[OT] Behringer A500 Power Amp Modification

[Sean Gibbins]

Hi Folks,

Apologies for the OT post but this thing powers my GS401s so I figure I can just get away with it!

I am attempting to modify my Behringer A500 power amp to work around an issue that has seen it in the workshop twice now.

Essentially I am removing the two volume controls on the front of the unit (these are tiny and one repeatedly gunks up causing the volume to drop on that side), replacing them with a resistor according to this thread here:

http://www.diyaudio.com/forums/solid-state/77959-behringer-a500-diy-project-7.html

(scroll down to linuxworks post if you are interested in the detail)

It all looks pretty straightforward and well within my somewhat limited soldering capabilities, however I could do with a little help on a couple of things.

The first is the 20K resistor. I went into Maplins today and they sold me this:



It looks nothing like the one that the author used in that it is much smaller (although granted he uses a spare 22K that happened to be laying around):

http://www.flickr.com/photos/linux-works/2650515068/lightbox/

Can anyone put me right here please? If I need something different it would be useful if you could supply detail as I know little or nothing about these things.

Also, what type of connector is this please:



They were attached to the board with hot glue stuff and I need to replace one that was damaged as it was removed (two of the cables came away).

Sean

-- 
music, film, comics, books, rants and drivel:

www.funkygibbins.me.uk

[Pete Wilson]

Sean

The value of a resistor is given by coloured bands, and the accuracy of the value is set by a different band, normally well- separated from the colours. For resistors of 10% accuracy, not much point having moe than ten per decade (such as 100, 120, 150, 180, 220, 270, 330, 470, 680, 820, 1000 [and repeat 10x higher values for next 'decade']....(I didn't bother to check this is a real sequence but the values are real even if I've left some out). Note that you only need 2 significant figures for these values. If you had 1% accuracies, there would usefully be more different values in the decade, and suddenly you need more values, and suddenly you need three significant figures.

The resistor posted in the forum has four bands in total. The gold band at one end says it's 5% accuracy, and so we can get away with two figures for the value plus a power of ten. The colour red has the value 2, and the colour orange has the value 3. With three value  bands, the first two are numbers and the third is a power of ten multiplier. So it's 22 X 10 to the power three, or 22K

Yours is a 1% resistor, so it's got more bands.

Go here to see what its value is:

http://www.hobby-hour.com/electronics/resistorcalculator.php

Find out which the tolerance band is - it'll be separated from the others. It's the brown band on the very left of your pic. Then start from the other end. I reckon the colours are red, black, black, red <space> brown.

The calculator says this is a 1% 20K ohm resistor, which is what you asked for.

(the fact that the guy who posted used 22K should not be an issue)

Dunno what the name of the connector is, tho :-(

-- P

Wikipedia

On Jul 4, 2012, at 12:23 PM, Sean Gibbins wrote:

The first is the 20K resistor. I went into Maplins today and they sold me this:

<res_400x600.jpg>

It looks nothing like the one that the author used in that it is much smaller (although granted he uses a spare 22K that happened to be laying around):

http://www.flickr.com/photos/linux-works/2650515068/lightbox/

Can anyone put me right here please? If I need something different it would be useful if you could supply detail as I know little or nothing about these things.

------------------------------------------------

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104 Forest Trail::Leander::TX::78641-9398::USA

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[Sean Gibbins]

Thanks for that Pete.

Any thoughts on what bearing the wattage of the resistor has? I note
they come in several flavours where that is concerned.

[Pete Wilson]

It's in place of a volume control, right?
0.25 watt should be fine....

-- P

On Jul 4, 2012, at 1:59 PM, Sean Gibbins wrote:

> Thanks for that Pete.
>
> Any thoughts on what bearing the wattage of the resistor has? I note they come in several flavours where that is concerned.
>

[Sean Gibbins]

On 04/07/12 21:12, Pete Wilson wrote:
> It's in place of a volume control, right?
> 0.25 watt should be fine....

Correct, thanks again Pete.

I modified the channel with the good connector plug and that is fine,
but the other one where the cables detached from the connector is broken
despite my best efforts to reconstruct it, so it looks like I will have
to find myself a replacement from somewhere.

[Sean Gibbins]

On 04/07/12 21:38, Sean Gibbins wrote:
> I modified the channel with the good connector plug and that is fine,
> but the other one where the cables detached from the connector is
> broken despite my best efforts to reconstruct it, so it looks like I
> will have to find myself a replacement from somewhere.

Replacement connector, that is!

[timhum]

Hi Sean,
Looking at the mod and how simple it is, I would be tempted to crack off the plastic around the socket and solder directly to that. It is unlikely you are wanting to undo tehy mod and even if you did, you could solder the wires rather than obtain the plug. Having said that it looks like a standard Molex 0.1" or 2.5mm header and plug. Look up Molex kk and it will lead yo in the right direction, they do both imperial and metric sizes it seems. They are available from the CPC Catalogue. www.cpc.co.uk and look up CN14232 in the search box. you will need the crimp terminals but not the crimper, a bit of luck and a small pair of pliers should do the trick.
Best of luck

[Sean Gibbins]

On 05/07/12 20:48, timhum wrote:
> Looking at the mod and how simple it is, I would be tempted to crack
> off the plastic around the socket and solder directly to that. It is
> unlikely you are wanting to undo tehy mod and even if you did, you
> could solder the wires rather than obtain the plug. Having said that
> it looks like a standard Molex 0.1" or 2.5mm header and plug. Look up
> Molex kk and it will lead yo in the right direction, they do both
> imperial and metric sizes it seems. They are available from the CPC
> Catalogue. www.cpc.co.uk and look up CN14232 in the search box. you
> will need the crimp terminals but not the crimper, a bit of luck and a
> small pair of pliers should do the trick.
> Best of luck

Cheers Tim, I may just do that, although to be honest my patience is
starting wear thin with this unit which has already been to the menders
a couple of times and I am not sure that the issue I have described is
the only one plaguing it.

Fortunately my NAD 325BEE is reasonably capable and certainly a lot more
dependable, so until something better comes along I will probably make
do with that.