Crate / GX-40C+

[SpareRibs]

Hello,
      Does anyone have a readable schematic of a Crate GX40-C amp, with the preamp laid out. I have one that Loud let me download but it is not clear enough to read the values of anything. I know it is an amp of
the previous owners, and that is probably all they have in their data base.

[DrGonz78]

Well not sure if you have a schematic of the GX40-CDSP or this older one. Maybe post the one you have so we don't post it again. I did see the DSP version online (at least the DSP page) was impossible to read. This one is not great but it does layout the preamp pretty good.

[SpareRibs]

Hello,
      Sorry you went to the trouble to post that schematic, but I don't think its the one. The one I have is on a now defunct computer. I cannot post it, however I can tell you it is the GX-40DSP / It was drawn on
11/04/92. Also on the bottom it says, page 2 of 2. I was trying to get a better view of the clipper circuit.
      Thank you. I guess I will leave that one stock, at least until I learn more about electronics. If you saw the thread about the Fender Squier 15, it really opened my eyes to how much better an amp could sound with so few changes. I thought the Crate could be improved on slightly with changes to the clipper circuit as well. Without the schematic for the preamp stage I am hesitant to go in changing things willy nilly.
       I guess you can see the beating Crate amps take on some of the other forums. I have a hard time something that sold so well could be considered so sorry by so many people. Anyway thanks again.
                                                                   
                                                                     
         

[Roly]

In this one the back-to-back clipper consists of a couple of LED's, D4 and D5.

As DrGonz has already said, the forward voltage of LED's differs with the LED colour, so if these LED's are the same colour you can get a similar effect to adding a diode simply by changing one of them for a LED of a different colour.  (n.b. they may already be different by design)

The "scientific" way of doing this would be to pull the two LED's and hook them up in turn to a 9V battery via a current limiting resistor, say around 1k (there should be a little flat on the skirt to indicate the negative end), and measure the voltage drop across each.  Now find a LED with a different forward drop, higher or lower, and substitute that for one of the originals.  In this case it doesn't matter which is which, just so long as they are opposite, flats in opposite directions.

Generically you can identify these clipper diodes by the fact that they will be together, and will be connected in opposite directions across the same two tracks.

[SpareRibs]

Hello Roly,
       I can maybe make out, 4 and 5. I am going to check it out. Thank you !!!!

[SpareRibs]

Hello Roly,
       OK, The GX40-C has no LED's. But that's all right I will pull it apart and look at it. At that time I may be able to see a pair of diodes close to the volume control.
        I tried to call Loud today but they put me on hold. I will try again tomorrow. I will ask them if they can do something about the resolution of the schematic. I will also inquire about getting both pages. I am sure
that they may not be aware they are only sending one page.

[DrGonz78]

Yeah it's completely horrid to try to read some of these low resolution scans. I saw the one page (page 2) of that schematic and I am sure it this same one here at elektrotanya. It almost seems like maybe they don't have a good one since the cut off from SLM to LOUD tech.  :grr

http://elektrotanya.com/crate_gx-40cdsp_guitar-amp_sch.pdf/download.html

[Roly]

I wouldn't expect LED's used as clippers to be on the front panel since they wouldn't be doing anything particularly spectacular, just giving a tiny blink on signal peaks.

When I say "close" I mean electrically, not physically.  If you look at the G-40C circuit for example you will see that they are just upstream in the signal flow from the "Shape" and "Level" (volume) controls.

[teemuk]

The basic operating principle of the "Flexwave" clipping circuit is similar in just about all GX-series amps:



Zener diodes create asymmetric clipping at specific voltage thresholds. A halfwave rectifier / filter circuit gradually modifies DC offset of the circuit, resulting into dynamic variation in clipping symmetry.

Crate amps usually have two of such circuits in series. First one (or was it the second) usually implements a gain control pot. The zeners may be ordinary zeners or zener equivalents built out of transistors and resistor networks. Google search just about any Crate schematic since the invention of the patented "Flexwave" circuit and the basic arrangement is almost the same in nearly all of them.

The GX-40CDSP schematic at Elektrotanya seems to be partial and only shows page 2 of the entire two pages.

[SpareRibs]

Hello Guy's,
      Dr Gonz 78. I copied the 2-G40 schematics you put in your post. I think it will help as the should be at
least similar. I have the one you told me about online.
    Roly,
      I am just going to have to take it apart. Only way to go.
    teemuk,
      Thanks for your drawing, that may help me to know what I am looking for when I get inside.

    I tried to call Loud Technologies yesterday and stayed on hold for a while. I just gave up on the phone. I found an email address and emailed them. I got the reply this morning. They only have the one sheet. and
there is no way to copy it any better. So that's just the way it is. Anyway I will just jump in.
       So this will likely be me later today.   :grr   :grr   :grr   :grr   :grr

[J M Fahey]

Thanks Teemuk for posting the Flexwave patent.

Now in that CG40 schematic, they use plain old diode clipping, only they use Leds.
You may replace one of them with a green one (Marshall does that) or if you want to go wild, replace one with an 1N4002 or similar standard diode.
And if you want to go *real* wild, just pull one Led and don't replace it.

[SpareRibs]

Hello,
       I took chassis out and there are at least 25 or 30 diodes on it, none of them are opposite each other so I think I will just leave well enough alone. Without a proper schematic I would have no idea where to start.       
       Everything functions, I only paid $20.00 for it because the electrical input socket was shot, and someone had dropped it or something fell against the knobs and broke the circuit board. I just melted the solder back together, adding more as needed. Good to go. I would hate to meddle with it and ruin it being stupid.
       So I guess if Eric Clapton ever decides to come over, he will have to bring his own amp if he can't make
do with one of the 15 watt ones. However I am quite sure before that ever happens, I'll probably have it up
and running.
        Thank each and every one of you for your interest and effort, it was not for the lack of trying this did
not work. It was lack of information being available. I am going to send some emails to service departments
of a lot of major company's and inquire as to why they do not include a schematic in their owners manuals.
It probably won't make any difference but someone has to start somewhere
         Thanks again!!!!!!!                                       

[Enzo]

They do not include schematics for several reasons.

First, the VAST majority of amp owners have no use for them.  Most people never remotely think of working on their own amp.

WHile some do and some do not, any amp maker that gives you the schematic is also thereby giving you tacit approval to work on the amp,  That means you would be exposing yourself to potential lethal shock.  And that adds them squarely to the liability loop.

Even people who do want to work on their own amp usually lack the skill or ability to do so.  And in many cases, those who try wind up screwing the amp further.

And look at the present situation, the company was willing to send the schematic to you, they just could not find the file.   This amp was from 1989, like 25 years ago, and the company has gone through ownership change, including moving across the country.  Easy for some files to be misplaced.

[SpareRibs]

Hello,
      Yes you are right. I can see that is why they require anyone requesting their schematics to waive the company of responsibility. It's just a little frustrating. Also it is 25 years old. I will just keep watching the forums, maybe someone will make a repair on that part of the circuit. Thanks for your explanation.

[Enzo]

I have the paper copies in my hand, and I will have them scanned when I get the chance.  My scanner will not do 11x17 in one piece.

But you have to tell us which amp you have.  Here you have been talking about the GX40C, but over at DIY you ask about the GX40C+, which is a totally different amp.