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Old 08-24-2018, 10:45 AM   #1
Sojke
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Default Onkyo A-8250 50V on DC Offset transistor

Can anybody help me identify the problem with my Onkyo A-8250? Instead of ~1.5V on Q517 offset transistor I am getting 50V.

I have replaced: Q511, Q513, Q517 (Was defective), Q515, D507, D509, D513, D515. (Diodes with 1N4148, transistors with exact models)

Also checked C601, 603, 509, 507 (all good), R523, R535, R529, R533, R527, R525, R537 (all good), and the front and back from there transistors. It is illogical to me why I am I getting 50V on Q517. On the other channel it is normal ~1.5V as it should be. With out Q515 i get ~3V, but Ive replaced it 3 times already, and all of them measure correctly. This is measure with the amplifier section transistors removed (as they all go up in flames due to this).

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Old 08-24-2018, 01:49 PM   #2
redwire
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Default Re: Onkyo A-8250 50V on DC Offset transistor

It's not making sense.

Q515 is a constant-current source, it should pull down the Q517 point to just below zero volts. Removing Q515 should not lower the 50V down to 3V, there is no path for that unless the diode string D505/D507/R527/D509/D511 has a problem. In a series diode string, if there was an overload, it doesn't make sense to replace D507 but not D505 etc.

I would suspect the next (driver) stage, because if Q517 had failed, then the output/driver transistors must have shorted. R603, R601 can get cooked too, they are usually flameproof/fusible.

What I'd do is diode-test every semiconductor, and compare to the other channel. You still have a bad component. It just takes patience and less smoke doing this for 15 minutes.

The current through R525 and R523 should be almost equal, unless the driver stage has a problem. You can measure the voltage across them and use ohms law, assuming the resistors have not been damaged.
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Old 08-24-2018, 02:27 PM   #3
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Default Re: Onkyo A-8250 50V on DC Offset transistor

BTW, Q517 is part of the active bias circuit.
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Old 08-25-2018, 04:43 AM   #4
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Default Re: Onkyo A-8250 50V on DC Offset transistor

Quote:
Originally Posted by redwire View Post
It's not making sense.

Q515 is a constant-current source, it should pull down the Q517 point to just below zero volts. Removing Q515 should not lower the 50V down to 3V, there is no path for that unless the diode string D505/D507/R527/D509/D511 has a problem. In a series diode string, if there was an overload, it doesn't make sense to replace D507 but not D505 etc.

I would suspect the next (driver) stage, because if Q517 had failed, then the output/driver transistors must have shorted. R603, R601 can get cooked too, they are usually flameproof/fusible.

What I'd do is diode-test every semiconductor, and compare to the other channel. You still have a bad component. It just takes patience and less smoke doing this for 15 minutes.

The current through R525 and R523 should be almost equal, unless the driver stage has a problem. You can measure the voltage across them and use ohms law, assuming the resistors have not been damaged.
Replaced D511 and D505 with same forward voltage red diodes 1.93V (only lesser capacitance 15pF instead of D511 44pF, D505 29pF), nothing changed.
Ive noticed that when powered on, D511 blinks and turns off, D505 continues to glow. When power is turned off after some time D505 turns off and D511 turns on, than after a shorter period of time it switches again D505 on D511 off. R525 564ohm , R523 6.89ohm and R550 44.36k ohm.
I have removed the transistors and resistors in the Amplifier section as they have all burned , even tried removing C603, C601 and C507. I have no idea where is the voltage escaping. (Its weird)
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Old 08-25-2018, 08:12 AM   #5
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Default Re: Onkyo A-8250 50V on DC Offset transistor

(Sorry if double posting is not allowed but I cant edit the previous message any more)
Solved. Amplifier transistors are necessary to be mounted in circuit in order for bias circuit to work properly.
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Old 10-20-2018, 05:32 PM   #6
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Default Re: Onkyo A-8250 50V on DC Offset transistor

When an output transistor has failed I temporarily remove all of the main output transistors from the faulty channel. By this I am referring to the large output transistors bolted to the heat sink. Then I connect a 100 ohm 2 watt resistor from B to E terminal pads for each spot on the PC board where an output transistor was originally connected. Do not connect anything to the C terminal pads on PC board. The 100 ohm resistors provide the continuity that is required to get the rest of the amplifier circuit working. And it should be possible to get enough audio output to hear something from the headphone jack.

When the rest of the amplifier circuit is working correctly there will be about 0.5 to 0.6 of DC voltage drop across each of the temporary 100 ohm resistors. And the speaker protectin relay should click in. DC voltage from either end of the temporary 100 ohm resistors should be < 1 volt in reference to negative black speaker terminal output post. This is circuit ground (earth) in most cases.



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Old 10-20-2018, 05:47 PM   #7
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Default Re: Onkyo A-8250 50V on DC Offset transistor

i use 1k resistors to complete the feedback circuit if they are needed . most times the drivers emitter resistors do this job anyway .. never had a problem with 1/4w resistors . no load at speaker terminals .
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Old 10-21-2018, 01:18 PM   #8
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Default Re: Onkyo A-8250 50V on DC Offset transistor

This model and some other Onkyo amplifiers have an unusual architecture for the output stage.

There are 3 low value emitter resistors connected to the output transistors. One or more of these often fail open-circuit after an output transistor shorts out. These resistors are both “flameproof” and “fusible.” They rarely show any visible signs of overheating. Because these 3 resistors are in a series-parallel arrangement it is necessary to disconnect one end of at least one of these 3 resistors in order to check that each individual resistor still has the correct resistance value.

In this circuit the 2 temporary 100 ohm E to B resistors must be connected in place of the 2 large output transistors to test the functionality of the driver circuitry prior to reinstalling new output transistors. This is necessary because the pre-driver and driver stages have no connection to the output line.

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Old 10-21-2019, 07:51 AM   #9
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Default Re: Onkyo A-8250 50V on DC Offset transistor

Hi all and Sojke. I have a similar problem with Onkyo a8270. 50v at Q517 transistor. Everything around is working properly. How did you solve the problem?
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