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Old 11th January 2011, 06:19 PM   #891
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Look carefully for any loose wire that could be causing a short - or mounting hardware that shifted

Pull and measure DCR from Source to Drain on your output devices. If 0, they are dead and need replacing.

Offset jumping to 140 mV without C1 indicates you are getting some DC on your pre-out. Take a look at that after you get the amp running again.

Last edited by BobEllis; 11th January 2011 at 06:32 PM.
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Old 11th January 2011, 06:33 PM   #892
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Quote:
Originally Posted by BobEllis View Post
Pull and measure DCR from Source to Drain on your output devices. If 0, they are dead and need replacing.

Offset jumping to 140 mV without C1 indicates you are getting some DC on your pre-out. Take a look at that after you get the amp running again.
OK. Will check the output devices. Can I check them without pulling them. I can turn on the circuit and measure with the light bulb on. I measured across R16, 17, R18 and R19. They are showing around 0.5V

I measured the DC on my preamp. It shows 1.2mv.

Thanks.
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Old 11th January 2011, 06:36 PM   #893
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Only .5V on the source resistors indicates that the transistors are probably not blown. (at normal operating voltage)

Look for something shorting a rail to ground or the other rail.
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Old 11th January 2011, 07:37 PM   #894
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.5V across the source Rs with a current limiter in the circuit should tell you 1 thing. The output fets are fully turned on. Look at Q4, your Vbe reading before was not what it should have been or was bogus and Q4 has most probably now expired.
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Old 11th January 2011, 07:46 PM   #895
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Turned on, but drawing 1A each. The amp should draw ~100W if at full mains voltage. What is your bulb's power rating? If only 100W it could look full bright, even though it is only at 50W.

I noticed Q4, too. No idea how the current source is working on the one good channel with that bias point.
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Old 12th January 2011, 01:39 AM   #896
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Originally Posted by Bill Fuss View Post
.5V across the source Rs with a current limiter in the circuit should tell you 1 thing. The output fets are fully turned on. Look at Q4, your Vbe reading before was not what it should have been or was bogus and Q4 has most probably now expired.
I will measure Vbe (means Voltage across Base and Emitter isn't it?) of Q4 again. What I measured before on the good and the bad channel is .32v and .32v This is from my previous noting. The good channel is still working perfectly.

I will also change Q4 on the faulty channel and check again.

Quote:
Originally Posted by BobEllis View Post
Turned on, but drawing 1A each. The amp should draw ~100W if at full mains voltage. What is your bulb's power rating? If only 100W it could look full bright, even though it is only at 50W.

I noticed Q4, too. No idea how the current source is working on the one good channel with that bias point.
You may be right. The bulb I am using is a 100w one. I may think it is bright and not necessarily full bright. What should be the bias point on Q4? According to Papa's schematic, voltage across Emitter and Collector of Q4 should measure 4.3v and in my case it is measuring 5.3V on the working channel.

Thanks.
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Old 12th January 2011, 02:28 AM   #897
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Do you have fuses in the rails? If not, can you rig up something in the 3A range temporarily? I think it is time to try without the bulb, or maybe a 300W bulb.

What do you have for inrush limiting and what is your line fuse rating? What is your mains voltage?

I would expect Q4 Vbe (yes base to emitter) to be a lot closer to 0.6V. Odd that the one channel works like that. The high Q4 Vce reading means that your FETs need higher than average voltage to turn on.
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Old 12th January 2011, 02:33 AM   #898
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If you are worried about the bulbs blowing and messing the amp up, it is possible. Use 2-100W bulbs in series or use one 240v bulb. They don't glow as bright and they won't blow.

Fcrawley
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Old 12th January 2011, 05:50 AM   #899
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Quote:
Originally Posted by BobEllis View Post
Do you have fuses in the rails? If not, can you rig up something in the 3A range temporarily? I think it is time to try without the bulb, or maybe a 300W bulb.

What do you have for inrush limiting and what is your line fuse rating? What is your mains voltage?

I would expect Q4 Vbe (yes base to emitter) to be a lot closer to 0.6V. Odd that the one channel works like that. The high Q4 Vce reading means that your FETs need higher than average voltage to turn on.
Bob, I am running on 220v power line. I do have a 10ohms, 10A surge thermistor in series with the power line to protect startup surge. Also I have a line fuse to the transformer primary of 2.5amps. My bulb is a 100w bulb abs have access to a 200w one as well.

I have replaced all the ztx and FETs in the system and there is n change to the state of the problem, the bulb continues to glow. I suspect the mosfets might have gone bad. I am rebuilding the amp from scratch on a new board. I have a new set of matched mosfets in anticipation of a aleph JX. I have discarded the problem board. I will visit it after I get the new one running.

Cheers.
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Old 12th January 2011, 11:58 AM   #900
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It's not likely that the FETs died while you were changing speakers. Especially since they seem to be drawing only an amp now.

I have a feeling that inrush had popped the fuse, even with your thermistor. You're getting 22A surge, I think that you just got lucky earlier not blowing them. The difference between the amps could be just the build up of tolerances. I'd try a 2.5a slow blow fuse or a pair of thermistors in series. Hope you haven't disassembled it yet.
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