B&K Reference 7270 Mk II amp - value?
- Thread starter Mingo
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Great amp. Runs pretty warm with 70-odd power MOSFETs cranking away. Built like a tank and handles both XLR and RCA inputs. Six-pack of humongous filter caps, but power supply board tends to cook since it's oriented horizontally with components underneath so heat can't escape. Giant toroid transformer, those big caps, and all those thick heatsinks make the unit pretty hefty so make sure you've got sturdy rack to house it. Weak point is the electrolytics on the amp boards. The heat cooks the electrolyte (85-degree caps from factory) and the caps degrade. Easily diagnosed by channel attenuators being at different settings to achieve equal output in all channels (look for this in eBay photos). Replacement caps are cheap and available from the usual sources (8 per channel as I recall), but a bit of a pain to install since you'll have to peel all those MOSFETs off their heatsinks to get at the back of the amp boards. Bias current and DC offset voltage are adjustable and set using separate pots for each channel. B&K, effectively out of business since the late 2000s, has stuck with the same basic amp circuit for decades, so you've got a pretty good track record to go on. The pinnacle of their product line was probably the Ref 50 pre-pro coupled with the 7-channel Ref 200.7 power amp, very similar to your 7270 unit. $100/channel is a decent estimate of value.
fendray26
New Member
I have one here to be repaired and from what I have learned it's not easy acquiring diagrams or really any helpful info for these things. You seem to know a lot about this amp. Would you mind telling me what belongs in R15 on the board that sits above the capacitors? This is how it came to me. I have only inspected it but wanted to make sure I start with correct information. Thank you so much.
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As I mentioned above, B&K stayed pretty much with the same basic design for their amps for many years. If you do a bit of "sleuthing" (keep your eyes peeled!) you'll find that the schematics available online from the usual sources are comprised of "recycled" drawings of assemblies from prior models. So, for example, a package of schematics titled as belonging to the Ref 200.7 power amp will likely include drawings whose date is from well before that of the 200.7 and whose titles directly reference the progenitor models, such as your 7270 (or 7250/7260). Using this somewhat circuitous method of info mining, you can come up with the schematics and bills of material for almost every B&K model. A little bit of surgery with Adobe or the like and you can put together your own "Service Manual" for your 7270 Mk II. A good part of the sense of accomplishment in acquiring, repairing, using, and maintaining older gear lies in this kind of journey of discovery. I wouldn't dream of depriving you of the pleasure in hunting down and bagging your quarry. Good luck.
Several B&K employees started another company for AV gear. They call the company Eastern Elite Audio and Video. They were doing repairs on B&K gear and may be a resource. I haven't had any B&K gear in a few years, but the last number I had for them is (716) 601-7308. Good luck! Of all their gear, I found the amps the most reliable and best performing. Definitely worth repairing!
fendray26
New Member
Thanks for the information. I’m looking forward to actually being able to see what this thing can do as I have heard nothing but good things about its capabilities. And one more thing I can definitely say is it is the heaviest amp I have ever worked on. I do not look forward to the day that I have to carry it out of my house.
Good luck on your repair. BTW, not to rain on your parade, but I don't like the look of those attenuator settings. A properly running amp will have all of them set to minimum attenuation with speaker levels controlled by the pre-pro/receiver's internal adjustments on its preouts. Since you know you're dealing with a hot runner (which audiophile lore advised to run even hotter than factory), you're gonna need to check ALL the caps on each amp board (8 electrolytics and 6 tantalums) as well as inspect all other components for signs of obvious damage. Heat kills and it's generally heat that starts the cascade that kills these amps. Word of warning: WATCH OUT for those filter caps—they're many and big and pack quite a punch (as you should be able to see from the lights in your house/shop dimming when you power it on...).
fendray26
New Member
Don't worry....I know better. My son came in and messed with them while it was waiting for the part.
fendray26
New Member
So that was a real bitch to figure out......only because whoever got their hands on it before me had the power supply board on backwards, all amp connections were haphazardly plugged into whatever was closest, the entire ground wire from the power cable connection was missing and channels 3 and 4 speaker outs were plugged into the each other. What a long list of trial and error but it was all worth it. This thing is scary powerful.
Big thanks to @awillia6 for his helpful guidance and support. Couldn't have done it so "gracefully" without him.
Big thanks to @awillia6 for his helpful guidance and support. Couldn't have done it so "gracefully" without him.
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Congratulations on your repair! I was beginning to wonder if things were going to work out; I'm very happy to hear that they did. And against such a headwind of things gone wrong—good for you.
Now that you're a seasoned pro on B&K amps, can you shine any light on the process you used to select and set output transistor bias levels? I'd be very interested in what you have to say.
