Harman Kardon AVR1700 not powering up

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I have swapped, with another one from the same board. I don't have a spare one.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

The VCC supply, why is oscillating?

[Agent24]

Re: Harman Kardon AVR1700 not powering up

Which did you swap? You may have swapped with another faulty one, if it was connected to the other IC which blew.

Did you try replacing C918? If it is bad, the VCC supply for IC91 won't hold up under load when the IC tries to start up.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I have swapped it with PC94.
I have replaced that capacitor right now and there is no change.
What kind of voltage should I have at pins 4 and 5, of the IC?
I'm getting around .200V. If I remember correctly, I used to get here 324V.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

PC94 connects directly to IC92 and if IC92 blew up, there's a high chance PC94 was also damaged. So you may have swapped two bad optoisolators with each other. Check them with a test circuit, if they're bad, buy new ones. I wouldn't swap them with the others in case they are faulty and cause more damage elsewhere.

That's odd about the voltage going down. Do you still have ~324V on the primary capacitors? Or has it also gone down to 200V?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I have tested three optos: PC92, PC94 and PC95.
Used a power supply and a resistor. As I increase the voltage, the resistance decreases.
The 325V is still there, at the main capacitors.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

What's the voltage across C902?

[Agent24]

Re: Harman Kardon AVR1700 not powering up

Can we get some more photos of the PSU in its current state?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I will take some pictures and measure that voltage, tomorrow.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

At C902, I have 326.3V.
Pictures are not the best, taken inside with phone.
In some spots looks messy, because I have tried removing, that white foam thing.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

What should we check next?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

Anybody has any more ideas?

[Agent24]

Re: Harman Kardon AVR1700 not powering up

It's very odd.

What voltage do you get on the diode side of the PC92 optocoupler? Both pins.

Where did you get IC91 replacement? Do you have another to try?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

Almost nothing. .001-.002V
On the primary side, with min/max I have measured 3V.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

I've been re-reading the thread. I notice this.

There is 253 Ohm, between D921 cathode and ground

I am suspicious of this value. I would have thought it would be something higher than that. Maybe there is a bad part on that rail somewhere. Can you start lifting parts that go across that rail to ground and see where the resistance changes? I wonder if something is a bit leaky. Maybe a capacitor, or the shunt regulator.

Or D921 itself is leaky. It could be that on test with low current it's OK, but when it sees the full load of the power supply, it's effectively a short. If you have something you can swap it with, I'd try that too.

From the schematic, there's nothing to suggest to me that the resistance to ground from that rail should be that low.

At C902, I have 326.3V.

Also I find this odd. Unless you are running at 230v line voltage, the voltage at C902 should be 160v DC or so. According to the schematic, it's powered from a full-wave rectifier (D901\D902) straight from the AC, so it doesn't have anything to do with the voltage doubler of DB91\C903\C901

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

This time, I could not find that low resistance at D921. I don't know, how did I measure that resistance that time.
I did something stupid, twice. I have discharged the capacitor C902, with two screwdrivers and blew two diodes D901 and D902. I need to buy more diodes, to be able to continue.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

Diodes are in.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

Just to confirm, IC91 was one of the ones which blew up, originally, right?

Can you try disconnecting: J931, J914\J906 (probably the board only has one of these) and J955?
Then see if there is 5v at D921 Cathode.

If that goes nowhere, check if there is anything coming out of TF91 Pin 3\R932 and getting to D911's Anode.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

Yes, IC91 it was one of the original blown one. I have replaced it again later one, because it was shorted again.
I have removed those jumpers and there is no 5V. Some milivolt variation only.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

Have you replaced IC91 twice now?

If so, it could be that a bad snubber network keeps destroying the IC.

Remove D910, C911, check that the resistance of R979, R980, R981, R982 in parallel is exactly 470k divided by 4, ie: 117.5k - If it's off, one of those resistors is bad. Replace the lot.
Replace with brand new parts, D910 and C911 (note: 1kV part), and of course, IC91 again.

D914 could also be suspicious, and if bad would cause IC91 to be damaged. Even if it tests fine out of circuit, it might be intermittently failing. Likely worth it to replace it as well.