Fender FM212 - Deluxe 90 PowerAmp problems - diyAudio
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Old 18th April 2015, 07:56 AM   #1
Tarzan is offline Tarzan  Belgium
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Default Fender FM212 - Deluxe 90 PowerAmp problems

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It's a problem I encountered a few times and this time I try to solve it.
It's allways in the same type of Fender poweramp.

Now it's a FM212R.
It uses the same power amp as the Deluxe 90 and even the same component numbers.

After different attempts over time the amp is still behaving bad.
I.o.w. - It plays great but within 10 - 20 seconds it starts to draw excesive current and Q20 becomes hot and hotter and hotter…
(Yes the heatsink is attached.)

When I use a freeze sparay on transistor Q14 and the surrounding components the amp behaves normal for 10 seconds or so.
It is a thermal problem but where?

I suspected a bad Q14, but that didn’t helped at all.
I finally changed the outputs, Q20 and Q21 and it helped a bit but finally it took 10 seconds more before Q20 became hot again.

So at the end you suspect every component and test them all, one by one.
Of course these from the power amp stage...
They all measure OK.

Very frustrating...
"I can’t see the trees anymore in the wood"

A hint can do wonders when you tried allmost everything with no results...
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Old 18th April 2015, 09:46 AM   #2
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I would check R101 and 105, they produce the on bias. If Q14 heats up the CE voltage drop decreases and the quiescent current should reduce. Check all the diodes!
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Old 18th April 2015, 02:15 PM   #3
brig001 is offline brig001  United Kingdom
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If Q20 gets hot but Q21 is cool, check the DC offset - the current is going somewhere and if it's into the speaker, it will cook eventually.

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Old 19th April 2015, 07:00 AM   #4
Tarzan is offline Tarzan  Belgium
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It's sunday today and I have other "Garden stuff" to do.
Tomorrow I’ll have a look at it again…

Allthough the different parts all measured OK, it will be a problem that when voltage is applied one of these will become faulty.
Aargh, these are pesky problems…

More details tomorrow evening…

Thanks for some hints.
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Old 20th April 2015, 03:05 PM   #5
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Check with a scope along the signal path to check whether there's some sort of non-audible oscillation that makes those parts run hot.

My 2c
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Old 20th April 2015, 04:19 PM   #6
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Check the base voltage of Q20. It will tell you if the transistor bias it is too high.
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