Rockford 800a2 problem with right channel

[Witwe]

I have trouble with the left channel of my Rockford Fosgate 800a2 amp.
It began with noisy/distorted sounds from the loudspeaker at normal volume.
When I raise the volume up the distortion disapears.
It also stays away when I lower the volume.
When I turned the amp of for about one hour the problem becomes the same again.
First I thought that it was a bad connection to the speakers but now the distortion won't go away anymore, even if I raise the volume up.

I have also checked the gainpots.
Today I send a sine tone to the amp and check the speakerterminals with a oscope:

right channel:

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left channel:

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can anyone tell me whats wrong with this amp???

 

[Perry Babin]

Does the waveform change when you connect a load to the channel?

Is this with or without a load?

Do you have a clean signal on C201?

 

[Witwe]

I have also checked this on every of the outlined mosfets with the same result, but always without load.
tomorrow I will check it with load connected. (I got the amp at work)

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[Witwe]

OK, I got the same result while a 8ohm computer speaker is connected.

Where should I grab the signal on c201? between c201 and r201?
-> there it is the same problem.

 

[Perry Babin]

Both side of the capacitor should have the same signal.

Are you saying it's distorted on the capacitor?

 

[Witwe]

I only checked it between c201 and r201 with the oscilloscope.
And the visual is the same as the visual I got at the speakerterminal.

 

[Perry Babin]

Drive a signal into the defective channel only and check the signal on the outputs pins (pins 1 and 7) of every op-amp. For the ones with signal, which are distorted and which have a clean signal.

 

[Dr Zeus]

What kind of scope is that? Looks like a heathkit or something home built. Looks cool.

 

[Witwe]

hehe

this is a good old RFT EO211 made in GDR
I think this scope is as old as me.

I checked the op-amps, u102, u104 and the a output (pin1) from u103 looks good.
but the b output (pin 7) from u103 got the problem.
does that mean that u103 is defective or is it something else?

Thank you for your help!!!!!!!!!!!!!!!!!!

 

[Witwe]

haa!

I think I have found the problem!

I checked pin 5 and 6 (channel b input and inverted input)
they also got this problem.
So I traced it a little back and found out that c107 is the part where the damaged sine wave comes from!
between r199 and c107 the sine wave looks correct, but between c107 and c108 it looks damaged.

do you think that simply changing this cap will solve the problem or could there be anything else that cause this cap to fail?

 

[Perry Babin]

I don't see an R199 that connects to C107.

That's part of the high pass crossover circuit. Does the amp produce a clean signal when set to full range or low pass?

 

[Witwe]

hmm, I thought that it already stays at fullrange...
I will check this tomorrow!
Then I will attach a photo with the resistor.
thank you!

 

[Witwe]

damn, my fault...
The high pass crossover was set!
I can't remember that I have ever switched it...
At Full and Lowpass the Sine looks good.

here is a photo of R199:

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As you can see, it is directly connected to C107 and Pin1 of U103

 

[Witwe]

I have desoldered this capacitor:

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you can see that it is broken!
Can you tell me what may cause this?
What replacement part should I use?
The capacitor is marked with 683.
683 = 68 * 10^3 = 68000pF = 68nF?
What does the color stands for?

 

[Perry Babin]

That's the correct value. The color is of no significance. It's a Mylar/polyester film capacitor.

I've never seen one crack in this type of circuit. It's generally caused by overheating which wouldn't be possible here. Are you sure it's defective?

 

[Witwe]

The cap is cracked and you can see the pin inside of it.
And you can easily pull the pin out.

It also matches with my fault description, because in the summer (when it was warm outside the problem disapears when the inputlevel/voltage gets higher.
Or am I wrong?
Should I pay attention to the rated voltage of the cap?
I have found this one: ECQE2683JF

 

[Perry Babin]

Sometimes, the epoxy can be cracked but there's nothing wrong with the cap. If the pin can be pulled out, it's definitely defective.

I don't know if this could be affected by temperature.

The voltage isn't important here. The lowest voltage mylar caps will be rated for 50v. The maximum voltage that can be applied in this circuit is 15v.

 

[Witwe]

OK Perry, thank you very very much!!!
I will buy this cap and see if it works.