Yamaha P4500 / P3200 Power Amp | Distortion

[Stoffie1974]

Hello Sloan,
Could it be a cross over distortion what you mean?
Of course this is hard to tell without a scope ...
I'd check/compare the DC voltages on Q103 & 104, if i'm not mistaken, they are for biasing the power transistors.
Compare the DC voltages over the output resistors too (should be several mV's)
This is indeed a complex amp, the right side of the power amp schematic is the "class H circuitry".
I'd also check/compare the breaking point (DC voltage where it goes to class H ) on L101 & 102 - Q133 & 134.
Just start it up, no signal, no load ... and avoid shorting pins with your probes !!
I wouldn't be surprised if you find a solder failure here, sometimes they are invisible to the eye, but when you bend a component, you can see the solder legs moving on the other side ... 1 component is enough.

Keep me posted, 'cause i'm about to buy a defective one for peanuts on Ebay !!

Grtz

[Enzo]

I am thinking he has some sort of gross distortion, he described it earloier as almost cant tell what the sound is. That sounds more severe than crossover to me.

[Sloan]

Thanks for all the help, I didn't get to mess with this thing tonight because of some other work, i would stay up and check it out, but i'm already fighting a sleep deprivation headache. too much to do!

I will report back hopefully tomorrow evening.

[Stoffie1974]

Hmmm, difficult to tel !
Maybe you could record the distortion and put it here as an attachement ?
Would also be great to attach the schematics with the measured voltages on critical components wrote down (good & bad board). just an idea ...

[Sloan]

Hmmm, difficult to tel !
Maybe you could record the distortion and put it here as an attachement ?
Would also be great to attach the schematics with the measured voltages on critical components wrote down (good & bad board). just an idea ...

Good ideas, I record music so I can throw a mic up to my speaker and post an example. I know you would rather not have a speaker attached while doing this stuff, but so far the outputs seem just fine, they're are amplifiying a bad signal is what it seems to me.

Working with the schematics is tough too since I can't print them out I have to scroll around all day in the .pdf. haha. I guess if this was easy it would be 'work' eh?

[Sloan]

In an attempt to rule out solder joints, I've reflowed/soldered every damn joint on the board. No improvements. more later...

[Sloan]

I'd check/compare the DC voltages on Q103 & 104, if i'm not mistaken, they are for biasing the power transistors.
Compare the DC voltages over the output resistors too (should be several mV's)
I'd also check/compare the breaking point (DC voltage where it goes to class H ) on L101 & 102 - Q133 & 134.

Q103/104, L101/102, and Q133/134 all test the same on both boards.
'P' transistors show similar mV
will check 'N' side soon...

My multimeter seems to have a real hard time measurnig these small mV readings, flashes through a bunch of values, so I just pick what seems to be showing most often. Is this ok?

[Sloan]

Didn't find anything strange on the other side of the board. Rather frustrating.
I'm just going through everywhere and trying to find a difference, not very efficient, but I'm not sure what else to do.

One thing to note is that the protection and temp LED's do not light up.

[Sloan]

Here is an mp3 showcasing the distortion.
Original audio on the right, distortion on the left. It has been compressed to bump up overall volume.

http://bestnetworx.com/uploader/file...ion_stereo.mp3

I've also attached this same file and can also upload each audio clip individually if needed (instead of panned hard left and right as above).


yamahap4500_distortion_stereo.mp3

[Stoffie1974]

Hi Sloan, sorry for the late reply ...
Great work you did there with the MP3, this helps us a lot!
The multimeter should give a stable value, even with mV.
This distortion seems to be a bad contact, loose joint (for what i'm hearing) and i think it should be comming from the preamp stage.
First of all, check the +18/-18 DC voltages on IC 101 & 201. Might be that just one of them is not getting it right, or that one IC is broken.
Which channel is the distorted in the schematic, the A or the B.
If B, check the stereo/bridge/parallel switch, it might be broken and result a bad contact.
I had a similar ditortion with an amp, and found the fault to be a 14-pin connector with just 1 bad contact due to oxidation.
These faults can be hard to find ... the easiest way is to inject a signal (if possible a clean sine wave, not music) and bend the cables & connectors one by one. When the signal comes out clean (normally measured on a scope) you found the source.
If no luck here, compare the voltages Q101-110 / Q201-210.
It also can be a shorted capacitor ...
Again, it would be much easier to locate the problem with a scope, you measure the sine wave on the input of a component, then you measure the wave on the output of that component, if it's bad, replace it, but we'll just have to do without it.
Maybe if you know somebody with a scope you could ask him ...
Can you record a clean sine wave 1KHz and post it here, this way i can open the file in cool edit pro, so i can "scope" it for you. Kind of a scope with very long probe wires ...

Good luck !!

[Enzo]

Scope the music anyway, it may not be a sine wave, but it should still identify something like missing half of a waveform or clipping.

[Sloan]

Ha, I thought about playing a test tone from my pc through it, but figured i would see if anyone recognized the character of the audio. When I get time, I'll record a tone through it. Thanks a ton for all the help so far, definitely a valued resource.

I've ordered all the electrolytic caps on the board since they're cheap and I figure it's worth a shot since i can't actually 'test' caps. Threw them in with a big order for some other stuff too, so don't worry i'm not wasting a ton on shipping just caps!

[Sloan]

I just finished swapping all the electrolytics for brand new ones, no change. I wasn't betting on an improvement, but hell, those were the one thing i can't really 'test'. I'm thinking about just ordering all of the small transistors on the board and changing them out. I've got to order a bunch of other stuff anyway, might as well throw those in eh?

[Stoffie1974]

Hi Sloan
Too bad for the capacitors ...
Replacing the small signal transistors, as long it doesn't cost too much, but this way you (and i) will never know which component caused this distortion.
I scoped your signal on my PC and the distorted channel shows a much smaller wave than the good one.
It's hard to tell what's wrong, because it's a stereo signal (L&R are not the same) so i cannot compare.
Try the test tone (1KHz) from your PC in double mono, this would clarify something i guess.
Did you checked the +/-18V supply's yet ?
You could try swapping the IC's 101 & 201 and listen if the distortion changes from board side.
This said, it's never bad to have some spare components ... just in case

Keep us posted
Grtz

[Stoffie1974]

Me again !!
I forgot to post the scoped images, so here they are.