Harman Kardon AVR1700 not powering up

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I have blew up a diode, twice a while back, when I have tried to use the scope in the primary.
You are right about the diode orientation.
I want to clarify something, that may be important, I'm measuring the PS, out of the receiver. I hope, that is OK. It is not connected at all, to the receiver.
I have managed to take some snapshots. I have touched the plastic wrapping, with the tip.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

The ST5V rail and circuit of IC91 (and also IC92) will run as long as it has power, it won't matter if the receiver is connected or not. The main control signal (AMP_VCC_ON) from the receiver only applies to IC93.

If you are not getting any voltage into the anode of D911, something is bad. Is the circuit feeding D920 still disconnected? If there is a fault there, it could be shorting the output of the transformer.
Can you also try with J931 or D943 disconnected?

Are those waveforms you measured all you get? Does that waveform repeat? Can you zoom out a bit and see if there's anything else?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I have mounted all together and applied 5V at D921 and the standby LED turned on, but the receiver, still did not turn on. The amperage started to go higher, from 0.07A and the standby LED turned off.
D920 is not disconnected. There is no short that way. Measuring over 5 Mohms.
I have to pay more attention, to those wave forms.
At the anode of D911, should I get AC right?

[Agent24]

Re: Harman Kardon AVR1700 not powering up

There may be no dead short but there could be a problem with something leaky\breaking down under voltage. Since this thing has been going nowhere for a while, I feel like we should try every possibility...

I would say, take the circuit down to the bare minimum it should be operational under. That means, disconnect D920, disconnect J931\D943, does it still fail to power up?

You should definitely get *something* at D911's anode, but it may be too fast for your multimeter. The switching frequency of IC91 is supposed to be 50kHz. You really need the scope on it to see what's there. But you should also have the isolation transformer.

If the supply was operating normally, when you hold the probe over the transformer core, you *should* see something like the attached image (from http://mmcircuit.com/waveform-of-fly...-power-supply/)

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

Definitely, I don't have anything like that on my scope.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I have removed D943 and D920. What should I measure and what should I expect to get?
At D911 cathode, I'm not getting anything. I measure AC, right?
I have tried again, with the scope and there is no change. It does not oscillate.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

On the cathode of D911 you should see DC with some slight ripple (it will be smoothed by C918)
On the anode of D911 you should see a pulsed waveform, likely similar to the one in the image I have posted.

If you have nothing at D911 anode, then IC91 is not switching for some reason, or TF91 is bad.

A faulty optoisolator could cause IC91 to not run, so that is why I suggest using a known good one. When the primary side IC blows, it can take out the optoisolator as well, since that is usually connected directly to it.

I assume you bought the new IC91 chips from eBay or Aliexpress? Unfortunately you may have received fake or faulty chips. That's the thing with such places, if it doesn't work, you have no idea if the part is faulty or if there is some other problem. That said, I have purchased several parts from there when I had to, and so far they have worked, although I still find such parts to be a source of doubt.

I wonder, could you post some new up close and clear photos of the board around IC91, both sides? Just in case there is some new problem I'm missing? It's hard when I can't see the board.


Another thing you could try, is powering IC91 from an external supply. Gianni posted such an idea in another thread:

When I have to repair a power supply I feed with an external adjustable power supply the control IC (just above the UVLO start threshold) and than I feed the PSU input voltage with another adjustable power supply with current limit set to a low level (200~500mA) to see if I obtain the output voltages incresing the input voltage slowly, from 0V to nominal value.
In this way, with an adjustable power supply with current limitation I can work safely and if there is a short circuit on the output I can avoid any problem just limiting the input current. Obviously you don't have to apply load during this test. If the output are present, than I bring the input voltage at nominal level (if input is 110Vac then I apply 155Vdc, if it is 230Vac I apply 310Vdc). At this point I disconnect the external supply from controller IC and if the PSU shut down I try to apply a minimum load because some PSU need it to stay on.

(https://www.badcaps.net/forum/showpo...06&postcount=8)

For current limiting, you could probably get away with a 40W lightbulb for the HV supply, if you don't have a high-voltage PSU with current limiting (I don't!)

If the circuit works from an external power supply, then we know for some reason that IC91 is not getting its supply voltage from TF91.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

At the D911 anode, I have the oscillating voltage, with a max voltage of 12.5V. On the cathode side, I don't measure anything.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

At the D911 anode, I have the oscillating voltage, with a max voltage of 12.5V. On the cathode side, I don't measure anything.

That doesn't make sense. I think you have the cathode and anode mixed up?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

At the transformer side, I don't have any voltage.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

That would be the anode. A diode pinout is like this: Cathode ---|<--- Anode
Easy to remember, the anode side looks like a capital A on its side.

According to the datasheet, 12.5v is the nominal value where the chip will exit UVLO (undervoltage lockout), so it should start up when the supply reaches that voltage.
The voltage there will drop when the chip tries to start up, but then the winding from TF91 that supplies power via D911 will bring the voltage back up (or it is supposed to)


Datasheet says also:

At overload condition when FB input voltage exceeds power limit threshold value for more than TD_PL, control circuit reacts to shut down the switcher.

It has this:

VTH _PL Power Limiting FB Threshold Voltage 3.7 V

You have 3.93v at FB pin, perhaps it is too high and shuts the IC down. I really wonder if the optoisolator is bad? But then I think this would happen if the opto gets no input, and it isn't. So could be either.


Did you say you had another one of these? Can you swap the transformers?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

Thanks for the clarification.
I have another receiver, but the PS is missing and the HDMI board is corroded, no sound.
I will measure that 3.7V again, just to be sure.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

You are right, I have bought the IC from Aliexpress. Nobody else sells these anymore.
Since I can't use the scope on the primary side, I'm out of luck seeing what is going on there.
Do you have a recommendation, on what should I buy for my Rigol DS1052E, to be able to use it on the primary side? Right now it i hacked to be 100Mhz scope.
The new optos, are on the way. I will replace it, as soon as it arrives, but I wouldn't think, that it will make a change, since I have tested it and it shows OK.
I'm measuring the FB voltage again and what I have noticed, that if I use the range + Min/Max I'm getting 3.83V and if I use the second range, I'm getting 2.99V.(hitting the range button once, on Fluke 189)
VDD on the IC, is 12.3V.
Top picture is not the best, shadow from the sun.
I can retake it inside, if you need it.
D943 and D920 are removed, at one side.
Viewed from the bottom, the bottom white strip is the transformer and on the left side of it, is the IC91. Opto is above the transformer.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

I have borrowed an isolation transformer.
At the D911 I see a nice 60Hz sinus.
What else should I look at?

[Agent24]

Re: Harman Kardon AVR1700 not powering up

Which side of D911?

You have ground lead of probe connected?

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

At the cathode side.
I have ground lead connected as well.
Scope/lead set at 10x. I have managed to see Vmax, at 136V. That sounds about right?

[budm]

Re: Harman Kardon AVR1700 not powering up

60Hz? 136V? that sounds like wrong ground ref is being used, the frequency should be in 40-50 KHz.
The unit is plugged into the Iso xformer, right?
The GND ref in the hot side is the Negative leg of the main filter cap, the (-) leg of one (c930 per diagram) of the two cap with the star GND, not the chassis GND.

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

You are right, now that I'm looking at the board, I have used that metal tab, right on top of the two main caps. I haven't even looked, what is on that side. I have just looked at, the bottom of the board. I have assumed that is a ground.
I'm using an isolation transformer.

[Agent24]

Re: Harman Kardon AVR1700 not powering up

If you're working on the mains side of something, don't assume anything!

[tibimakai]

Re: Harman Kardon AVR1700 not powering up

OK, I have used the - of the first main capacitor and this is, what I have got at the D911 cathode.
How could I add a picture of this file here?