Emotiva UPA-500 Distorted Channels

[AlexRivera]

Hello Everyone!

I have a little Emotiva UPA-500 with 4 out of 5 channels distorted, this is my first time troubleshooting an Amplifier with more than 2 channels, can you suggest what are the culprits in this scenario?

I'm checking Filtering caps in the power supply but at the same time I think, wouldn't all be distorted if that was the problem since they all share the same power supply?

Thanks in Advance.

 

[rayma]

I'm checking Filtering caps in the power supply but at the same time I think,
wouldn't all be distorted if that was the problem since they all share the same power supply?

That's right. Check in case there are individual fuses per channel.
Otherwise (with no power and unplugged), check the resistors and semis in-circuit,
comparing with the good channel if necessary.

 

[Mark Tillotson]

Would the working channel be subwoofer by any chance? That's likely to have different PSU voltages.

 

[AlexRivera]

Otherwise (with no power and unplugged), check the resistors and semis in-circuit,
comparing with the good channel if necessary.

I checked the 4 filtering caps in the powers supply and as expected they are all good.

I will compare the channels tomorrow and see what I find.

Thanks

 

[AlexRivera]

Would the working channel be subwoofer by any chance? That's likely to have different PSU voltages.

No, this amp doesn't have a subwoofer channel

 

[AlexRivera]

Folks,

So for a reason I don’t know, now only 2 out of the 5 channels are distorted, what I did so far is:

1. Desoldered the filtering caps on the Power Supply just to test them and then soldered them back.

2. Disassembled the Amplifier board and, you know, twisted it around a bit to look at the bottom, etc.

3. I noticed the board was dirty mostly with signs of excessive soldering flux so I sprayed some DeOxit and brushed it a bit cleaner.

4. Measured some resistors, continuity, etc. But haven’t changed any parts.

I’m surprised to see how many SMD components this amplifier has on the bottom part of the amplifier board, hundreds of them...

Then I decided to put a 1kHz sine wave and analyze it with the scope, that’s when I noticed that only 2 are bad now, then I played some music and connected the speakers and yes, confirmed, they sound good with music as well...

What could be causing this???

Attached are some pics from the scope, there you can see how a good channel (blue) compares with the bad channel (yellow) both playing music and with the 1kHz wave.

And because I have all the testing equipment it doesn’t mean I’m a Pro by any means, not even close, I’m just an Audio Enthusiast and this is my hobby.

 

[llwhtt]

Does the bad channel have that sawtooth pattern with no input signal? Sawtooth is usually raw power supply voltage. Are those signals with a load or without a load?

Craig

 

[AlexRivera]

Does the bad channel have that sawtooth pattern with no input signal? Sawtooth is usually raw power supply voltage. Are those signals with a load or without a load?

Craig

Thanks for your reply Craig!

No, w/o signal is completely flat.

And the signals are without a load.

I’m attaching the same measurements but with an analog scope, the first with both signals in and the second with the bad channel without signal in.

 

[llwhtt]

I don't suppose there's a schematic?

Craig

 

[AlexRivera]

I don't suppose there's a schematic?



Craig

Unfortunately I don’t have one and I have not been able to find it

 

[rayma]

Do the bad channels have any DC output voltage? Do you even need the bad channels anyway?

 

[llwhtt]

You''ll probably have to ask Emotiva for one, fingers crossed.

Craig

 

[AlexRivera]

Do the bad channels have any DC output voltage? Do you even need the bad channels anyway?

I’m glad you asked, I didn’t measure this before!!

Yes, the bad channels have DC output, one has 876mV and the other 650mV, all others have anywhere from 15 to 65 mV.

 

[rayma]

the bad channels have DC output, one has 876mV and the other 650mV, all others
have anywhere from 15 to 65 mV.

Might be leaky electrolytic capacitors. A bad transistor probably would cause much more DC output.
There may be more problems than capacitors, if you get no sound in those channels.
See if you can get a schematic, it would help a lot.

 

[AlexRivera]

Quick Update,

I tried finding a schematic but nothing, Emotiva didn't even respond (yet) my email where I asked about the price for the repair which should be very easy since they have a fixed price policy.

So I did remove the electrolytic caps for that channel and tested them, they were all good, even then I replaced them with new Nichicon caps and nothing the problem was still there, then I removed the output transistors to check them and they were also good, not even leakage, so when I soldered them back, voila.... no DC offset, it went down to 1.1 mV, then I tested with the speakers and no distortion.... Cold solder huh???

I tried re-heating the solder in the other channel's output transistors but distortion is still there, DC offset is gone too, that channel is also down to 1.1 mV.

I will keep looking around and re-heating solder here and there and see what happens.

I will also run the 1kHz sine wave and analyze it with the scope like I did before and see what happens.

Cheers!

 

[rayma]

Good work, keep at it but be careful and don't get discouraged.