Onkyo TX-NR828 - Repair assistance wanted

[roman-hartwig]

Object: Onkyo TX-NR828
Problem: Overheating / Burned off
Service manual: available

Runs all the years without problems, day on day off. Then all of a sudden with a bang and a small flash one of the final transistors burned out. This is the left front channel. Since the original Toshiba were no longer available, I replaced them all with the equivalent:

Toshiba TTA0001 => 2SC1962
Toshiba TTC0001 => 2SC5242

This also included replacing the thermal paste and the very own insulators with pads (1.3W/mk).

After I now put everything back together and wanted to do a first test, it turned out that the heatsink on the left side (Channels : SBL - SL - L - C - R - SR - SBR ) quickly became very warm. So it is obvious that the burnt transistor was only the result of another original problem. This is the point where I would like to ask you for help. Unfortunately my knowledge is limited / and also a little rusty. Every hint for troubleshooting is welcome.

[petehall347]

Re: Onkyo TX-NR828 - Repair assistance wanted

you selected both npn replacements when one is npn and the other pnp .. ttc npn tta pnp .
you should be checking the bias circuit and driver transistors and the power supply .

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

Sorry, I made a typo when writing down the types. Of course, the correct ones are installed:

Toshiba TTA0001 => 2SA1962OTU-ND
Toshiba TTC0001 => 2SC5242OTU-ND

Without really having a concrete idea, I have already started to measure the left channel. Currently Q5410 and R5470 are striking and show too high values / results. Am I on the right track with this?

[petehall347]

Re: Onkyo TX-NR828 - Repair assistance wanted

yes check everything including the bias adjust pot which should be turned to zero after replacing anything then adjusted to correct idle current after repairs . .

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

Thanks for now, I will then measure further tomorrow (it's already late). Can you deduce what could have caused it? The procedure for setting the idle current is described in the service manual as in the attachment. If I understood correctly, I should set/measure in series to a 100 Ω resistor the mentioned voltages at the mentioned time. The measuring points for e.g. the left and right channel are shown in the circuit diagram above.

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

This is the current status of my measurements. The two marked in red are the ones that need to be replaced. Now I looked on Digikey but found replacements with different values for the transistor.

2SC2412 (a) https://www.digikey.com/en/products/...2KT146R/650455
2SC2412 (b) https://www.digikey.com/en/products/...2KT146Q/650454

The difference here is "DC Current Gain" with once 120 at 1mA, 6V and once with 180 at 1mA, 6V. Which one should I prefer?

Would it also make sense to replace the 2SA1037 (Q5460) at the same time?

[petehall347]

Re: Onkyo TX-NR828 - Repair assistance wanted

https://cdn.badcaps-static.com/pdfs/...bb9809e8c3.pdf
looks like you want R gain

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

That was my guess too, but I wasn't sure. Thank you for the certainty. However, I still have the question of how the defect occurred. Material fatigue? All other channels have no deviations or obvious defects. Do you think there is another potential defect that can be deduced?

[petehall347]

Re: Onkyo TX-NR828 - Repair assistance wanted

if the idle adjust pot was bad it might have caused it . but saying that the 10k resistor should stop that happening . a shorted speaker can cause same damage .

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

Have checked the potentiometers again individually, but they all seem to be okay. A short circuit at the output is excluded at least on the speaker side. Now I have to wait until the spare parts arrive, then I'll get back to you.

[tibimakai]

Re: Onkyo TX-NR828 - Repair assistance wanted

When I work on an Onkyo receiver, I check all the parts on that channel, before and after the repair. I compare measurements to another channel, just to be sure.
When you turn it on after the repair, with the 100 ohm resistors connected, make sure that the measured value does not increase to fast. If it does, that means that there is still something wrong. Turn it off right away.

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

After the arrival of the spare parts, I immediately set to work. I replaced the two transistors and the resistor. After 1 minute with electricity, the amplifier could now be switched on again. So I started to measure the idle current. It was noticeable that some deviated nevertheless already clearly from the target (up to +10mV). I readjusted everything appropriately then. The primary (large) heat sink is no longer hot after 5 minutes. The final sound check is still pending. However, I noticed that there is a noticeable heat radiation from the "larger" transistors (e.g. Q5250, Q5260 for the left channel) on the preamp board. I mean warm the AMP has always been. Should I be worried here?

[tibimakai]

Re: Onkyo TX-NR828 - Repair assistance wanted

Most of the time I have found more bad parts, then just two transistors and a resistor. I would check all the parts on the preamp board-at that channel only, and around the final transistors, near the (big)heatsink.

[petehall347]

Re: Onkyo TX-NR828 - Repair assistance wanted

measure voltage across each 2,2 ohm emitter resistor then work out the current using ohms law .

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

A day later and after normal use, everything works as usual. I also measured again and could no longer find any deviations. The heat generation still takes place. Said "larger" transistors ( first post => preamp-pcb-front.jpg ) are still a bit warm for my taste. If I would build something that has such heat development, I would at least add a heat sink. Even if this was only small or thin. Does anyone have such an amp and can confirm that the heat generation is common?

@petehall347
I may have missed something, but what 2.2 ohm resistors are you talking about? Do you mean the 0.22 ohm ceramic resistors?

[lotas]

Re: Onkyo TX-NR828 - Repair assistance wanted

@petehall347
I may have missed something, but what 2.2 ohm resistors are you talking about? Do you mean the 0.22 ohm ceramic resistors?

If the circuit you posted is correct, then we are talking about these resistors.

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

You're right, I overlooked them. Thanks! But since they are marked green, I have measured them as intact. Don't get around to it today and won't tomorrow either. So on Sunday I will measure the voltage drop and then calculate the current. But what can I deduce from that? Whether they are running within their specification, or is the set point of the current known?

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

I have now measured the resistors and their voltage drop and calculated the resulting current. For comparison, I measured the right channel.
The results are as follows:


Left Channel:

R5370:

R = 2.3 Ohm
U = 25,7 mV
I = 11.17 mA

R5380:

R = 2.4 Ohm
U = 25.6 mV
I = 10.66 mA

Right Channel:

R5371:

R = 2.3 Ohm
U = 26.6 mV
I = 11.56 mA

R5381:

R = 1.8 Ohm
U = 20.7 mV
I = 11.50 mA

[petehall347]

Re: Onkyo TX-NR828 - Repair assistance wanted

they are looking much the same so shouldn't be any hotter so long as they are thermally connected to the heatsink

[roman-hartwig]

Re: Onkyo TX-NR828 - Repair assistance wanted

These are not mounted on heat sinks. These respective pairs stand freely on the board without heat sinks. You can feel the heat emission with your hand if you only hold your hand over the open AMP.