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Old 7th April 2004, 01:38 AM   #1
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Default A strange problem with Rod Elliott's P3A

I'm helping my friend build a P3A amp, one channel works reliably and the other's really confusing us. MJE340 and 350 were used in place of MJE15034 and MJE15035, and the output transistors are MJL4281 and MJL4302. The amp seems to be working correctly except the output stage draws 200mA at idle (this is somewhat unstable) for reasons I don't understand - bias voltage is about 1 volt which should correspond to zero bias current. In this case, 20 ohm safety resistors are connected in series with each supply.

Here are the voltage readings for the defective channel:
Click the image to open in full size.

And the working channel, which is biased to about 80mA:
Click the image to open in full size.

Thanks in advance for any insights !
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Old 7th April 2004, 02:14 AM   #2
Muz is offline Muz  Australia
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Quick question: have you got C5 around the right way in the faulty amp?

Murray
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Old 7th April 2004, 02:35 AM   #3
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Yes, C5 is correctly oriented. Also, the output devices and all the other transistors still work but have not been matched for Hfe.
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Old 7th April 2004, 03:28 AM   #4
mcp is offline mcp  United States
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Quote:
MJE340 and 350 were used in place of MJE15034 and MJE15035
Perhaps the orientation of the MJE340/350 is wrong. They are different from the MJE15034/5.

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Old 7th April 2004, 04:13 AM   #5
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If all devices are tested and ok then you must consider that they are dumb devices and the configuration is wrong. I would (if on a protoboard) start over.

The shem is solid and works so if everything is + to + then it's an assembly error and a careful look should solve your dilemma.

Cheers,
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Old 7th April 2004, 04:23 AM   #6
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I assume you are sure your devices are not counterfeit? It's just that they seem very leaky, or worse, the circuit is oscillating and sucking juice for that reason. Did you use his board? The layout is critical for complementary pair outputs. You would most likely have problems when the amp is clipping, not at rest as I assume it is.

Check for leakage on the drivers. I prefer a 100 ohm resistor across the output base/emitter junctions, or better 47 ohm.
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Old 7th April 2004, 04:37 AM   #7
sam9 is offline sam9  United States
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You could take the channel that works reliably and measure the voltage a key point. Then compare this to the one that doesn't work. Start the input section constant current source, then the differential pair and just work your way forward to the output. If you find voltages that don't agree you will as least have naropwed down the search.

Consider something a simple as a bad solder joint or wrong resitor value.
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Old 7th April 2004, 04:59 AM   #8
AKSA is offline AKSA  Australia
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The problem likely is in the upper CFP. Check R11 is the right value and correctly mounted; likewise Q5 and Q7.

Q7 has too much base/emitter voltage, and Q5 insufficient. Something funny there.......

Check also that R13, 0R33, is good and correctly installed.

Cheers,

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Old 7th April 2004, 02:38 PM   #9
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Thanks to everyone for the suggestions. I have checked all the resistors on the board (which was bought from Rod Elliott), they are all correct - and I re-soldered many joints. All the transistors are genuine and inserted correctly. The first time my friend built up the boards, he did have Q4-6 backwards. We then flipped them around on both channels.

It appears that the input stage works correctly, the bias circuit works (and is operating like it should for zero bias) but something causes the output stage to draw current. Since the base to emitter volltages on Q5 and Q6 are so low (0.43V or so), wouldn't that mean they are not conducting at all and the 3-4mA current through R11/R12 is just going to the bases of Q7 and Q8? If that is true, why is the Q7/Q8 collector current only 200mA?

By leakage, do you mean Q5 and Q6 are defective? It's strange because the junction drops measured by a multimeter are still .611 and .619 volts.

What else might I check to see if the circuit is oscillating? It seems sort of unlikely given this is ESP's board and the zobel is in place.
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Old 7th April 2004, 03:09 PM   #10
Duo is offline Duo  Canada
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Well, a fast way to see if it's oscillating is to connect a scope. Hopefully the scope you use will have enough bandwidth to see the oscillation frequency if it's present.

Also, are you sure about C4 and C6? If these are omitted, or the wrong value, the amp could easily start oscillating.


EDIT: Also, what is the voltage drop across each of the output emitter resistors? R13 and R14
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