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Old 28th May 2003, 12:35 AM   #1
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Location: Beaver Falls, PA
Default amp died while biasing

I have a MCS 3233 receiver. It is a stereo 33w @ 8 ohm receiver. It has a single power suply for both channels. So it's nothing special. But it sounds great and I want to keep it running strong. The power supply is underrated and the transformer fries all the time, but it's never died on me, I use it ever night.

Problem is, I was trying to bias it. I tried going for 125ma on the .33 ohm emitter resistor. It has a MJ2955 and a 2n3055 pair of transistors for each channel. The right channel died, but nothing smoked. The current on the resistor disapeared when I was in the middle of ajusting the pot. I measured the voltages on the base and the emitter. Instead of being around ~2, and a few hundred millivoltes, they where close to rail voltage (plus and minus 32v). They mesured 31v. I really don't know what happened. I was under the impressing I broke a solder joint by accident and got an open circut. Did I kill the transistors? If so, what transistors will improve a push pull amp like this, that I could just mount on a heat sink and wire up?

Thanks,
Nick
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Old 28th May 2003, 11:20 AM   #2
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Depending on how the bias adjustment is implemented "things might get interesting as soon as the contact of your wiper's pot fails"*.

Regards

Charles


* copyright by Nelson Pass
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Old 28th May 2003, 11:39 AM   #3
yeti is offline yeti  Germany
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Default reply on "dead amp"

I think the emitter resistors have blown with the transistors.
Those pots used for biasing usually are of the cheapest kind and if they weren't used for many years they'll loose contact while turning them.The result will be that for a moment the bias current raises and destroys the transistors and the emitter resistors.
But it doesn't seem to be a real problem here because these types of transistors are cheap and easy available.... and don't forget to change the pots too.
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Old 28th May 2003, 11:45 AM   #4
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If it is done wellm the failure of the pot would result in UNDERbiasing. If it's done well ..........

Regards

Charles
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Old 28th May 2003, 11:45 AM   #5
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Default Safer bias

Things could be safer if a different arrangement was implemened on every Vbe.

First of all put a parallel resistor with the trimpot.

Second add a series resistor with the trimpot.

In such a way contact failure shouldn't be a deadly potential problem.

So if you change the pot you cold quite well do this mod.

On both channels.



Carlos
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Old 28th May 2003, 11:49 AM   #6
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Default Re: reply on "dead amp"

Quote:
Originally posted by yeti
I think the emitter resistors have blown with the transistors.
If nothing smoked, as Nick said, why do you say the emitter resistors may have blown?

Can that happen? Blown resistor and no blackening?

The transistors may have died with no external show though.



Carlos
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Old 28th May 2003, 12:28 PM   #7
Freddie is offline Freddie  Sweden
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Quote:
If nothing smoked, as Nick said, why do you say the emitter resistors may have blown?

Can that happen? Blown resistor and no blackening?

The transistors may have died with no external show though.



Carlos
I have seen resistors that have died with no external show, so it can happen.

But it's easy to check..

The resistors are probably OK though.
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Old 28th May 2003, 02:54 PM   #8
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The resistors seem to be good without taking them out. They have not changed value, and have not smoked. So if I change all the power transistors, am I going to have to change the driver transistors also? I guess I should only work with the blown channel first. Where can I get matched transistors from? Radio Shack stoped carrying them a year or so ago and my local electronics store is jake. Maybe I can grab the phone book and start looking for a better store downtown.

The trim pots are 2K ohm. I'll get some replacements. I was reading the article on www.tnt-audio.com and they don't really mention anything breaking because of bad trim pots, he just says they sound bad.

It sucks not having a schematic. But the board for the power amp in this receiver is rather simple. I could trace it in a couple hours, but i've never traced anything before.
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Old 29th May 2003, 03:58 AM   #9
Diode is offline Diode  United States
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Hey Man,

Do as the others have said..... Replace the outputs, drivers and I'd go ahead and replace the bias transistor as well. I usually replace most, if not all semiconductors in a blown amp. you never know what else has been stressed or blown or leaky........

Good luck,

Chris
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Old 29th May 2003, 04:12 AM   #10
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Crap, ok.
I guess I should find a better store to go to. Or just find someone cool online to order from. I might need some alternative transistors. I think I'll keep the same output transistors. I don't have the power to do anything more anyways. Any good places to shop online, or to order from I mean (don't care if they have an online store)?

Another question, is it normal for an iron core transformer to get hot enough to burn yourself on, but not melt itself or short? The transformer gets freeking hot in this amp. But then again, it sounds good. It dosn't distort at all, it pretty much a set headroom. You hit it and you know you've gone to far. So it's not like I'm listening to a song, a bass note hits and it runs out of power. I hate amps that do that. haha, we got computer speakers at school that clame they have a 200W amp, get real, 200w on an amp that is probably built with a power supply that can bearly handle 50... Sounds like crap....

- Nick
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