Harman Kardon AVR 3700 blown standby power supply

[petehall347]

Re: Harman Kardon AVR 3700 blown standby power supply

did you ask why they upgraded part number ?

[rievax_60]

Re: Harman Kardon AVR 3700 blown standby power supply

I'm worried that the transformer might have been damaged by testing with mains AC. This would mean that there could still be another fault still present. We should be able to figure it out from new oscilloscope waveforms if there is.

[tibimakai]

Re: Harman Kardon AVR 3700 blown standby power supply

Most like, all these will help me soon, when I will get back to it.
Thanks, for all that you have done.

[Agent24]

Re: Harman Kardon AVR 3700 blown standby power supply

I called Harman Kardon tech support and they were willing to sell me the transformer. It took a bout a half hour to get it all taken care of, but in the end it was only about $15 shipped and my time is worth that much.

I'm pleasantly surprised that you were able to get a new transformer for such a good price!

[jacampb2]

Re: Harman Kardon AVR 3700 blown standby power supply

did you ask why they upgraded part number ?

The short answer is no, I didn't. The long answer is that when I said this call took a bit more than a half hour, it wasn't because of trouble convincing them, or having to go through some "tech support trouble shooting steps" before they were willing to entertain selling parts. It was simply because the support person had a thick accent and we were having trouble communicating. She was very helpful, and very patient, but it took the majority of that time to get the different part numbers transferred across accurately. I told her it is board designation TF91, and the AVR3700 service manual I had says part number CLT9Z076ZE. That also matched the number on my pulled part. She came back after looking in her service manual, which I presume is current, and told me that they have the part number as CLT9Z079ZE. There is no part matching that number in my manual. I did not try to get further details just because it had been a difficult conversation already. They had stock of my transformer still, so I was happy to just get one that should be the same as existing. If it solves the problem, I may call back and order a second to have in reserve, and also ask about getting a new copy of the service manual, even if it has to be purchased.

I'm worried that the transformer might have been damaged by testing with mains AC. This would mean that there could still be another fault still present. We should be able to figure it out from new oscilloscope waveforms if there is.

I agree with you, after I pulled the trigger on that "test" I almost immediately realized the flaw in my plan. I did have a 10:1 stepdown on the mains though, so I was only giving the little guy ~12Vac, but it still could have pooched it. That said, I have literally pulled and tested every part in this standby supply, unless the fault lies in the +/-12V op amp supply which is powered from the TF91 bias/bootstrap winding, I don't think I missed anything. As mentioned earlier though, I did not think to isolate that other supply until the advice earlier in this thread suggested it. When I get the new transformer, I will report back either way. I am sure that I will need more help if there is further damage to track down, and if not, I will share the good news. You and the others who have tried to explain these tests and concepts have been invaluable to me. Thank you!

Most like, all these will help me soon, when I will get back to it.
Thanks, for all that you have done.

I hope so, I have been fighting with this thing for a month and a half now. If the leg work we did here helps even on other person get their back working, then it was worth it, even if I never do get mine going again. You are welcome, the other members here who have been assisting deserve far more thanks than I do

I'm pleasantly surprised that you were able to get a new transformer for such a good price!

Me too! Honestly it restored a little bit of faith in me for this company. I have used Harman Kardon gear for somewhere between 15 and 20 years. This last 3-4 years with new owners has had a lot of quality slide away IMO. However, even when I had to get in touch with tech support about my old AVR 2750, they were more than willing to help out. Customer service goes a long ways, and it is sadly mostly missing anymore.

Thank you all again for your help and direction. I hope to be back with good news soon!

Sincerely,
Jason

[R_J]

Re: Harman Kardon AVR 3700 blown standby power supply

TF94 "CLT9Z079ZE" is the transformer for the VFD display Page 180, good thing you did'nt get it

[Th3_uN1Qu3]

Re: Harman Kardon AVR 3700 blown standby power supply

I doubt you damaged that transformer by testing with 12Vac at 60Hz. I'm quite sure it was already broken.

Primary in such a small transformer is going to be wound with very fine wire. Not as thin as an equivalent 60Hz transformer as there needs to be less wire, but still quite thin, think 0.2mm or thereabouts. If a big surge hits, it is not so unlikely that it can arc over a bunch of turns. Please do keep us posted.

[tibimakai]

Re: Harman Kardon AVR 3700 blown standby power supply

I'm trying to test mine as well, but i don't have the knowledge, on how to do it.
If I measure the secondary, with the ohm meter, I don't have continuity all the way across.
I have seen a video on Youtube, that the person hooks up an oscilloscope CH1 on the primary and the CH2 on the secondary and uses a transformer, to generate electrical field. As it gets closer the signal gets stronger.
I have tried this, but I don't see any difference. The transformer, that I have tried, it was in an Onkyo receiver. This one seems insulated pretty well. It is a good choice for testing?

[tibimakai]

Re: Harman Kardon AVR 3700 blown standby power supply

I have asked a friend to test out the transformer for me with his LCR meter and the winding between pin #1 and #2 shows 0 uH. Ohms are under 0 as well.
It seems like, my problem could be the transformer as well.

[jacampb2]

Re: Harman Kardon AVR 3700 blown standby power supply

My meter would not measure the inductance of the primary either. Initially I assumed it was just below the range of my inexpensive tester or that my tester had a problem. I later measured several different transformer primaries with it and it worked fine. I also tested on known inductors and it worked as well. It may well be that that was my first indication that the transformer was bad.

As for testing, I did not complete all the ring testing that rievax_60 recommended earlier in the thread. The only function generator I had access to was either soundcard based, or using the little AVR transistor tester. The tester has some built in pre-selectable waveform frequencies, but is not arbitrary, so I could only do the test at one frequency, and not what I believe would have been the resonant frequency (if I understood his instructions correctly). That said, it was pretty easy really, I put a 10nf capacitor in series with one side of the primary coil. The other side of the primary went back to the function generator. Then, I connected the scope across the primary, basically right at the transformer. Once you get a good waveform triggered, you are basically looking for several decaying oscillations. From what I understand, more oscillations are indicative of a high Q and a good transformer. Any shorted winding in the transformer will highly dampen the oscillations. You can test this (and I did) by putting a known good transformer under test, and short any winding in the transformer and watch the ringing basically die off immediately.

If you have a problem in the primary windings, I suspect it is unlikely that you will be able to induce any current in the secondary.

Good luck!
Jason

[jacampb2]

Re: Harman Kardon AVR 3700 blown standby power supply

I'm pleased to say that I have the receiver back up and running. I bought a proper function generator and it arrived yesterday. I re-ran the same test I did on the old transformer with new function generator and on both the new and old transformer and have attached the waveforms below. It was definitely a bad standby transformer.

I also have the waveforms of the functioning power supply in case it will help someone else.

Thank you all so much for your help. It was a bit of a long road, but I am glad it is up and running again.

Sincerely,
Jason

[budm]

Re: Harman Kardon AVR 3700 blown standby power supply

Good news, you should post the pictures of your transformer test setup so people can see how it is being done.

[jacampb2]

Re: Harman Kardon AVR 3700 blown standby power supply

you should post the pictures of your transformer test setup so people can see how it is being done.

Great idea! I'll try to take some pics with the bad transformer when I'm out in the shop tomorrow. Thanks!

**edit** I forgot to mention, I just finished up rewiring this think in the home theater cabinet. It is indeed working again. Seems to be 100% functional.

Thanks,
Jason

[Per Hansson]

Re: Harman Kardon AVR 3700 blown standby power supply

Awsome repair! Looking forward to seeing your testing setup

[jacampb2]

Re: Harman Kardon AVR 3700 blown standby power supply

Thanks, I'll definitely take some pics tomorrow. It's pretty simple setup, and was based on riveax_60's ring test description much earlier in the thread. I can't take any credit for this at all. When I stared this repair I didn't even know that you could "ring test" an inductor or transformer.

Thanks,
Jason

[rievax_60]

Re: Harman Kardon AVR 3700 blown standby power supply

I'm pleased to say that I have the receiver back up and running. I bought a proper function generator and it arrived yesterday. I re-ran the same test I did on the old transformer with new function generator and on both the new and old transformer and have attached the waveforms below. It was definitely a bad standby transformer.

I also have the waveforms of the functioning power supply in case it will help someone else.

Thank you all so much for your help. It was a bit of a long road, but I am glad it is up and running again.

Sincerely,
Jason

The CS waveform is now the expected ramp shape. The measured amplitude is high, possibly a measurement error. Do you use a switchable probe?

[jacampb2]

Re: Harman Kardon AVR 3700 blown standby power supply

Yes, it's a 10:1 switchable probe, set on 10x. I'm not sure what you are seeing, that it is peaking at nearly 4v? I don't have enough experience to know what to expect.

I don't know if it makes a difference, but the power supply was not in the receiver when I took those measurements. I believe I also had the 22R that powers the op-amp supply feed still as well. It's a 1.5hr job to get this thing out of the cabinet. Should I be concerned and pull it again?

Thanks,
Jason

[rievax_60]

Re: Harman Kardon AVR 3700 blown standby power supply

Yes, it's a 10:1 switchable probe, set on 10x. I'm not sure what you are seeing, that it is peaking at nearly 4v? I don't have enough experience to know what to expect.

I don't know if it makes a difference, but the power supply was not in the receiver when I took those measurements. I believe I also had the 22R that powers the op-amp supply feed still as well. It's a 1.5hr job to get this thing out of the cabinet. Should I be concerned and pull it again?

Thanks,
Jason

I dont think there is anything actually wrong with the repair.
Is it possible that the probe was set to X1 for the CS measurement? The 4V reading seems way too high.

[jacampb2]

Re: Harman Kardon AVR 3700 blown standby power supply

I kept an eye on it as I went because I have inadvertently flipped it before. I can say that I took the readings on CS immediately after the ring test, and then I went on to do the FB and Drain waveforms. I know for certain I didn't switch it back, so if I fumbled it to 1x, I fumbled back to 10x. Not at all outside the realm of possibility, but I don't think I did.

Thanks,
Jason

[tibimakai]

Re: Harman Kardon AVR 3700 blown standby power supply

Thanks for all this, it will help me right away. I have found out, that the transformer is a bit over $6. I was just waiting for you, to confirm that fixes the problem. I will be placing an order, right away.
I'm waiting for your test pictures as well. Just to be sure, that I have tested it correctly.
Great job.
Which generator did you buy? I need one as well.