B&K 200.7 blown up capacitors

TLS Guy

TLS Guy

Seriously, I have no life.
Yes it is a cap, apparently a tantalum capacitor. they popped and smoked when it turned on. They may have been cracked before and I just didn't notice but they're real cracked now. I have sent a message into steve at stargate electronics to see what can be done. Also have an email into Rick's home theater repair here in St Petersburg who said they too can take a look at it for 35 bucks to assess the damage. Mostly worried about mosfet outputs being damaged, which apparently are no longer available or difficult to get the exact replacement. I have had some good responses and solid information given to me here and on another audio forum and I've learned a lot in the little time I've been dealing with this. I admit that I am very new to this kind of equipment and do understand that I've already made a fair share of mistakes here.

Going to get some quotes but if it seems like an amp I just wont ever be able to trust not to cause damage, it will be sold as broken / for parts or repair only.
You have a difficult problem as there is a grave issue here. Tantalum caps are known for this when not used properly. They have low internal resistance and small form factor. However if their rated voltage is exceeded by a small amount they explode spectacularly and have set fires. If voltage is not exceeded they have long life.

So something likely occurred to cause an over voltage.

So the best solution to make the amp even safe to use in your home is to replace all those tantalum capacitors with solid aluminum ones. There may be a problem fitting them in. This is a must for safety.

Now a big problem is that I can not find a schematic or service manual. I have got close and run into no longer available when other B &K manuals are available. It is certainly possible they have been pulled for safety reasons.

One thing has been clear to me is that you have to be an expert on these amps to work on them. This is another red flag as it is a strong hint there are stability problems with it.

I have come across people who have had problems not being able to keep the DC offset stable. This seems a particular problem with B & K amps. The service procedure seems the same across the range.

These procedures should be performed by a qualified technician.
1. Plug the amplifier into a variac.
2. Connect an 8 Ω RMS @ 20 Khz.
3. Slowly turn the variac up to operating voltage while monitoring the rails for excessive
current draw. The meters should not read more than 400 mA DC. If one or both read
more than 400 mA, turn the bias pots (P2) to minimum and retry.
4. Set bias of the amplifier by adjusting the bias pot (P2) mounted on the solder side of the
board to achieve a reading of 200 mA on each channel for models ST 140, ST 140M,
ST 202, and ST 202+. The bias for the Pro 600, EX 442 and M 200 amplifiers is 250 mA.
Setting the bias higher than stated only causes the amplifier to run hotter and burn out
faster. It does nothing for the sound quality of the amplifier.

5. Set the DC out of the amplifier by monitoring across the outputs. Adjust the DC pot (P1)
for 0 VDC ± 5 mVDC.
Thank You,
B&K Components, Ltd.

Now you still have the problem of the output devices being NLA. I did ascertain that these need to be closely matched. So if there already is trouble here or it develops later the chances of finding a closely matched pair is close to none.

Certainly this amp is not now safe to use as a five channel amp.

To me there seem far too many issues to make this a sensible and worthwhile project.
I never like touchy products and these seem it. I like amps stable as rocks, and have really zero tolerance for gear with these types of issues. Probably good they are extinct.

Basically what I'm trying to tell you is that pursuing restoration of this unit is probably not a wise use of funds.
I have a strong suspicion these units were not that good from day 1.
 
AcuDefTechGuy

AcuDefTechGuy

Audioholic Jedi
Ugh. I'm really feeling like crap right now. Thanks TLS guy. I know now I was taken advantage of and bought a time bomb masked as a diamond.

Lesson learned the hard way unfortunately.
At least your screen name isn't "B&K Guy". :D
 
Out-Of-Phase

Out-Of-Phase

Audioholic General
In the 90's, I used to own an Acurus A250 power amp and an Acurus R11 preamp. No meltdowns.
 
lovinthehd

lovinthehd

Audioholic Jedi
Sometimes that just isn't possible when shopping online. This is the model the world has done to as local shops are becoming more and more scarce. Years ago we had a awesome used audio video shop that got in all kinds of interesting stuff but the internet killed his business model and now we are all left with crossing our fingers.
Sure it is, don't buy used gear online without protections and willingness to deal with what comes with untested (by you) used gear. The consumer's need to save via used gear simply comes with risk.
 
highfigh

highfigh

Seriously, I have no life.
TLS Guy scared the heck out of me a long time ago with the Acurus 200X3 amps.

Shortly after, I sold all 4 of my Acurus amps. :eek:
I know someone who has a couple of Acurus amps and they work fins, but I have doubts about their repairability, although I have very little experience with them, other than going there to remind her how the system operates (even though I didn't install it, it was easy enough to use).
 
highfigh

highfigh

Seriously, I have no life.
For B&K service info, have U tried this contact..

Stargate Electronics
824 Conklin Street
Farmingdale, NY
631.465.0900

Steve
stargate3103@hotmail.com

Just my $0.02... ;)
And for documentation, I think ATi bought the assets and manuals- I needed one and it was in my inbox very quickly.
 
B

Brendan944

Enthusiast
User manuals or service manual? (If something like that actually exists) I can find user manuals online all day long.
 
Out-Of-Phase

Out-Of-Phase

Audioholic General
I know someone who has a couple of Acurus amps and they work fins, but I have doubts about their repairability, although I have very little experience with them, other than going there to remind her how the system operates (even though I didn't install it, it was easy enough to use).
They are very simple in design. My old A250 looked like this:

1544480987520.png


Coke can filter caps. I loved the fact that the heat sinks were placed on the outside of the enclosure instead of the inside. That thing never got hot, ever.
 
TLS Guy

TLS Guy

Seriously, I have no life.
User manuals or service manual? (If something like that actually exists) I can find user manuals online all day long.
A user manual is useless in your case. It is a service manual and circuit you need, and I can not find what I want.
 
M Code

M Code

Audioholic General
And for documentation, I think ATi bought the assets and manuals- I needed one and it was in my inbox very quickly.
Correct, ATi did buy B&K in 2011.

According to the North American federal trademark registration database, the following are currently active and registered to ATi.

Just my $0.02... ;)
 
TLS Guy

TLS Guy

Seriously, I have no life.
I know someone who has a couple of Acurus amps and they work fins, but I have doubts about their repairability, although I have very little experience with them, other than going there to remind her how the system operates (even though I didn't install it, it was easy enough to use).
The real issue is that they have zero protection against DC offset. That means that if an output power transistor blows then you get the full DC rail voltage, around 100 volts right through any woofer VC connected to that channel. That makes smoke and is very dramatic. A lot of amps did not have DC offset protection back then, in fact in the first years none did. I have suffered this problem twice. The worst was when a Crown DC 300 A took out 8 expensive drivers and smoked them in less than a second. It was brand new and only been operating about an hour. So that unit was replaced with a BGW amp that did have protection.

So yes, I really discourage using old amps that do not have DC offset protection. They really have no business being used. It is a safety hazard and no longer allowed.

It was strange as the hazard was not thought about when direct output coupled amps were introduced. The early amps like the Quad 303 have a big output capacitor that will not pass DC.

Anyhow the hazard was soon recognized and this was a common incident and dangerous. Various schemes were introduced to prevent damage.

The early fix was a thyristor clamp circuit that pretty much sacrificed the failed amp channel. It made a short circuit across the speaker terminals in the event of DC offset and blew the line fuse.

The best way of dealing with it now is with an optocoupler circuit.
 
TLS Guy

TLS Guy

Seriously, I have no life.
B

Brendan944

Enthusiast
Ati may not service B&K but I sent an email to them about getting the service manual and it was sent to me within 24 hours. Pretty damn good service!
 
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