LED strip replacement

Hi there,

I'd love to change my GM40 color scheme. I am total noob when it comes to electronics so I came here for advice. What I plan to do is to open up the amp (following the warranty safe method posted by Bordonbert), take a look at the LED strip/strips on board, purchase similar strips apart from the color, swap them et voila!

Should I be fine or am I missing something? The last thing I want to so is to damage my amp, or worse, affect its tone.

Thanks for the advice! If I ever get it done I'll post pictures.

Rico

No, don't worry about altering tone, that won't happen, unless you are talking about the tone of your mood maybe?   I don't have access to the GM40D service data so I can only speak from the perspective of the GM36 which I do know but in most things the two are exactly the same, or the same in principle, so hopefully this should help.  This won't be the easy job you may expect it to be.

Firstly, the LEDs sit on a custom PCB strip only a few millimetres deep and the full width of the amp.  Its dimensions are actually 6mm x 345mm.  It is fed current from a miniature socket on the mainboard in the lower section of the amp.  I would be extremely surprised if you can source one of those PCBs even without different LEDs fitted.  You will most likely have to make a new one for yourself from scratch.  The Service manual for the GM36 shows this PCB as part number HU0211-LP-R03-1A but you won't get H&K to supply one as they only send spares to a recognised repair workshop.  If you are thinking about replacing the blues on the original PCB then I would say, good luck!  I have 45 years experience in the electronics industry designing and building all sorts of highly complex kit.  When I have to make up anything unique completely from scratch I steer well clear of surface mount devices like the plague unless it is absolutely essential to use that technology.  Soldering SMD components manually is a maddening experience even with a full hot air soldering station.  With a basic iron it is well nigh impossible to do well without a lot of experience of soldering.  You may have that of course.  Or you may not!

Secondly, you really had better take this in despite the atrocities we see online bodgers do with LEDs out in goonieland.  Every non-technical person imagines that LEDs are just like light bulbs.  You just stick a voltage across them and they glow don't they?  That is totally wrong!  LEDs are current controlled devices, not voltage.  If the voltage across them is increased even very slightly the current through them increases massively and out of control.  This means that trying to control them by applying a fixed voltage is not going to be anything like reliable or safe.  For the tiniest fluctuation in your voltage the current will jump around disastrously, and that is without temperature effects which make this worse.  The LEDs will eventually be damaged, perhaps even immediately.  You don't just stick a set voltage across them as you would a light bulb, you push a chosen current through them while you give them the conditions to then settle at their own voltage.

The driving circuit, though it can be simple, has to be designed to set this current correctly.  For this reason the ones fitted as standard are fed from a 22V line through a 680ohm 1Watt resistor.  This has the job of setting the current through the LEDs and limiting it to your chosen value and it is specific to the type of LED in use.  Changing the colour changes markedly the voltage that the LEDs will settle at when the correct current is pushed through them.  This is very different for different LED types.  As a rough guide pretty general types can differ by as much as 1.6V for red to 4V for blue.  Those figures are a general guide only, each LED type will be slightly different and they will all alter as current is increased.  When you have 5 of them as we do that would mean some blue types could drop as much as 20V, 5x4V, while some red types could drop as little as 8V, 5x1.6V.  The rest of the supply's 22V, (2V-14V), is dropped across the resistor so the current would vary between around 3mA-20mA.  And that wide range of current is under well controlled conditions with the resistor in place.  I would expect the original blues will sit at around 3V each for about 10-15mA current.  That 680ohm resistor will need recalculated and replaced to suit your new LEDs which means breaking down the whole amp assembly and desoldering the original from the mainboard PCB to fit a new value.

You have to ask yourself, is it really worth going to all of this difficult and perhaps expensive process just to change the colour of the LEDs in an amplifier?

Thanks for the detailed reply. Good I asked first... Your conclusion killed me

I obviously do not have any sort of solderingexperience so I am left with 3 options.
-Give up. Not too interested in this one.
-Disconnect blue LED strip, install another one on top, power it externally. Far from ideal.
-Find a way to turn blue to pink (the intended final color). Filter maybe?

Hahaha!  Turn blue to pink!    That's a good suggestion but sadly not applicable I'm afraid.  If you try to filter a well defined coloured LED you will end up with nothing but virtual black.  With the exception of white versions the wavelength of the light they produce is generally closely defined and specified and is pretty much one colour.  Effectively, there would be nothing left if you filter that colour out.  The attached picture of the wavelength curve is taken directly from a Super Bright Red LED datasheet.  I added the arrows showing the limits from Red across to Blue.  Above 700nm is into infra red which we can't see and Violet is about 400nm so below that it goes into ultra violet and is again dark to our eyes.  This LED emits red light only in a narrow band around 660nm which is what it is deliberately designed to do and this is pretty much what all LEDs will look like centred around different colour wavelengths.

I don't know if you are aware of this, I assumed you would be but just in case, the LEDs in the GM/TM amps are not acting as simple "illuminating lights" shining down into the amp interior.  They are sitting directly on top of the edge of the acrylic, square on to its thick edge, so they actually shine down into its interior.  The acrylic then acts as a light guide feeding the light everywhere inside itself and picking out any discontinuities where it can spill out and brighten.  That is why the H&K logo is illuminated in the way it is.  You can't just add another set of LEDs inside the case lighting up the whole of the interior and get the same effect in the lettering of the front panel.

It has been pointed out in a previous thread where this sort of thing was asked that "[I am] making this more complicated than it needs be."  I promise you I'm not.  The correct reliable way of controlling bare LEDs is the way I have described.  Try to put a voltage directly over them and they will either not light up correctly or they will fry.  There are commercial packages out there of LED strips which are described as "12V".  I don't think these will fit on top of the acrylic as they would need to do, (6mm wide remember).  These have the current limiting resistors built into them so they are working exactly as I said.  You cannot then just stick these onto a 22V supply, you would need to have a separate 12V supply or do a little testing and calculating to add a further resistor to them to drop the 22V down to close to 12V at their input.  That would need to be reasonably accurately worked out.

This project can be done and has been for other H&K amps but some posts I have seen online about it are real bodges.  I would honestly recommend you don't just take a "come Hell or high water" attitude to doing this, it needs doing properly or not at all.  The needs are pretty specific for the GM and TM families.  I'm not sure what else you could do other than use a different version of that thin PCB sitting on top of the acrylic front plate shining into it.  Your only real choice is to mod or replace that I'm afraid and as I pointed out that requires making a PCB and good soldering skills, or finding a source of a new one.  You may be able to use a thin strip of something like Veroboard but you need a long piece to take just two tracks off the edge.  Yes, you could power a commercial strip externally but you still need to have the LEDs sitting on the acrylic to get the full effect and I don't see them fitting as they should.


(N.B.  This is a bit pedantic but needs to be said.  When I described the H&K GM LED supply earlier I said it was 22V.  I really should have specified -22V.  It doesn't make too much difference but it must be taken into account when doing this sort of work.)