold, I get high voltage, then with the time, the voltage decrease to good values.

This is near-guaranteed symptom of bad electrolytic capacitor(s). Bad capacitors get better when they warm up.

EDIT: if you feel bored, you could try replacing those smaller ones (big ones rarely fail), there aren't many of those anyway..

[gibs]

Hi Toni,

No I'm not bored, I have ordered all PSU's electrolytic capacitors.

I'll keep you updated

[cpiac64]

hello, it's my first post and sorry if exist a section for newby and sorry for my bad english.

my problem is similar, one channell (red - right) distorcion with game and cd of music, at the moment don't use any floppy (i have one but is broken), next time i measure tension on psu and change any capacitor

p.s.
i am electronics and i have a tester and oscilloscope

[Loedown]

Originally Posted by cpiac64

hello, it's my first post and sorry if exist a section for newby and sorry for my bad english.

my problem is similar, one channell (red - right) distorcion with game and cd of music, at the moment don't use any floppy (i have one but is broken), next time i measure tension on psu and change any capacitor

p.s.
i am electronics and i have a tester and oscilloscope

If you have an oscilloscope, you can trace the signal path and find where it fails, if you write back where it starts to clip, then we can suggest which component(s) to change.

[Toni Wilen]

CDTV audio signal path is: (get schematics!)

Paula pin 30 -> U5 pin 9 -> U5 pin 8 -> U5 pin 12 -> U5 pin 14 -> U6 pin 13 -> U6 pin 14 -> Right channel output.

Paula pin 31 -> U5 pin 4 -> U5 pin 7 -> U5 pin 3 -> U5 pin 1 -> U6 pin 2 -> U6 pin 1 -> Left channel output.

Check all above pins and compare waveforms, you should find the point where sound gets bad very quickly.

[gibs]

@cpia64 Musicians are using 2 channels (of 4) for the drum + the bass. In most of case (for mixing reasons) those Instruments are on the same channel because they produce low freq sounds.

If you are using your scart cable for audio output, the sound could soundlike less saturated than if you are using an external amp, I mean that one channel could soundlike "ok" on TV output. So... be sure that you have the problem in only one channel (try different musics for example, where the Drum/Bass are on the White RCA)

Toni, My Psu was delivering more than 16V in charge...more than 20V in standalone :-( poor CDTV. Of course I don't use it anymore.

PS: I still haven't received my capacitors

[cpiac64]

at the moment i have connected the speaker of my computer on hear connector and the audio is perfect, next time i connect oscilloscope on pin of paula

[Shadowfire]

Thats a total loss of regulation, and is probably the source of all your problems. Too bad you didn't have an oscilloscope, you could have found all the ripple on your audio supply rails.

[cpiac64]

i have a dubt, i connect cdtv to another tv (plasma) and the audio is ok, next i have connect to previuos tv (crt) my 1200 and the audio is ok, mumble

[gibs]

Hi guyz,

As I haven't received my capacitors at this time, I have removed the capacitors from the CDTV PSU. Then with I did some measurements and only one was 1560uF instead of 1500uF.

So maybe the PSU's capacitors are not faulty.

So, to win some time, I have done an adapter so I can test the CDTV with my ATX PSU modded for Amiga.

http://dl.dropbox.com/u/1566495/CDTV/P1020388.JPG

It's almost finished.

[Toni Wilen]

Originally Posted by gibs

Hi guyz,

As I haven't received my capacitors at this time, I have removed the capacitors from the CDTV PSU. Then with I did some measurements and only one was 1560uF instead of 1500uF.

So maybe the PSU's capacitors are not faulty.

Electrolytic capacitors, at least large ones can have -20/+20% tolerance...

They can still be faulty. Old electrolytic capacitors may appear perfect, both externally and when measured but they can still fail to work correctly in high frequency (like switching powersupply or CRT TV) circuits.

(many years ago I fixed P3 mainboard that didn't boot, it had known bad capacitors, most of them had leaked but they all still measured perfectly fine in capacitance meter. Replaced capacitors and it booted normally)

[gibs]

ok Toni. good to know.

I was about to try with my ATX PSU. but it's not working anymore lol.
It was working yesterday...I'm unlucky. Unbelievable.

Well I have another one... Just need to do the mod again...

[gibs]

I checked with my adapter and a new ATX PSU and the screen display was anomaly bright (in sleep mode), so I shut off immediately. I checked the voltage on each pins and all seems OK (on the 3 connectors) if:

All RED are 5V
All BLACK are GRD
All Yellow are +12
Orange is -12

Someone can confirm ?

http://dl.dropbox.com/u/1566495/CDTV/Pins.jpg

.

[rkauer]

Orange is +3.3V.

-12V is the blue wire.

[gibs]

Originally Posted by rkauer

Orange is +3.3V.

-12V is the blue wire.

I don't ask for the ATX side.
I ask to someone to get the voltage on the 3 connectors of the CDTV. (there's no blue wire but thanks)