AKG BX 20 problems

[Rob Flinn]

Hi folks

I`ve been looking at a friends BX20 today which has a problem with one channel. I believe this is a problem with the amp card since both channels work. The box is in good cosmetic condition.

I`ve been looking at the manual & it has a section relating to echoes in the decay. One channel has this happening. The manual states that this can be tuned out by adjusting mainly preset 2 but sometimes preset 3 needs to be tweaked as well. I can reduce the echo to a single repeat, but it won`t completely go. I`ve got this so the channels sound much more matched than before, but they`re still not identical.

Has anyone had experience with this & is able to point me to why the echoes happen in the first place ?? Jakob, I think you`ve worked on these before any ideas ??

 

[gyraf]

Oh yes, they're a nightmare..

Start out by checking that ALL your pickup-coils are alright - there's two in each end of each spring - eight total.

If a pickup is open-circuit, you can get symptoms like those - and believe me, they're not funny to replace (that is, if you can get them at all nowadays - I got mine directly from AKG after confering with them over the phone, persuading them that this WAS in fact an AKG product..).

Other than the pick-up issue, it's a good idea to try to get a grip of what the circuit actually does - it's a bit more clever and complicated than what I expected at first.. :grin:

There's a short service manual (excluding schematics) at Berklee: http://classes.berklee.edu/mpe/pdf_files/manuals_pdf/akg_bx20.pdf

I think I have full schematics somewhere if really, really needed..

Jakob E.

 

[Rob Flinn]

Jakob

Thanks for the info. Do you know roughly what resistance the pickups should read if they are good ??

Also I`ve been trying to understand the circuit. Is there some sort of delay chip in there so the signal gets sent to the spring several times to make the sound more dense ?

 

[gyraf]

Nope, but they use the springs from both ends - simultaneously.. :shock:

 

[SSLtech]

[quote author="gyraf"]...after confering with them over the phone, persuading them that this WAS in fact an AKG product...[/quote]
...which is a stunning example of why Jakob's contains:

don't let Harman run your company

:green:

Keef

 

[haima]

I'm dredging up this OLD topic as I'm having similar problems with my BX20... thought it'd be best to keep all the info in one spot.

For me, both coils at one end of one side have gone open circuit - so that side has the dreaded "mostly dry, discrete echo" sound rather than the smooth even decay it should have.

I've just emailed various AKG service email addresses - hopefully someone who knows what a BX20 is will be at the other end  :

Anyone got any other tips - perhaps I should find some microscopic litz wire and rewind them!?!

 

[dandeurloo]

I just spoke to Dan Kennedy about this.  He knows a thing or two about them, I think he has done some repair on one or two in the past.  I have a Manual if you need it.  They sure do sound great so do whatever you have to in order to get it back up and running!

I'm currently borrowing one from a friend and I don't want to let it go.

 

[haima]

Do you know roughly what resistance the pickups should read if they are good ??

The service manual says resistance between pin pairs (+/- 10%):

11 and 12, 21 and 22 = 260 ohms
13 and 14, 23 and 24 = 180 ohms
15 and 16, 25 and 26 = 260 ohms
17 and 18, 27 and 28 = 180 ohms

 

[haima]

I just spoke to Dan Kennedy about this.  He knows a thing or two about all them.  I have a Manuel if you need it.  They sure do sound great so do whatever you have to in order to get it back up and running!

I'm currently borrowing one from a friend and I don't want to let it go.

I LOVE this place!

Thanks dandeurloo!! (and hopefully Dan-deur-Kennedy  )

Yes, I'm a big fan of the BX20. It WILL be returned to it's former glory - even if I have to rewind the coil myself!

 

[haima]

Here's a picture of the guilty party - found online at http://www.villaraspafactory.org

** Just got the first email reply from AKG:  "Sorry.  We no longer have parts for this product." - Customer Service Admin

Hopefully something might come back from the service department emails...?

 

[guy_4]

Hello Haima,
Can you explain more of what is shown on the picture of the " guilty part " ?
Yesterday I opened a BX20 for the first time in my life.
It has the known problem of the single echo repeat.
I have the service manual, but mine says " around 200 ohms " for each coil...different edition than yours ?
I checked them all on the male connector, 3 coils over 8 are open :'(
I have not yet opened the cylinder with the springs inside.
Any advice or suggestion for me to go ahead safely ?
Best,
Guy

 

[haima]

Hey Guy,

That picture shows the "coils" which, AFAIK are both "driver" and "pickup" for one "spring" - there are 4 such spring ends inside the main suspended tube. In the documentation the coils shown as two oval loops - which they appear to be, based on this photo.

The orange/red coating would be the enamelled, fine winding wire - possibly "litz" wire, which is 4 strands twisted together. Either way, it's very fine wire!

The oval loop "formers" that the wire is wrapped around are probably some kind of ferrite/iron/mu-metal.

The whole thing is suspended in the magnetic gap - as shown in the photo.

It looks like the fine wires then travel up to the "gold can" which forms a connection point for the 4 driver/pickup wires.


RE: "around 200 ohms"  - both 180 and 260 ohms are "around 200" - I think they either changed the design slightly or got more specific with their measurements. Here's a link the the latest documentation i've seen from AKG:

http://www.akg.com/mediendatenbank2/psfile/datei/23/BX204055d1e1a05c1.pdf

I've only measured the open circuit resistance at the connector coming from the tube on my unit - I haven't yet opened mine up to see at what point continuity is lost.

I'm hoping it's just somewhere in the wire run to the pickup/drivers - not within them!

Failing that, perhaps the fine wires are "just" broken at some point on their way up to the "gold can" - and not somewhere deep inside the winding of the coils - that may be slightly easier to fix. I'm sure others here can correct me about how "easy" it is to fix these things.  ;D

Guy - I guess that would be my advice to you - follow the wiring back and confirm that it's damaged at the actual coil, not somewhere else in the line... hopefully there's just a break somewhere that can be repaired.

 

[haima]

So i've heard back from every AKG service contact listed on their website - they don't have any parts for the BX20 any more...

I asked if they had any info on the winding of the coils (can't blame a guy for trying can you?!) but apparently they don't.

So we're on our own!

 

[gyraf]

..measure physical size, inductance and resistance of a known-good coil - that will extract the needed parameters if you just add theory.

Changing these coils is no fun - I did it once, and it took forever to get it right. It was some 15years ago, and AKG told me that they had less than 10 replacemant coils in stock at that time, with no plans of making more.

Jakob E.

 

[haima]

Thanks Jakob!

I'm under no illusions that it'll be an easy task, but if we can figure out the winding spec (CJ style 8) ) we might just be able to nurse some sick reverb units back from the grave. Hopefully.

I personally know of 3 units that have dead coils, including my own - i'm sure there's many out there around the world.

I'm pretty busy this year, lots of studio work, moving house, moving studio, going on tour... phew! things are looking pretty crazy. But, if I, or anyone else, makes some progress let's post it here.

One question - when you got the replacement coil from AKG, do you remember if they supplied a new spring also, or was it just the small coil that goes in the magnet gap?

 

[zebra50]

I have a 'mono' unit, so am watching this with interest.

I've been too scared of damaging the good channel to go inside and see what's wrong with the bad one!

 

[gyraf]

The sparepart was only the two glued-together coils, with (miniature) flying leads.

Jakob E.

 

[haima]

Thanks Jakob, that's good to know.

I wonder where/why the fail... there's obviously some weakness - it seems like every second one I encounter has at least one dead coil.

I guess that's a question for anyone who has repaired a BX20 - was it clear where the fault lay in the coil? was there a broken flying wire? or did it seem to be an internal break?

Zebra50 - is you the "dead" channel completely dead? or is it partially working?

Haima

 

[gyraf]

the error in the coil wasn't obvious, i.e. not a broken connection to the coil, but inside it.

 

[haima]

Thanks Jakob... not the best news, but good to know!

Did you keep the broken coil??