B&K AVR 307 Receiver center channel blown- may convert to power amp - AVS Forum
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post #1 of 13 Old 02-12-2013, 01:34 AM - Thread Starter
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I've owned nothing but Rotel since the late 80s. I stumbled onto a B&K Reference20 and was impressed with the build quality and functionality. Also made in the US is so rare! I needed a second system so got a B&K avr 307 on FleaBay. Volume knob mostly broken, display is dim and quirky, but no big deal for what I paid. Unfortunately the center channel is dead. Well, wish it was dead. After it torched my center speaker I noticed center put out 36v. I thought amp section may be the problem. After figuring out the design etc, I swapped the center (green and blue) preamp output from center amp to rear surround amp. I only use 5 speakers so I have two spare amps. It was a very easy swap- just unplugged from center amp and plugged into rear surround. But alas the 36v moved.

So the preamp section is bad. I found MTAtech's pictures on Google and he was converting the AVR to a power amp. Interesting. Not sure if it worked or not- the thread went dead. He thought (and makes sense to me) if he just pulled the (green and blue) connectors that run from preamp to amps, put an RCA on the end, he could run RCAs from an external preamp and use the BK just for power.

I wonder is the voltage compatible? Is it moot as some easy fix for 36v from a channel (i.e. would EE101 know that's a single simple component)? I thought I may throw caution to the wind and go for it- maybe using the BK DVD input RCAs and wire those into green/blue wires to amp sections. I actually have an awesome Rotel 985 preamp to use. Great preamp, and I don't use it anymore (replaced by BK ref20) and really worth next to nothing to sell. And if I can do some quick wiring to get a 5 channel BK amp for it...

PLEASE, any thoughts, suggestions, hurdles....?
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post #2 of 13 Old 02-26-2013, 07:42 AM
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I am interested in how this went. As you know I started a thread (mentioned above) on this since I own a B&K 307 that has an obsolete pre-amp section (no HDMI connections) but a first-class amp section. Selling the 307 would only yield about $400, while a B&K ref. 200.7 amp is worth a couple of thousand. Thus, turning the 307 into a amp makes sense.

However, I got chicken with performing surgery on a mint condition receiver -- especially with not much electronics knowledge. I have the service manual and diagrams but haven't gone forward.

The only hurdle seems knowing what you are doing. Otherwise, it seems straight forward.

One can see from the picture below how massive the amp section is:


My theory is that by finding the place where the pre-amp board connects to the amp, I can separate that connection and install RCA inputs to feed the amp directly.



Note the markings, such as "RF" (Right Front), "LF" (Left Front), "CF" (Center Front), etc.
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post #3 of 13 Old 02-27-2013, 11:19 AM - Thread Starter
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I use 5 speakers, so have spare amps channels. I simply popped connector off at the center channel amp board and ran (green and blue) to the right rear amp board input and it works great. Relabelled back and removed fuses so no risk of blowing another speaker. So... MTAtech is right I think. The green and blue pairs can be mixed and matched and it appears they are directly connected to the DVD rca inputs (not 100% sure). So I think it'd work fine as a power amp. Just connect RCAs or other output from external preamp to the green/blue wire pairs to the individual amp board channels. I would examine the 'DVD' rca inputs- maybe they could be used (just for physical connection. Alternately I'd remove the top rear boards (for access and they'd be unneccesary) and use the empty 'holes' to insert my own rca jacks. Point is, it could easily be a very neat build. In the menu one can set the turn on volume I think (which could be 3/4 up or something).

Truth is, if I had a working unit I'd likely just use it as a receiver. I have a Fluke meter with transistor tester so I checked the blown channel. I can't be sure if it's the only thing wrong, nor if it's a result of other components going bad upstream. Whatever the root cause- the amp output transistor is bad. It should read open (0 ohms) but doesn't. SO I was wrong in original post. The preamp is fine- the amp channel is bad. I first thought this but when I swapped wires it didn't work- so I thought preamp bad- it then occurred to me that as I had 36vdc at speaker output I had likely cooked my center speaker. I did. SO when I went back and swapped green/blue from center to rearsurround, with new speaker, it worked fine.

Great lesson- ALWAYS check for DC at speaker outputs when buying used gear. There should be zero, but at most maybe 1vdc.
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post #4 of 13 Old 05-27-2013, 01:16 PM
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Hello,

I bought an AVR 307 just over a year ago, and thought that it was excellent value for the money, american made, great amp, and adequate features for the low low price (I prefer the analgue audio)

Just this past weekend the front left channel blew out - almost destroyed my Paradigm 11SE, but I was close by and unplugged the B&K before any lasting damage occurred. The woofers in the speaker went straight out as far as they would go, and it made a screeching noise. I think this means the channel went DC, possibly a blown transistor? In your opinion is this something I should have fixed, or should I cut my losses and move on? I almost want to take a look into the service manual and see what kind of mess I'm in before I send it off. Where did you find your copy?

Any help would be greatly appreciated.
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post #5 of 13 Old 05-27-2013, 04:28 PM - Thread Starter
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BKcomp has a lot of info including:
http://bkcomp.com/support/schematics-and-service-manuals/index.html

Also just try Googling it. My 307 had bad transistor. Lucky I have Fluke meter (trans tester). But any $10 meter will tell you if you have DC on spkr out. Quite sure you do. Should be like under 1v AC. My blown channel had 35V DC. Also has bad volume knob and bad display screen. Also noticed a Power transformer buzzing noise- physically from box and through speakers when no music is on. And if one transistor blew (is it the cause or the symptom of other components blowing???) others likely to follow. I think one important factor is that this is a receiver - not separate components. It runs VERY hot, which everyone knows kills electronics. My new spot for the 307 only has like an inch clearance above it and it's REALLY HOT. So I just think as beautiful as BK design is, these just don't cool well. And honestly for a not too old rcvr to have as many issues as mine... it dimmed my view of them a bit. Think I may go back to Rotel separates. I have had like 20 Rotel items, mostly second hand (or 3rd, 4th?) andf have never had a single failure. of any kind. That said, never had a receiver... Seen like 20 972 rcvrs on eBay! Or stick to BK preamps (not rcvrs).

You can get a nice 307 for like 300 bucks. I wouldn't fix it. And again, 1 blew (almost popped spkr), so you may have others go soon too. Cut your losses and move on. Get a BK ref 20 or ref 30 or Rotel RSP985 and a 5 channel Rotel 985 amp. My thoughts anyway. My 307 likely gonna get shelved, I have a Rotel rsp985 (old but 5.1) and built like a tank. I'll get a 5 channel amp for it. And/or I have an older Harman Kardon AVR 5.1 I just got back. But either way- much as I hate to do it- think I'll decommission the BK307. Like 80 frickin lbs., maybe the copper in it's worth more than the unit... But most important, don't want to roast good speakers.

Good Luck
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post #6 of 13 Old 05-27-2013, 07:19 PM
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When plugged in and turned on (nothing connected) my multimeter reads 60v DC on the channel that went bad. Oddly though it also reads 120v AC which does not make any sense. I'll try a different meter tomorrow. All other outputs read 0 for both AC and DC. Back to the day it happened: I was lucky to be right there when it failed. A bit of smoke wafted out and smelled of molten solder. When I tested it with the meter, that location on the heatsinks was generating far more heat than the rest of the unit. I kept this piece on the floor with nothing above it. Perhaps it was its time. I have a nice old onkyo integra set up RL so I have time to research a nice pair of separates. Thanks for the advice.
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post #7 of 13 Old 05-27-2013, 11:50 PM - Thread Starter
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Hmmm. The 120vac is pretty strange, but I don't know enough to say for sure what that could be. Other than the obvious- that output circuit is fried- really fried... That's funny- I have an older Integra rcvr too- took it's temp place. It's from the year or two it was actually branded Integra (by Onkyo). Doesn't look like much but those are under rated I think, good stuff. Integra and Sony ES are the two top hidden cheap-as-dirt ebay finds. I think my THX Ultra Integra 5.1 was like 25 bucks and 25 shipping. I have a BK Reference 20 that I like a lot (also VERY cheap). I'd highly recommend one if that's the way you go. BK ref20 and Rotel rb985 is my main setup. Full functions, tons of power, sounds great. Cost like $400 total (used of course).

Take it easy- good luck.
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post #8 of 13 Old 12-12-2013, 12:08 PM
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Did any of you guys get this to work as a power amp? Just wondering because I was given a 307 recently and was thinking of using it as a power amp with my Onkyo 818. It looks like the 307 is a more powerful amp than the 818 but I would like to use all the bells and whistles the 818 has.

 

Thanks,

S3N

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post #9 of 13 Old 12-27-2013, 02:12 PM
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Have any of you guys tried using the DVD-Audio IN RCA jacks as inputs from your pre-amp? It has 5 channels plus the sub input. My Onkyo 818 has RCA pre-outs for 9 channels. Seems like I would be able to use these for at least the LCR and SLSR channels. Would this turn it into a power amp?

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post #10 of 13 Old 01-11-2014, 04:45 PM
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A bit late to the table, but here is my experience with the AVR-307

I do not think the DVD audio-in turns this into a power amp.
You just bypass the surround sound processing by the receiver.
I use the inputs for four channels only. (Stereo speakers, external crossover, driving tweeters and bass separately - Audio Mode 4B on receiver).

Repairs:
These run hot?! It seems the amp sections on the sides are not getting awfully warm, neither does anything on the processing board.
The real killer is the power supply board on top of the big caps. There is little ventilation / convection possible.
One AVR-307 I bought used already had bulging filter caps on this board (plus shipping damage, DOA) ... another just got fixed.
http://www.audiokarma.org/forums/showthread.php?t=565034

The repair was successful, the diagnosis by one of the Audiokarma members was correct. AC leaked past the rectifiers, causing the output relay to buzz, tested with borrowed oscilloscope.
Now that at least one unit is up & running again, I am worried how long the repair will last. Could you see how hot your rectifiers close to the filtering caps are running?
Here, one is doing fine, just a tad warm, the other is probably ~100degC. Replacement rectifiers are name-brand and same spec.
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post #11 of 13 Old 01-11-2014, 09:44 PM
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Quote:
Originally Posted by Primergy View Post

A bit late to the table, but here is my experience with the AVR-307

I do not think the DVD audio-in turns this into a power amp.
You just bypass the surround sound processing by the receiver.
I use the inputs for four channels only. (Stereo speakers, external crossover, driving tweeters and bass separately - Audio Mode 4B on receiver).

Repairs:
These run hot?! It seems the amp sections on the sides are not getting awfully warm, neither does anything on the processing board.
The real killer is the power supply board on top of the big caps. There is little ventilation / convection possible.
One AVR-307 I bought used already had bulging filter caps on this board (plus shipping damage, DOA) ... another just got fixed.
http://www.audiokarma.org/forums/showthread.php?t=565034

The repair was successful, the diagnosis by one of the Audiokarma members was correct. AC leaked past the rectifiers, causing the output relay to buzz, tested with borrowed oscilloscope.
Now that at least one unit is up & running again, I am worried how long the repair will last. Could you see how hot your rectifiers close to the filtering caps are running?
Here, one is doing fine, just a tad warm, the other is probably ~100degC. Replacement rectifiers are name-brand and same spec.
I never did build that amp out the 307 but it woulf have bee ausome. Any body with the skills wanting a project?. Remember it belong to me.
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post #12 of 13 Old 01-12-2014, 07:50 AM
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I never ended up converting the 307 to a power amp. Lack of time and electrical knowledge were the reasons. I wish I could get someone with experience to do it for me.
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post #13 of 13 Old 03-24-2014, 01:49 PM
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New to the group and new owner of an old AVR 307 that I picked up for a song.

 

I'm interested in knowing if the amplifier section of this model is similar to other B&K models. Does anyone know?

 

Also, I do have a slight buzz on some of the channels (maybe all) that is annoying when the room is quiet.

 

Any chance that the people who took over B&K would look at this model?

Otherwise, as an amp only, if I can deal with the buzz, it would be nice to have around.

 

Thanks.

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