Onkyo TX SV727: Found bad transistors, what else?

[budm]

Re: Onkyo TX SV727: Found bad transistors, what else?

You should check the idling bias by measuring the Vdrops on the Emitter resistors you just replaced, once you have the Voltage (it will be in mV range) then you calculate the idling current through the output power transistor.
You can also check the DS offset at the speaker terminal since it sounds like the speaker protection relay (the one with delayed on) does come on.

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

Ok. Im just checking i take it from center to outter on the resistor pins?
What do i do with these numbers? Ie what am i looking for/to check? Is this like biasing a tube amp and making sure you're running within the proper current rating for the tube?

Edit: for anybody else wondering, this is a good read
http://www.tnt-audio.com/clinica/bias_e.html

Do different transistors make a big difference in sound like tubes? I bought a second set of transistors in case the answer is yes

[tibimakai]

Re: Onkyo TX SV727: Found bad transistors, what else?

I'm not familiar with that Onkyo, but usually there is a small two pin connector on the mainboard, at each channel.
You should check the service manual, on how to do it, if it needs to be done at all.
On the later models, I had to make a two wire setup, with a matching connector, that plugs in into the other one, on the mainboard and each wire has to have in series, a 100 Ohm resistor. There is an instant on measurement and then after like 5-6 minutes, a final one.
This voltage may vary, from front main channels to surround channels.

[budm]

Re: Onkyo TX SV727: Found bad transistors, what else?

Read the service manual.
https://www.badcaps.net/forum/attach...2&d=1489074703
You can see two test points at the Emitter of the output transistors
It does not look like it has adjustment pot for Bias, but the pictures is not showing the whole diagram so read the service manual..

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

I see nothing in the service manual about biasing. This does have the pin headers tibimaki mentioned. There's also no pots anywhere in here.

Welp. Tried to take the voltage from the output terminals. Bad idea. Its stuck in protect mode and the secondary relay no longer clicks on. Ive disconnected everything but same result.

Back to the drawing board.

Edit: the new transistors checkout ok. Whew!

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

Take a look at the top of the schematic. There is circuitry that I have not seen in any other amplifier. This circuitry automatically sets the output transistors' idle current. The idle current should be checked.

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

I see nothing in the service manual about biasing. This does have the pin headers tibimaki mentioned. There's also no pots anywhere in here.

Welp. Tried to take the voltage from the output terminals. Bad idea. Its stuck in protect mode and the secondary relay no longer clicks on. Ive disconnected everything but same result.

Back to the drawing board.

Edit: the new transistors checkout ok. Whew!

Is there DC at the output now?

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

Nope. No DC output. No secondary relay click.

Edit: im a bit fuzzy on this automatically setting the bias current. Even if i check it, is there anything i can do about it? Ie instead of a pot is there a spot where I'd solder more resistors in?

I broke something lol. She sat fine until i hooked the meter probes up. Yes it was set to DC voltage, not amps. I need to get it out of protect mode before i can worry about biasing.

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

The only things that are able to cause a protection trip in the amplifier outputs are DC offset or over current. Q525 and Q526 sense over current by sensing the voltage drop across the emitter resistors. each channel has one of these over current sensing transistors. All their Collector pins tie to the Base of a PNP somewhere via the 47K resistors.
The controllers in many amplifiers treat over current as a serious condition and will disable the amplifier from being turned on again until the condition is cleared by a certain button combination press.

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

You might have accidentally shorted something which can cause either a temporary fault or a permanent one. The idle current setting circuitry should be carefully checked for cracked tracks and solder joints. A fault in this area can cause the output transistors to conduct destructive rail to rail current.

Does the amplifier appear to turn on normally?

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

What are you calling "secondary relay" ?

[budm]

Re: Onkyo TX SV727: Found bad transistors, what else?

Nope. No DC output. No secondary relay click.

Edit: im a bit fuzzy on this automatically setting the bias current. Even if i check it, is there anything i can do about it? Ie instead of a pot is there a spot where I'd solder more resistors in?

I broke something lol. She sat fine until i hooked the meter probes up. Yes it was set to DC voltage, not amps. I need to get it out of protect mode before i can worry about biasing.

When you say output, are you referring to the speaker terminal on the back of the unit or right at the output of the amplifiers that go to the speaker protection relay input side contacts?
The one you call secondary relay is the speaker protection relay.

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

Amp turns on normally.

I refer to the "secondary relay" as the speaker protect relay i guess. I turn it on, it goes click, the VFD powers on and a few seconds later another relay would click.

I took measurements at the speaker terminals for the Front Left (i think left) i'll check when I'm not at work. Didnt think a meter would cause it to short.

I agree with checking my solder joints.

Maybe i didnt short it and it was timing from another problem?

Thanks guys!

[budm]

Re: Onkyo TX SV727: Found bad transistors, what else?

If the speaker protection relay is not on then you cannot test for the amp DC offset at the speaker terminals, you will have to test it right at the output of the amplifier.
But right now if all the speaker relays are on then you can test the DC offset at the speaker terminals.

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

Nah testing will have to be done at I'm guessing the test point terminals on the board. But I'm guessing these arent even going to be active if its stuck in protect mode?

[petehall347]

Re: Onkyo TX SV727: Found bad transistors, what else?

check dc voltages on the output transistors emitter resistors where they join together .. should be 0v ideally but likely a few mv ..this is dc offset ..
to check bias idle current measure mv between outputs emitters . or just right across one or both emitter resistors .. will somewhere between 15 and 30mv or so . or should be somewhere like that .
do testing with vol at minimum no inputs connected .

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

Yes, check the DC offset at the emitter resistors. If there is a problem, you will see many volts of offset.
The idle current can be checked by measuring the voltage across one or both emitter resistors then calculate the current. Dont bother with this for now.
Dont drive the amplifier with audio.

[rievax_60]

Re: Onkyo TX SV727: Found bad transistors, what else?

Check the DC offset from between the emitter resistors and ground, I use the amplifiers metal case as ground.

[petehall347]

Re: Onkyo TX SV727: Found bad transistors, what else?

if speakers tied to ground i use speaker post as probe stays in nicely

[Zeppelin1007]

Re: Onkyo TX SV727: Found bad transistors, what else?

Ok.
collector on the 2SD2xxx transistor was shotty so i resoldered it. I do get a secondary relay click quickly after powerup, but immediately back into protect.

I do believe i smelt the smell nobody wants to smell last night. Ie. Burn components :-( cause i dont smell it now.

Borrowing a thermal cam monday.

There isnt much on this damn board i should be able to check it out pretty quickly.

Is there a computer reset on this? I feel as if the service manual is a bit incomplete.

So what could i possibly have broken that has made it stuck in protect? Before the meter i had it on for a couple minutes without speakers and it stayed on just fine. Or is the problem now with biasing?

Checking emitter resistor voltage should be easy. I'll do that when I'm back home. Still no voltage output at the speaker terminals