B&K AVR-202i DIY repair possible? Relays buzzing (I think) - otherwise normal? - diyAudio
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Old 18th June 2009, 03:51 AM   #1
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Default B&K AVR-202i DIY repair possible? Relays buzzing (I think) - otherwise normal?

Hello all,

I'm faced with a dilemma. I have, in the last year, been trying to round up a respectable sound system for my music and home theater needs. This is quite difficult on a limited budget (as a college student), but I managed to find a used B&K AVR-202i on ebay for $280 shipped and I had to get it. The "i" in this case refers to the fact that it is a prototype model of the AVR-202. The 202i includes component video ins/outs where the regular 202 does not. Other than that, I'm unsure of the differences. I would assume that most all of the other specs are spot on with the regular 202.

SO.... It worked flawlessly for several months. One day, after having it on for a while listening to music, it started to buzz very loudly from the unit itself. This terrified me, so I quickly switched off the main power. I turned it on the main power again (not yet bringing it out of sleep), hoping that it was a fluke, but to my dismay the buzzing returned in full force as soon as the unit settled into standby.

That was two months ago, and since then I have been testing the various inputs and searching for the source of the irritating buzz. As far as I can tell the buzz is coming from a few of the relays near the back of the unit. There are 5 relays; 4 of the same model, and 1 other very close relative. The relays in question are on the "motherboard" (so to speak) to which all of the other dsp chips and input boards connect. Also as a part of this board are the multitude of RCA inputs (red,white,yellow), Zone 2 outs, Zone 1 pre-outs, pre-ins, the AM/FM antennas, IR controls, and a few other minor things.

I called B&K a day or so after the buzzing started (I hadn't found the source of the buzz yet), and they told me that it was probably a faulty transformer. They told me that I would need to send it in for service. Well, I've heard what they charge for repair service, and combined with the shipping costs (both ways) I just didn't want to jump to that conclusion...

I really have no idea what a faulty transformer would do, but there's something that leads be to believe it's not that dire... I still get full and clean power output to the speakers and all of the inputs work (to an extent, which I'll explain). The FM tuner and optical inputs are all working flawlessly and the sound is still as crisp and powerful as I remember from the time before the buzzing. The RCA inputs work as long as the buzzing continues. During my testing, the buzzing would periodically die down to an (almost) inaudible level. At that point, most of the RCA inputs would stop receiving signal. As I haven't been able to control the buzzing by choice, it has returned (seemingly for good) and I can't test this phenomenon again. The relays do stop buzzing for a second when I switch modes on the receiver (speaker config, source input, surround mode, ect...)

Another interesting clue related to zone 2 operation; when in standby, if zone 2 is powered on the relays buzz. If zone 2 is off (in standby), the relays are silent. In full operational mode with zone 2 off, a couple of the relays are buzzing. In full operational mode with zone 2 on, the buzzing seems to be coming from several, if not all, of the relays.

The relays in question are:

4x >>> HASCO HS 212 DC 5V
1x >>> HASCO HS 212S DC 12V

It's hard to tell exactly which ones are buzzing because they are so close together and I didn't want to take any out yet (never done that before, and didn't want to ruin it).

My questions are these:
What could be causing the relays to buzz like that?
Is this something that I would be able to fix?
If not, is it worth sending it to B&K?
What do these relays do? What happens if they are removed?

As far as the source of the buzzing goes, I'm not POSITIVE that it is coming from the relays, but I've read recently that relays are prone to buzzing in the described way under certain conditions. If it's something else, then I have no idea what. I had been living in an apartment complex with very shady wiring and we had voltage fluctuations like crazy. I was stupid and ran the receiver without a UPS, so that could have contributed to the issue.

If more information is required, I have no problems dismantling the unit to search for specific clues (I have already done so, several times) I am pretty tech savvy, but I don't have the experience required to troubleshoot this on my own. I AM, however, very good at following directions!

It's strange because, other than the annoyingly loud buzz, the rest of the unit seems to function as normal (which is why I wasn't sure about B&K's suggestion of a transformer failure). I really hope that I can save this unit, as it's the best one I've owned thus far and I'd hate to have to downgrade to a "cheapy" brand. Eventually, I would like to be able to build my own but I figure I'm a LONG way from that yet. At least I can learn by making repairs (hopefully). ANY help or suggestions greatly appreciated! Sorry for the long post, but I like to be as thorough as I can be. Can upload pictures if needed.
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Old 18th June 2009, 06:04 AM   #2
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Length of tubing --> ear ought to nail down which one, but just because the relay is buzzing doesn't mean it's bad. More than likely it isn't getting enough current to fully engage.
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Old 18th June 2009, 08:33 AM   #3
Enzo is offline Enzo  United States
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Or the filter cap for the power supply the relays run off of have either failed or cracked free of their solder. Checking that is easy. Set your voltmeter to AC volts and measure the voltage at the relay coil - there may be a diode next to the relay wired in parallel with its coil, that makes a handy test point. If teh DC voltage on the relay coil measures very little or no AC voltage, then my scheario is ot correct. But if you measure several volts of AC or more, then there is no filtration on that power. And the relay would buzz.

Also possible the transistor that engages it is failing.
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Old 19th June 2009, 07:14 AM   #4
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Thank you both for the useful insight! I'm sorry if I seem clingy, but I wonder if you would clarify some things, as I'm kind of a newbie to electronics tinkering and I have no schematic of this unit.

*First though, I tested the relays:
Because of the way the board with the relays sits in the chassis, I couldn't access the terminals of the relays without removing the board altogether. After doing so, I tested each relay with a set DC voltage across control coil terminals. All of them proceeded to click once when I applied the voltage and once when I removed it. They seem to be switching normally. Clearly, you were correct when you suspected that the cause was a power filtration issue.

You mentioned that I should test the filter capacitor for the power supply connected to the relays. I'm having a difficult time figuring out exactly what section of the power supply board I should be checking. I see many capacitors, though all of them seem to be connected properly to the board. How can I identify a filter capacitor? Also, you said that I could test for an AC potential, which I assume means that the amp has to be powered on. I haven't yet tested this for fear of getting shocked, but I suspect that I would find exactly what you described (an AC voltage across the coil terminals).

You also suggested that one of the transistors that controls the relay might be malfunctioning. I run into the same issue here, where I can't identify the transistors that control each relay specifically. How would I do this? Also, I'm not sure how to test if a transistor is working correctly. Any hints here?

I can upload some pictures of the board(s) if you think they will help you.
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Old 23rd June 2009, 03:18 AM   #5
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Default PROBLEM FOUND - need confirmation and guidance!

Ok, after inspection of the PSU board, I found a burnt spot. This happens to be a bridge rectifier which leads to (I'm assuming) the board with the buzzing relays. I tested the leads of the rectifier to find that they did indeed conduct in 2 directions.

The rectifier model No: KBP204G


Now.... Here's where I need some assistance in deciding what to do.


If I replace the bridge rectifier with a new one, will it solve the problem?

Can I use any bridge rectifier with similar specs, or does it have to be the exact model?

Could I use a "stronger" model to prevent this from happening again?

Does this rectifier failure hint at a bigger issue, or is it self-contained?

If the the rectifier failure was caused by something else, what?


I'm hoping that I can just replace the rectifier and be done with it, but I would really like to know if there is something else possibly wrong.

I would be very grateful if someone could address these concerns! Thanks very much in advance.

P.S. I'm learning a lot about amplifier design from this ordeal. I'm still a LONG way from even attempting to build my own, but I see a logical progression ahead!
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Old 23rd June 2009, 03:24 AM   #6
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The power supply failure is probably a symptom of the larger problem. Something is drawing a lot of current (shorted output tranny most likely) and pulling the rails down causing the rectifier to overheat and the relays to go into oscillation.

Sorry to be a downer, but I think you need to remove all the o/p trannies and test them. Maybe all the drivers as well. Testing feedback amplifiers is difficult at best and sometimes the easiest approach is to desolder all active devices and only replace known good devices.

Take lots of photos and meticulous notes
come back here and we'll talk you through it.

edit: the fact the relays are trying to kick in at all says to me that the rectifier is working fine (just overheating ) -your protection circuit is working perfectly. Just need to find the problem.
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Old 24th June 2009, 10:50 PM   #7
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Default Uh oh...

Oh boy... Thanks for the response Iain! In my excitement at possibly fixing my poor amp, I went ahead and ordered a couple of replacements for the burnt rectifier. At least they won't go to waste (I will have to replace it anyway right?), but now I can look at the bigger picture.

Electronics, in general, have always fascinated me, but it has only been a couple of months since I have seriously started to take action and learn some things. Please excuse my ignorance, but I don't have much of the electronics jargon down and I want to make sure we're on the same page. By "o/p trannies" you mean output transistors? "drivers" and feedback amps are one and the same? HAHA, boy I sound pathetic....

As stated in the first post, the signal to the speakers seems to be okay through the optical inputs. Does that mean that those circuits are working correctly? Where should I be looking for failed transistors? On the board with the relays? I realize that these questions might be difficult to answer without being able to see, first-hand, what I'm dealing with.

I haven't got time to look at it any more at the moment, but I will be sure to venture into all possibilities later this evening. I've attached what photos I have, though I don't have time to properly caption them. I have taken closeups of the relays and rectifier in question, as well as many other sections. It might be hard to see where everything goes, but I'll try to clear things up later.

For now, you can see (on the PSU board) that the fried rectifier is right next to the CON5 connection from the transformer. I'm not sure exactly what this supplies power to, but the identical rectifier connected to CON6 seems to be ok. If I'm not mistaken, the larger silver square is also a rectifier right? I believe this is the one that (from the VIOLET/GRAY leads) powers the amplifiers, while CON5 and CON6 possibly send power to the DSP and input chips. These are just my ramblings though, and could be dead wrong! I see that the BLUE/YELLOW transformer leads do have connection points on the PSU board, but I don't think they actually go anywhere (that I can see, anyway)???

I'm not sure what else you would like to see, but just say the word and I'll take pictures of whatever you want. I really do appreciate your kindness and patience I may not know a lot (yet), but I can follow directions and I'm a quick study.

I've uploaded the zipped picture folder to MegaUpload.com at the following link:


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Old 24th June 2009, 11:07 PM   #8
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Are the relays for selecting the inputs or are they on the speaker outputs?

I'm not familiar with this model so apologies if I made false assumptions.

Your remark about fluctuating mains supply could be important. If there was an overvoltage or surge that can take power supplies and other circuits out.

A good first step would be to make and break all internal connectors and cables and clean with deoxit or similar if you have it. Just disconnecting and reconnecting will usually clean most contacts. Let's check the simple stuff first.
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Old 24th June 2009, 11:23 PM   #9
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Hmm, that's not a burn mark under the rectifier, that's just running hot. I'd say the rectifier is working fine.

Looks like you've already given all the connectors a good working over. and audio passes through to the speakers OK? (why not find noisy relay and remove it - it's obviously not in the signal path)

Unfortunately, my guess is a logic problem, maybe a front panel button problem (the controller is switching inputs as fast as it can)

You're going to need a meter, a scope and a schematic to fix this one I fear, unless there's a B&K engineer here?
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Old 25th June 2009, 02:58 AM   #10
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As far as I can tell, the relays control the RCA inputs (which I don't use). The signal to the speakers does appear to be loud and clear through optical inputs. I thought about removing the relays all together, but then I wouldn't learn anything! I would like to find the root of the problem if I can; I think it will be a more rewarding experience, both in learning and keeping a fully functional receiver.

I did finally test the voltage across the control terminals of each relay while the receiver was turned on (and the relays were buzzing.) I found, not surprisingly, that the voltage was AC which is probably causing them to oscillate (they are DC controlled relays). The relays are rated for 5 V DC and 12 V DC, and I tested them by applying the corresponding voltages with batteries to confirm that they are switching correctly.

I also tested the resistance across the terminals of the bridge rectifier connected to CON5 (the one that I think is toast) and found that most of the terminal pairs let current through in both directions. A rectifier is just (basically) a bunch of diodes in a loop, correct? Doesn't this dual-direction conductivity mean that it isn't working properly? The condition of the CON5 bridge rectifier compared to the one on CON6 (faded, and writing burned off) ,and the brown spot on the terminals right beneath it, leads me to believe that it is toast.

I'd be a little disheartened if it truly is a logic problem, as that would probably require sending it in to B&K, which I really can't afford at the moment. I have neither a scope or a schematic for this model, unfortunately . For now, I'm going to assume that the rectifier stated previously is indeed toast and I will test as much as I can in relation to its apparent failure.

Assuming that the failed rectifier is allowing AC power to travel through the board to the relays, what could have caused the rectifier to fail? Perhaps over-current due to another shorted component? What are some possible culprits? Filter caps? Transistors? I'm not sure how to check either. At the moment, all that I have to use is a digital multimeter.

The replacement rectifiers that I ordered should be arriving on Friday. I ordered 3 since they are cheap ($0.60/each); so I plan to put a new one in to replace the burnt one. I'll let you know how it goes. I hope that by some fluke this rectifier is my only failed component, but I fully expect it to fail again if something else caused the failure in the first place.

Thanks again for your insight Iain! I'll report back when I can test my hypothesis, or if you have any other suggestions. Cheers!
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