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Post subject: Mustang 3 Ver:1 Power Amp
Posted: Sun Jun 21, 2015 8:37 am
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Location: Stockton NJ USA
Finally couldn't take that buzzing brittle, sitar Fizz any longer.
Took the amp down into my basement workshop and opened it up.
I took the clamp that holds the power amp chip down onto the heat sink so as I could find out what was being used.
Turns out that the 3, and possibly the 4 and 5 all have this same chip. Its a TDA 8950J 2 X 150 watt class D amplifier. For the 3 they only use the right side output. The left hand output is not used.
I was quite shocked to see so many cold solder joints. Very dull, not shiny and the component wire ends loose, barely contacting the solder around them. More in the power amp section than the power supply end of the board.
So I carefully re-soldered the joints that looked poor.
Under the power amp section of the board are two power resistors. The type you see in CRT power sections of TV's. Square and about 1.5 inches long. These are heat sinked to the metal chassis via large amounts of white thermal paste.(Horrible stuff to get on your hands or clothes) They had cold solder joints. So they to were re-soldered and had the thermal paste reapplied.
Blew all the boards off with Dust Remover Spray from RS, replaced the power chip clamp and bench tested.
Buzz/Fizz/Brittle sound Gone!!. Back to the great sounds of before.
Buttoned it up and now its back in my music room (Minus all the dust from 3 years)

Here are a few pics.

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Power amp section.

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Power amp with clip.

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Power amp part number.

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Power amp left channel output not used.

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Power resistors under power amp section. Note the white thermal grease and how its burnt out in places on the unused channel. This probably explains why it took over 3 years to start the awful fizz sitar noises. Lets see how long it lasts now that I have replaced the grease insulation properties.

Edit.The cold solder joints were on the power resistors under the preamp section of the board R472 and R449. Plus other electrolytic caps on the board. Possibly a bad run that day due to solder temp or solder quality.


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Last edited by JohnR242 on Sun Jun 28, 2015 1:06 pm, edited 1 time in total.

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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Sun Jun 21, 2015 6:42 pm
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Thank you John for providing this, great information.

Bob


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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Mon Jun 22, 2015 2:54 am
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+1, great find. I wonder how much of that Fender has known all the time (and decided not to tell their customers).


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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Mon Jun 22, 2015 4:08 am
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this is just brilliant John. Nice work!


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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Mon Jun 22, 2015 5:47 am
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Congratulations John - very impressive work.
Surprised no one else stripped one done before - but maybe they didn't know what cold solder joints look like.
So you could have converted the output to 300 watts - or added a stereo channel. I bet you thought about that for a while !


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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Mon Jun 22, 2015 6:09 am
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Yeah, soldering on another 1/4" jack (or better yet, a Speakon jack) for a speaker out wouldn't be that hard. Then you could buy another defect MIII, put them side by side and have a 2x12 if you need it and a 1x12 if you don't.

Hmmm...


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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Mon Jun 22, 2015 7:44 am
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Thanks for the kind replies. I was contemplating doing some updates, like change out those flimsy phones and aux in connectors. However, if it's not broke I err on the side of leave it be.
The one thing that surprised me was the fact that the amp doesn't have a star ground point. (Where all the boards run a ground wire to the ground lug for the incoming power) The lug is there, but only has the ground from the power input attached to it.
All the grounds for the boards are through the metal chassis.
Image[/URL][/img]

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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Mon Jun 22, 2015 10:13 am
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All the grounds for the boards are through the metal chassis.

I guess the simple fact is, the cost is managed by cutting corners at every possible intersection.

I'm sort of amazed,, with the stuff I've now learned about these amps, the my 2 Mustangs have been very stable.

the III doesn't fiz, sounds great and the four, well for now,, it just works.


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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Thu Jun 23, 2016 2:27 pm
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Well it has been a year since my power amp repair, and I'm glad to say......no fizz.
Just thought you all would like to know.

Hope everyone is enjoying their Mustangs and making a lot of good music. Enjoy the summer everyone!!
Cheers!!


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Post subject: Re: Mustang 3 Ver:1 Power Amp
Posted: Sat Jun 25, 2016 7:56 am
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JohnR242 wrote:
Well it has been a year since my power amp repair, and I'm glad to say......no fizz.
Just thought you all would like to know.

Hope everyone is enjoying their Mustangs and making a lot of good music. Enjoy the summer everyone!!
Cheers!!

Thanks for the update John and for your original posts. Really appreciate them. Cheers!

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