View Full Version : Just got a DBX BX-3 MKII and CX-3 and Question


Whaleman
08-12-2009, 11:48 PM
Just got this set home and cleaned up quite nice. I set the amp BIAS and DC offset.

Do have one issue, one of the 4 amp sections is tripping the protection when increasing the volume at anything over about 4 watts, turn the volume down and it comes out of protection mode. The other 3 sections are fine.

Only clue I have is that the amp section in question had very low bias current that I dialed up to spec. It is the left rear. I want to use it in bridge mode to obtain 400 watt per channel. So I do want this section to work.

Anyone have any info or experience on this amp problem? Thanks. Any other info or opinions on this setup are welcomed.

I was looking for watts, but maybe I stumbled on to something good.

http://www.audiokarma.org/forums/attachment.php?attachmentid=164361&stc=1&d=1250135808
http://www.audiokarma.org/forums/attachment.php?attachmentid=164357&stc=1&d=1250135808
http://www.audiokarma.org/forums/attachment.php?attachmentid=164360&d=1250134894
http://www.audiokarma.org/forums/attachment.php?attachmentid=164358&stc=1&d=1250135808
http://www.audiokarma.org/forums/attachment.php?attachmentid=164359&stc=1&d=1250135808

ManyMoonsAudio
08-13-2009, 12:21 AM
Wow what a monster! What is your DC-Offset for the problem channel under load? Measure it also while playing it under load and see what it gets to before it trips.

Whaleman
08-13-2009, 08:18 AM
Wow what a monster! What is your DC-Offset for the problem channel under load? Measure it also while playing it under load and see what it gets to before it trips.

Good idea Paul, it dialed to 0 mV pretty easy, so I am starting at a good point. I am suspecting one of the ouput transistors is flaky since it plays fine at low volume.

I will report back.

Man, can wait to feel the horsepower in this thing. :smoke:

Jon_Logan
08-13-2009, 08:48 AM
Check to see if the problem channel causes the protection relay to open when the volume is raised but the speaker/load is disconnected.

If the relay does not open under this condition, it is likely a current related problem.
bad/shorted load
open transistor, open emmiter resistor
open cap in the over current protection circuit.

One note on these... they eat power switches. It's best to turn them on with a remohte master switch.

jhoyt
08-13-2009, 09:25 AM
Awesome amp! I truly miss mine...

Mine was flaky also, one channel would not play by itself. However, in bridged 2-ch. mode it worked fine. I can't help you on specifics, but just know that this is IMHO one of the great underrated amps out there.

However, I never liked the CX-3 preamp very much. It just didn't sound very good. There are several gain stages, and if I remember correctly, each stage was op-amp driven. Replacing the original crappy op-amps with something modern would make a huge difference.

Whaleman
08-13-2009, 10:24 PM
Check to see if the problem channel causes the protection relay to open when the volume is raised but the speaker/load is disconnected.


If the relay does not open under this condition, it is likely a current related problem.
bad/shorted load
open transistor, open emmiter resistor
open cap in the over current protection circuit



One note on these... they eat power switches. It's best to turn them on with a remohte master switch.

Thanks for the advise, here are some findings:

1/2. The protection kicks in with no speaker attached
3. Could not find any open transistor and the emmiter resistors are not open
4. The over current protection circuit acts normal with the other channels not matter how loud.

5. Nothing seems amiss on the right half of the amp board pictured which is the one in question. All electrolytics test fine.

So, I am leaning toward a misbehaving output transistor. They all test fine, but the channel seems to work fine to a certain point in the power (volume) and then takes a dump.

http://www.audiokarma.org/forums/attachment.php?attachmentid=164478&stc=1&d=1250217540
http://www.audiokarma.org/forums/attachment.php?attachmentid=164477&stc=1&d=1250217540

gary7
08-14-2009, 11:43 PM
I had the same set up a year or so ago. Both were MK II models. The CX-3 would occasionally drop the right channel. It is a killer amp. Hope yours works out.

Whaleman
08-15-2009, 08:46 AM
I had the same set up a year or so ago. Both were MK II models. The CX-3 would occasionally drop the right channel. It is a killer amp. Hope yours works out.

I am looking forward to finding out. 400 watts per channel, 600 watts dynamic output sounds like a manly serving to me.

After wading through a bunch of fake Toshiba vintage power transistorson on the net, I decided to buy the current Toshiba replacement set from www.mouser.com. I should have them in by middle of next week and hope this pup is back on its feet. It is one well laid out amplifier, easy to work on.

Any other comments on what I can expect performance-wise welcomed.

koseltri
10-15-2009, 10:13 PM
Update...?

Whaleman
10-15-2009, 10:30 PM
Update...?

Sorry about the delay, I had to work some issues out with the amp. Once sorted, this is one killer amp. 400 watts RMS/600 dynamic per channel, will satisfy at any volume. It's quite impressive really and beyond my vocabulary to describe what it does with my Bose 901 when we are hitting 200-watt peak bursts. The cooling fan came on a low speed for a little while, but no sweat. It has more detail than my hi end Denon. A total sleeper and scarce amp, especially in the MKII form. I keep wondering why not many folks have discovered this beast. When I want audition some new CD, this where I do it.

koseltri
10-19-2009, 11:43 AM
Sorry about the delay, I had to work some issues out with the amp. Once sorted, this is one killer amp. 400 watts RMS/600 dynamic per channel, will satisfy at any volume. It's quite impressive really and beyond my vocabulary to describe what it does with my Bose 901 when we are hitting 200-watt peak bursts. The cooling fan came on a low speed for a little while, but no sweat. It has more detail than my hi end Denon. A total sleeper and scarce amp, especially in the MKII form. I keep wondering why not many folks have discovered this beast. When I want audition some new CD, this where I do it.

I just let a Mk. II slip thru my fingers, went for less than $300. :mad::rant:

Whaleman
02-27-2010, 03:23 PM
More on my DBX. This has been a work in process as the other night without warning the unit went into protection for a bit. Then after a meter bulb LED conversion, the power/protection LED went completely out but the unit kept operating. After a while, a R519 a 330 ohm resistor on the protection circuit board smoked and stunk up the house.

Many hours of head scratching, poking, and replacing every part imaginable that could cook this resistor, and thinking about dumping it on eBay for parts, turns out the power/protection LED supply leads where shorted to the case behind the faceplate. These have long 1" bare leads that I trimmed and shielded with tape.

This plus replacing resistor R519 and the two 4.7/50V electrolytic caps in harms way fixed the unit like new. Now it is very stable. Apparently, this LED has been shorting to the case ever since I had the unit, this explains its odd behavior and lots of unnecessary parts that went in.

Hope this help someone with one of these out there. The protection circuit is on the right board as you face the back plate. Take all the screws off the backplate and it pulls away from these boards.

Damage:
http://www.audiokarma.org/forums/attachment.php?attachmentid=197979&stc=1&d=1267302937
http://www.audiokarma.org/forums/attachment.php?attachmentid=197980&stc=1&d=1267302937

Now fixed:
http://www.audiokarma.org/forums/attachment.php?attachmentid=197981&stc=1&d=1267302937
http://www.audiokarma.org/forums/attachment.php?attachmentid=197982&stc=1&d=1267302937
http://www.audiokarma.org/forums/attachment.php?attachmentid=197983&stc=1&d=1267302937
http://www.audiokarma.org/forums/attachment.php?attachmentid=197984&stc=1&d=1267302937

Whaleman
02-27-2010, 03:30 PM
http://www.audiokarma.org/forums/attachment.php?attachmentid=197988&stc=1&d=1267302617

brutal
02-27-2010, 03:38 PM
Nice work. Nice looking amp.

They must source their binding post setup from the same place Yamaha did on some of their amps (it's a crap design IMHO).

EDenton925
01-14-2011, 01:57 PM
Got my DBX BX-3MKII from a great guy:thmbsp: who runs his stereo business out of a flea market. He sold it to me for...$150.00!!!:deal::

God Bless :music:

Whaleman
01-14-2011, 02:15 PM
Welcome EDenton, great to hear on your DBX score. You will enjoy its power.

Unfortunately my DBX BX3 MKII sits on a basement on a shelf. I have not been able to find any information on how to properly set the BIAS (only for the non-MKII) and it does not seem to like 3 mV.

It plays for some time and then goes into protection. Hope this problem stays away from your unit.

Congrats!

Just Lucky
01-14-2011, 04:12 PM
Have you tried this French site?

I downloaded an English manual from them and they seem to have a service manual as well.

http://vintagedbx.free.fr/dbx.html

baco99
01-31-2013, 12:20 PM
@ Whaleman

Would you be able to repost the photos of your DBX amp? I just picked one up from Todd and he said it has an intermittent display light. Wondering if it's the same shorting issue you describe. Before I plug it in, I want to take a look, but need to know what to look for.

Thanks!!!

michigannick
09-04-2014, 07:24 AM
bump

and ;-) unlikely that you remember but do you remember the toshiba transistor sets you used to replace the outputs