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Old 2nd October 2002, 04:44 AM   #21
Jean is offline Jean  United States
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Well, I just pulled all caps and tested them. I also pulled all transistors and tested them with DMM. Pulled transistors measure the same as new unused transistors. Caps measure fine, no shorts.

I went ahead and pulled VR 2k pot and measured it also, and it seems ok. I am now stumped, the only other thing left to do is to pull all resistors out
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Old 2nd October 2002, 05:40 AM   #22
Jean is offline Jean  United States
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I am getting +36 volts on the output with 0 load. I just finished checking all resistors, pulled one side off and measured them that way. Replaced C2, C4, C6 with panasonic ceramic caps just in case. Replaced all transistors except for Q1 and Q2.

If I am getting +36 volts on the output, which is full rail voltage what is going on ? Is something on negative rail completely wrong or else ?

Edit < Thought maybe I got a blown fuse, both + and - rail side checked out ok >.
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Old 2nd October 2002, 04:48 PM   #23
sangram is offline sangram  India
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Jean, is there a schematic somewhere I can look at?

I am unfamiliar with this amp, and also new to the entire DIY audio web scene. If I look at a schematic I could probably try to figure out where the problem could be.
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Old 2nd October 2002, 04:58 PM   #24
Jean is offline Jean  United States
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Hello Sangram,

Here is full schematic of the amplifier. Now keep in mind that one channel works just fine so schematic is not the problem.

Click the image to open in full size.
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Old 2nd October 2002, 07:05 PM   #25
MikeW is offline MikeW  United States
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Try changing your 5 watt resistors. I built a couple of 3a and had the same problem with one.
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Old 2nd October 2002, 10:36 PM   #26
Jean is offline Jean  United States
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Hello MikeW,

I will try that tonight and see if it makes any difference. I'll just swap a pair from the working channel.
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Old 3rd October 2002, 08:24 AM   #27
sangram is offline sangram  India
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Hello Jean

Thanks for the schematic.

OK the problem if it is only in one channel means that there is a contruction issue (shorted solder joint, etc.) or a bad device. A lot of times the device may measure correctly but will not work correctly once full operating voltage is applied. It is imperative to test dynamically, with a load.

You're getting a full rail voltage on output means one of the output devices is locked into conduction. My guess is Q7, so a quick check on the voltage drop across Q7 (though I suspect it will be 0 volts) and R11 (if this is 0, check the condition of the BD139 at Q5) will be in order. So that you can connect load, I would second the suggestion of using a high-power resistor across the output. If you are scared of putting in too much DC current through the output stages, you can add a capacitor in series with it (this will allow you to connect a speaker without the attendant risk of blowing it up) for testing purposes.

Also check the negative rail side. In particular the bias voltage at the base of Q6, and the 0.7 volts between base and emitter of each transistor. I suspect the issue is a bias issue, and it has to be actively tested, I doubt passive testing can reveal the problem.
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Old 3rd October 2002, 01:38 PM   #28
saurus is offline saurus  United States
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Default Can you be a little more specific??

Hi,

I'm having exactly the same problem right now with one channel of my P3A. I know its not a construction error, because it worked fine before a malfunction in my preamp blew the one channel. I am at the "20vdc in the output" stage.

For the benifit of those of us just getting started in this hobby, what exactly do you mean by:

"put a 10-ohm high-power WW resistors in the collectors of the output transistors "

I mean, where exactly do I connect the resistor leads?? To the collector and ??

Thanks!!
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Old 3rd October 2002, 06:00 PM   #29
sangram is offline sangram  India
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I would assume he meant as a load dropper off the rails (this would for example substitute the fuses) so you don't keep blowing fuses but are able to get some level of operating voltage so that you can run tests on the amp.

saurus, what was the preamp malfunction?
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Old 3rd October 2002, 06:38 PM   #30
saurus is offline saurus  United States
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Default That's a good question!

I'm still trying to figure that out. The preamp routes the output from an ESP P88 preamp board to a 2P2T switch. One side of the switch goes to a pair of RCA jacks (for full signal out), and the other side goes to an ESP P81 active xover board that splits the signal at +/- 60 Hz, and goes to two pairs of RCA jacks for routing to the P3A and a sub.

The output from the P88 board works fine, but the xover started making random loud poping noises when the volume was adjusted (the pot is fine with just the P88), and the -60Hz signal was way too loud and distorted. One of the loud pops fried the P3A channel. I was trying to adjust the P88 sensitivity DIP's but it just kept getting louder no matter what setting I used. Finally it blew. I'm pretty sure it must be a bad opamp, so I have ordered more and will replace all 5 on the xover board.

The sad thing is that I got a taste of the P3A for a few blissful hours before it blew! Now I HAVE to fix it!
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