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Old 17th January 2013, 04:32 AM   #51
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Hi.

I found out by just measuring the resistance on thouse Resistors and found that there was no resistance.

Q3 is 2N5415.

I will change thouse resistors today.
BE Diode was fine.

I will read about oscillation today as I dont have a good understanding of it.
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Old 17th January 2013, 01:59 PM   #52
JMFahey is offline JMFahey  Argentina
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Quote:
I found out by just measuring the resistance on thouse Resistors and found that there was no resistance.
What does "no resistance" mean?
Open or short?
Did you measure them on the PCB or out of it?

Quote:
BE Diode was fine.
Not an answer.
I asked:
Quote:
It is impossible that you have 1.6Vbe there
Measure again, on the transistor legs themselves.
If still anything above 0.7V, pull that transistor (with amp off) and measure the BE diode in the multimeter diode scale.
Red to emitter and black to base.
Just for confirmation, read it the opposite way.
I need a number, not an opinion.
Thanks.
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Old 17th January 2013, 04:37 PM   #53
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Hi.

Desoldered D2.
Diode D2. Multimeter in a low resistance mode.
Anode-Red Cathode-Black: 707
Anode-Black Cathode-Red: 1787
Its a reading in resistance, sorry, don't have a diode mode.
It was difficult to understand if you ment D2 or you wanted me to measure Q3 BE in a diode mode. I measured D2.

Resistors were open. I first measured them on the board and then took them out, reading didn't change.

Last edited by served; 17th January 2013 at 04:56 PM.
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Old 17th January 2013, 07:35 PM   #54
JMFahey is offline JMFahey  Argentina
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Quote:
Resistors were open. I first measured them on the board and then took them out, reading didn't change.
Ok, thanks, now I see.
Use "open" or "shorted" instead of "no resistance" which can have both meanings.

They must have blown open by a *strong* overload.
Since gates are insulated and do not pass current, those MosFets must have turned into blobs of molten metal, plus ballast resistors must have blown open too.
Talk about "overload".
That amp was probably "repaired" with a 20 or 30 Amp fuse pulled out of a car , a ball of crumpled aluminum paper or something like that.
Only way to achieve such a meltdown.

I wanted you to
Quote:
measure Q3 BE in a diode mode.
, both ways.
But don't know whether I can trust your multimeter or not .
The Diode Scale *often* but not always, matches the 200 ohm scale, hope yours does.
To test it, get a clean unused diode (any one: 1N4002 , 1N5402, whatever) and measure it both ways.
I can trust that.
Quote:
low resistance mode.
Does it have any label?
As said before, I trust numbers more than descriptions, if that makes sense.
Thanks.

EDIT: by the way, having R27 and Q6 burnt is consistent with having R40/41/42/43 burnt open, all point to a massive short on the negative side MosFets.

Last edited by JMFahey; 17th January 2013 at 07:43 PM.
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Old 18th January 2013, 02:34 PM   #55
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Hi.
So I truned the amp on. After changing resistors I got +49V on the output.

My DMM is looking like this http://www.ehighend.de/pics/peaktech...10-product.jpg
I measured it not in the 200ohm mode but this mod has a diode drawn on it and you can make it beep if leads are shorted. (sorry for this bad description)

Some observations (sorry, no number at the moment as I didn't want to risk it)
The Bulb was very dark now. Turning the Bias pot didn't affect the bulb.

Q3 BE.

Red Emitter; Black Base : 638
The other way around OL. (Open loop, over-Limit)

DMM test 1N4007
Forward 573
Backward OL

DMM test 1N4148
Forward 600
Backward OL

Last edited by served; 18th January 2013 at 02:57 PM.
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Old 18th January 2013, 03:17 PM   #56
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Reconnected Feedback and output has 0V.

I would like to leave this on my desk for couple of hours with the bulb connected, but I will measure everything and post here before I do that.
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Old 18th January 2013, 03:20 PM   #57
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by served View Post
My DMM is looking like this http://www.ehighend.de/pics/peaktech...10-product.jpg
I measured it not in the 200ohm mode but this mod has a diode drawn on it and you can make it beep if leads are shorted. (sorry for this bad description)
DMM test 1N4007
Forward 573
Backward OL
DMM test 1N4148
Forward 600
Backward OL
No, the description is fine.
I also found your Multimeter Manual, but even without it, your diode (1N4007/4148) measurements confirmed it
NOW I can trust your measurements
Quote:
Q3 BE.
Red Emitter; Black Base : 638
The other way around OL. (Open loop, over-Limit)
Well, *this* is what I wanted to check (but I had to trust your Multimeter first): Q3 BE junction *now* measures loke a regular diode, around 600mV (similar to what you measured on those 2 others) .
BUT I was worried about what you had posted earlier, showing 1.6V (or 1600mV) BE at Q3 , which is impossible, unless the transistor is open. (Or "is not there" , as in cracked solder or tracks).
As you see, besides measuring parts, the faster and more accurate way is to "make the amp work" (or try to) and measure abnormalities.
Yes, they will probably point to a "bad part" ... or not ..... : bad connections, bad biasing, missing rail voltages, you name it.
You may spend a month pulling and measuring *everything* in a complex preamp .... while the real problem was a missing -15V rail or 1 broken pin or wire in a ribbon connector.

Quote:
Some observations (sorry, no number at the moment as I didn't want to risk it)
The Bulb was very dark now. Turning the Bias pot didn't affect the bulb.
Well, a dark bulb is a dark bulb, in this case it's acceptable info, but later I'll suggest some voltage measurements.
Now my lunch break is over, will post later.
But it will be similar to your earlier post where you showed some key voltages .... which now will be different, of course.

EDIT:

Quote:
Hi.
So I truned the amp on. After changing resistors I got +49V on the output.
Quote:
Reconnected Feedback and output has 0V.

I would like to leave this on my desk for couple of hours with the bulb connected, but I will measure everything and post here before I do that.
GOOD!!!
Looks loke we are real close now.
Only stone in the path will be to check Bias to avoid both your original problem (crossover) and the last one (fireworks) but I feel we almost are done

Last edited by JMFahey; 18th January 2013 at 03:25 PM. Reason: Read simulpost.
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Old 18th January 2013, 03:56 PM   #58
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Yes the bias. I will not bias it before I know what I am doing, so I will spend some time reading.

After everything is successfully done...
I was thinking to write an article based on how you guided me through it.
Do you have any place in mind where it should be found in the future?

This has teached me so many new things that I could not even describe what kind of enthusiasm is bulbing inside me.
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Old 20th January 2013, 01:14 PM   #59
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Hi.

I think there is still something wrong.
If I turn the BIAS pot then voltage on the output rail starts to rise again. I will make some measurements.
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Old 20th January 2013, 01:55 PM   #60
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Quote:
Originally Posted by served View Post
Hi.

I think there is still something wrong.
If I turn the BIAS pot then voltage on the output rail starts to rise again. I will make some measurements.
you need to be looking at the speaker output terminal with an oscilloscope. Input and master volume levels turned all the way down to zero. while watching the scope. if you turn the bias pot too far you will see oscillation very high in frequency. this is bad. turn the bias pot the other way. a scope is the only way you will know for sure!

To simplify what oscillation means. the amp has become unstable and has started to "squeal" for lack of a better word, very high in frequency, often way above the audio band. your meter may not be able to read that high in frequency so it just looks like DC. the amp may even produce some DC offset. The oscillation can be very high in level. can cause the output devices to overheat and die. which is exactly what I suspect happened to your amp (i have seen it many times). these amps are notorious for this!
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