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Old 31st October 2009, 10:08 PM   #1
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Default Explosion in Forte 1A...

Yesterday, I was pulling an all-nighter (graduate school...) and at about 5:30 AM I heard a loud, but muffled, explosion off to my right followed by weird and terrible distortion from my speakers. In case you are wondering, I noticed no smoke from the amp or speakers. I have looked inside the Forte 1A and saw nothing noticeably wrong. The output transistors are all intact, no burned spots on any of the PCBs, the power supply caps all appear to be fine (no electrolytic blow-out). Fuses are all also fine. I only have a multimeter, so the only testing I could do was to check power rail voltages, and all appear normal.

As a test, I hooked the amp back up and turned it on. The speakers hummed and the woofer cones sucked in, so it is now throwing out some DC to my speakers. There was no smoke from my speakers, so I am guessing the amount is relatively small...

Does anyone know what has gone wrong with my amp? As I mentioned before, I am a graduate student, so I don't have tons of funds laying around for repairs if something is terribly wrong... Is it just one of the power supply caps? If so, what should I use as a replacement?

Thank you all for any suggestions you may have.
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Old 31st October 2009, 11:08 PM   #2
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Perhaps this is a good time to explore learning how to
repair an amplifier. It's not terribly complicated.

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Old 31st October 2009, 11:15 PM   #3
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disconnect speakers - and measure DC on speaker terminals

give us few pictures of inside

have you schmtc ?

Mighty Papa just wrote you invite .......... I'm bloody sure that he'll help too ;

remembering few times that he helped even to some pretty rude and thick headed ones .... this time must be piece of cake
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Old 1st November 2009, 01:20 AM   #4
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Before you go any further, and in case it happens again in the future, I will suggest that you go to Radio Shack and buy a couple of power resistors. These are in the ball park of 10 watt & 10 Ohm and cost less than a dollar.

When ever you need to diagnose or double check something, do not have the speakers hooked up. Rather, substitute the power resistors (wire them across the output terminals). This way you never need to rely on fuses or other protection circuits to protect your speakers.
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Old 1st November 2009, 02:38 AM   #5
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I definitely agree WithTarragon. You can fix an amp easier than you can fix burned out speaker drivers.

As far as troubleshooting, I think a good first question to ask yourself would be whether both channels are humming or just one. If it's just one, you know it's probably a problem on just that one channel. If it's both, it's likely a problem in the power supply.
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Old 9th November 2009, 11:26 PM   #6
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I apologize for the delay in responding... I have been rather overworked this past week and am taking a break now (even though much remains to be done).

I spent a little time with the amp and a multimeter, and the problem is not in the power supply. Only the right channel hums. I adjusted the bias potentiometer slightly and the amp normalized... For a while, at least. Having thought I'd fixed the problem, I brought the amp home and hooked it back up. Everything went well that first listen. But, after trying it again last night, the same problem is back - the right channel hums terribly. DC offset across the right output terminals was sitting about 0.3V.

So, it looks like the bias potentiometer is bad, correct?

Thanks all
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Old 9th November 2009, 11:32 PM   #7
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Only a couple volts is needed to "suck a woofer in", but you'd better keep that amp away from your speakers until you get it fixed. 0.3 volts can turn into full rail in no time, depending on what is wrong.

It would be a little more odd than normal for the problem to be just the pot. I'd look for bad solder, then wonder about semiconductors.
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Old 9th November 2009, 11:36 PM   #8
Zen Mod is offline Zen Mod  Serbia
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Default schematic for start

you can try to replace bias pot , but first you must check few things - solder joints as already mentioned ......

few pics can't harm , in any case
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File Type: jpg forte1.jpg (89.9 KB, 580 views)
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Old 10th November 2009, 01:54 AM   #9
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You will notice that Q11 is the bias transistor, providing
a DC voltage to the Bases of the output stage for bias.

R15 is the bias pot. If the wiper on that goes bad, then the
bias goes down without causing damage, and without
making noise or having offset.

So it is unlikely to be the bias pot. Also, don't adjust this
pot unless you know what you're doing - this is the quickest
way to blow up and output stage.

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Old 10th November 2009, 06:10 AM   #10
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If there are no signs of physical damage to any components, could it be that the "muffled explosion" was really sound created by the loudspeakers responding to a transient caused by whatever fault in the amp?

You should check all of the output transistors individually, as well as look for intermittent connections. Sometimes it is easier to just reflow all the solder joints than try to find the bad one.
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