Onkyo 577 repair

[tony359]

Hello all,

I am having fun repairing an Onkyo TX-SR577 AV receiver. It was shorted so it was powering down immediately once the ON/OFF button was pressed and my current limiter (light bulbs) was confirming a short (bulbs were flashing bright).

I found two shorted transistors and a few more bad components on the AMP board.

I've replaced them all and the short is gone (no more lightbulb flashes) BUT the Onkyo still refuses to start.

Let me explain.

The Onkyo powers a small Stand-By transformer when mains is applied. That powers up the CPU and enables the ON/OFF button. When ON/OFF button is pressed, a CPU line goes to ground, the main 240V relay energises and voltage is applied to the main transformer.

What happens is that the main 240V relay engages but disengages after about one second. But my "current limiter" (light bulbs!) does not show a short anymore.

So I attempted jumping the main transformer to 240V directly - so it gets powered regardless (yes, I do have a current limiter connected!).

Weirdly enough, the AVR works! It powers up ok (even though it tries to power down by opening the mains relay, but mains is jumped to the main transformer) and all channels work, including the one I have repaired.

Now, the service manual has all the schematics but does not say what is the "logic" behind the scenes.

In my case, the APOWER line from the CPU is controlling a transistor which in turn grounds POWERD which drives the mains relay.

There must be something the CPU is sensing which triggers a full shutdown.

Does anybody have any ideas? Is there any documentation on how that works?

Thank you!

[petehall347]

Re: Onkyo 577 repair

its probably your dbt .. go higher wattage bulb or full mains

[R_J]

Re: Onkyo 577 repair

You likely still have a high dc offset voltage on the channel you repaired and that is placing the amp into protection.
Check the DC voltage on each pin of ID+ and ID- (P6080, P6081 etc.) Or at the common point of the dual emitter resistor R6100, R6101 etc.
Also check the voltage on the VPROTECT line

[tony359]

Re: Onkyo 577 repair

PeteHall

I admit I made that mistake at some point - mistaking a similar behaviour for an issue but it was just the current limiter dropping the voltage too much.

However, I have a "bypass" switch on my home-made limiter and yes, I have tried feeding the amp with full mains. The issue remains.

R_J
Excellent point. I did check ID+ ID- to adjust the BIAS - I found it a bit high (9mV) and tweaked that down to the recommended 4-6mV. I plan to do a proper adjustment with the unit fully warmed up once it works ok!

I will check the rest shortly, thank you for your advice!

[tony359]

Re: Onkyo 577 repair

R_J

Thanks again for your time on this.

I have checked all ID+/ID- and they all read around 2mV DC (when unit is cold). That includes the repaired channel.

I did not get any voltage on VPROTECT but I am also a bit puzzled on what GND to use with that! I think the CPU ground should be ok which I see is called GNDGD.
So if I reference my multimeter to that ground on pin 8 of the FW Upgrade terminal, I get no voltage.

To double check whether VPROTECT was going to ground, I referenced my multimeter to the +15V line on the positive lead and I probed VPROTECT, IPROTECT and also SEC1H with the negative.

VPROTECT and IPROTECT read +13V (confirming they go to ground I suppose) and SEC1H goes to +11V - I suppose that means the same?

[R_J]

Re: Onkyo 577 repair

Check page 49 of the service manual, The ground to use to check VPROTECT is the negative side of C6901, or JL6603A any pin from 3~9
The protect lines should be LOW if they are at 3.3v that line is in protect

[tony359]

Re: Onkyo 577 repair

Thanks again!

My multimeter reads only 1.42V on VPROTECT and IPROTECT between those points when I attempt to power up before the unit shuts down.

To confirm I am on the correct GND, I have checked +15 and -15V on the PC board connector and I read +15.07V and -14.72V respectively.

I checked with the oscilloscope in case my multimeter was too slow to show a 3.3V peak and no, it peaks at 1.42V and then it shuts down.

Would that be a normal behaviour?

[tony359]

Re: Onkyo 577 repair

Quick update: I have another Onkyo I am trying to repair (the infamous Dolby/DTS chip, I reflowed it but it lasted a couple of weeks - I'll try reballing it) and indeed the PROTECT lines read 0.something volts. So I suppose something is wrong with those lines.

What would you recommend checking? I have checked the transistors and diodes and resistors involved and they all seem ok

[R_J]

Re: Onkyo 577 repair

Are BOTH VPROTECT and IPROTECT at 1.4 volts? or only one of them. It could be that the transistor (like Q6073) for IPRO or one of the resistors (Like R6143,R6153) around it was damaged when the bad channel had the problem.

[tony359]

Re: Onkyo 577 repair

Yes, bot VPROTECT and IPROTECT read 1.4VDC.

I have checked Q6073, R6143, R6153, D6003, R6173, R6183 and R6193 (and relevant ones on all other channels) and I cannot see anomalies.

I've compared most (all) of the components of SBL (the damaged channel) with its twin SBR and I do not observe differences.

Can I remove Q6163 and test without it? All IPRO lines are tied together so I suppose it's difficult to see which channel is sending the signal. I'm wondering if it's worth isolating a channel at the time - but I have no idea whether removing Q6163 would cause any damage.

It also seems impossible to power up the unit with the boards "vertical" as the front panel cannot be connected to it? That makes difficult to probe components while they are powered.

[tony359]

Re: Onkyo 577 repair

Are BOTH VPROTECT and IPROTECT at 1.4 volts? or only one of them. It could be that the transistor (like Q6073) for IPRO or one of the resistors (Like R6143,R6153) around it was damaged when the bad channel had the problem.

After some troubleshooting, here are my findings.

I measure 40V (or whereabouts, I've noticed the supply voltage changes over time?) which is the supply voltage at both the BASE and EMITTER of Q6713. That should not give an output, if not mistaken, but I do read 1.06V at the collector. If I remove the transistor, I see that I still measure 1.06V where the collector would go - so is that voltage coming from the processor? Is that supposed to be there? Is that a floating reading? I checked R6716 and it reads ok.

I've also removed and checked R6712 and C6713 and they check ok.
Also, if I lift one leg of R6701, I still read 40V on the base of Q6713 but I read about 4V on the "amplifiers" end of that resistor (left hand side on the drawings).

This is for IPROTECT, I suppose that VPROTECT is the same.

Any help appreciated...

[tony359]

Re: Onkyo 577 repair

I still haven't lost the hope that @R_J (or anybody else) could chime in and push me in the right direction!

I have received some replacement transistors (Q6713) but I doubt they will fix the problem tbh.