CD32 very low audio and noisy

[Rochabian]

I've got a CD32 Rev4.1 board that has bad audio from the RCA & headphone

Very low audio and noisy on the two channels R & L

I've replaced already U38B & U15A with no success

U15A is just behind the PAULA chip U3
U38B is just before RCA & headphone output

Wich component is the sound amplifier ?

Is PAULA dead ?

I don't know what to check, change U31 or U38A & U15B ?

Can someone gives me another diagnostic on this problem ?

Audio schemes attached,

Thanks

[demolition]

Check that AVref is ~2.5V. If it is not centered between Gnd and Vcc, you will experience the symptoms you describe.

Do you also have problems with CD-Audio? If no, then it is either Paula, AVref (or a supply), U15A or U15B. If yes, then you're pretty much down to U38B (which you already changed). I don't think it is common for the opamps to die though and likewise with Paula which is why I would check the supply voltages for all the opamps as well as the bias voltages like AVref.

[theq]

Check Q341 or Q342.
Had similar problems myself were I had to replace one of those. Basically those will mute audio out at power-on and reset to prevent random noise spikes from blowing up your speakers.

[Rochabian]

i have removed Q342 and get normal audio volume but noisy,

i'll try to replace Q341 & Q342 first and see how it works

Then i'll try audio CD and check AVref

Thanks

[demolition]

Originally Posted by Rochabian

i have removed Q342 and get normal audio volume but noisy,

Q342 was probably bad then, but the audio should sound normal without noise with Q341 and/or Q342 removed.

Noisy in what way? Distorted audio or just constant noise? If the volume is normal and not distorted, then AVref is probably fine.

[Rochabian]

Replaced Q341 & 342 and it's the same
CD audio very low volume but not distorded
other audio low & distorded.
I'll check AVref

EDIT: I have Vdac = 2.27V & Avref=1.15V

On pin 8 U38A U38B U15A U15B = 12.30V

Could a bad capacitor do this symptoms ?

[demolition]

Vdac should be 5V, created by the 7805 voltage regulator U32. Check that you have 12V on its input pin 8 and 5V on the output pin 1. Seeing as you have 12V elsewhere, you probably also have 12V here and then I might suspect either the voltage regulator being faulty or U31 or C790 shorted it out.

[Rochabian]

I've replaced C790 ->idem
I'll try tomorrow to replace U32 as i have 2.27 on pin 1 instead of 5V, with C790 replaced

Thanks for your advices

[Rochabian]

Replaced U32 ->idem
It seems there is one last component to test -> U31
I've found an old Rev4.1 board wich has audio working
I'll compare voltage tomorrow

[demolition]

No need to desolder U31 from the working board - you can check if Vdac increases to 5V when you remove U31 from the faulty board. If it does, then Paula audio should now also sound normal and you will have a board that just has no CD-Audio until you put in a new DAC.

[Rochabian]

Well. I've desoldered the U31 and it's the same Vdac still 2.27

I don't understand why Vdac is not 5V, as it's generated by U32 and i've replaced it, and i have 12V on pin 8

It's sure that if Vdac is not 5V, then U15A and U38B can't work normally, because their inputs Vbias and AvRef are based on Vdac

Perhaps the 78L05 that i've replaced is faulty too.....This would be unlucky

In the pic attached i can see Vdac Generating Avref in PAULA/AUDIO DECOUPLING
And Vdac generated by U32 in CD/AUDIO DECOUPLING

I can't see what can be wrong with U31 desoldered......

[demolition]

Originally Posted by Rochabian

I can't see what can be wrong with U31 desoldered......

That is odd indeed.. Vdac isn't connected to much - C31 perhaps?
Also inspect the PCB closely in that area in case there are anything that could cause a short.

[Rochabian]

I'll try to lift pin 1 on U32 and check if it really produce 5V
Then i'll check for a short

Thanks

[Rochabian]

U32 is ok, C31 was bad

Now i have not distorded audio but low level on CD and paula

Vdac is 5V now

Perhaps U38B dead, i'll check the voltage

EDIT: Voltage are on the same range on this board and a working one
I think i have to deal with the headphone circuit because because whenever i plug the headphone or not, RCA are still at the same level but on headphone the volume is too high
Deconnecting the headphone PCB doesn't change anything, so i have to check where goes those signals

[theq]

Has the CD32 been re-capped?

If you have a oscilloscope - now is the time to use it

A very crude troubleshooting tool you could use for audio issues is a "mini-jack to RCA"-audio cable connected to an audio amplifier/stereo equipment. Be careful, though! Using this might cause static discharge or bridging between pins, so use with utmost caution.
If you have a TF- or SX-board I would recommend using AmigaTestKit to produce a 500Hz sine wave for troubleshooting.

[Rochabian]

It seems that the problem is after the amp.
Now i have good CD audio & Paula in headphone but very low on RCA, as if it was mute

I don't know if there is a system muting RCA when headphone is inserted or not, perhaps that's the problem here. It acts as if headphone were always connected

Perhaps i have to retest without Q342 or Q341 now

So no amp problem at the moment, it's between U38B & RCAs

Connecting the headphone to an amp via minijack to rca gives me good sound

It seems there was more than one problem on this board but now it's only the level of the RCA output that doesn't work, everything else is working great

[demolition]

If it is the same on both channels, I think it is either Q342 or Q341. The problem is not necessarily with the transistor, but could also be the signals controlling them, i.e. if C341 shorted out it would cause the transistor to turn on and mute the output.

[Rochabian]

Yes!!! Both of you are right!

It finally works. Pheewww.......

I've replaced Q342 or Q341 at start with transistors that doesn't match ( NPN instead of PNP ) ( it doesn't work without Q342 & Q341 too at the beginng of the repair, so i don't have seen that) )so reset was always activated i think

I've removed them for now and it works

Thanks to both of you for your diagnostics, very appreciated

There was multi problems on this board but now it works flawlessly

[theq]

[demolition]

Good to hear.
Although if reset was active, the machine should be stuck in reset and not be able to boot. Was it Q342 that caused it?