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I have a Roland KC 500 Keyboard amp. The tweeter is making a piercing sizzling sound, which is strongest at a certain frequency but still exists to a lesser degree in the surrounding frequencies. A blog I found suggested replacing the 5 watt 15 ohm resistor on the main circuit board (прим.- в фильтре пищалки) with a 5 watt 18 ohm resistor...
> Hi I can help you with this. Replacing that resistor may indeed help, but it may be a capacitor that is the source of the problem. The filter caps will reduce distortion and if you have one that's failing it may be the fault. Please look at all the caps in the circuit that drive the tweeter for bulging lids. You can also add a cap to the tweeter itself or even make a crossover network to help reduce this unwanted noise...
I will work with you on this but it may take a while, and since this question may continue for some time, if you will rate my service as good or better before you close this window, the system here will allow me to schedule free follow ups with you, so your question won't close or time out and you can contact me anytime at no additional cost.
I've got both a 5w 15ohm cement resistor (the original spec) and a 5 watt 18 ohm cement resistor coming in on Thursday. I'll hold off on replacing them until I've inspected the caps. Thanks again - I'll be sure to rate you highly...
I'm on the unlimited questions free trial now.
Getting back to the amp, I opened it up last night. None of the capacitors on the "main amp board assembly" are bulging or leaking. The only thing out of the ordinary that I saw is that there is a browned area (as if from heat) on the circuit board underneath the transistors.
I put my multimeter on the resistor in the circuit board now and it registered 15 ohms.
> This problem could be just about any cap in the system, but mainly the ones that are filtering the output as they are removing distortion. The reason for my focus is usually it is the caps that aren't filtering out unwanted harmonics, but the resistors play a key role as well.
You could try changing the values but its likely a cap situation that will fix this
... I may have sorted the issue of how to get you a pdf of the schematic. I uploaded it on Scribd
> Hey, I got it!
I would use a decade capacitance box to try different caps easily on across the speaker leads, to see which value might minimize the unwanted harmonics without losing what is desirable.
Are you thinking that I could install a new capacitor on the line to the tweeter, once I find the correct capacitance? I've been looking for a cheap solution for a decade capacitance box...
> It s always good to have this equipment so you can fix your equipment. It will make all the difference! You can scan through all the values while running the unit with a song that spans the spectrum of sound.
May I make a suggestion? Queensryche - "Silent lucidity". Play that while testing.
Do you anticipate that I may need a decade resistance box as well? If so, please let me know so I can order one.
> I started looking at the schematic and what I would try the decade cap box on the tweeter leads first to see if different cap values help. Resistors will help as well but they remove a lot of wanted sound.
I tried that fix that I found on the internet which was replacing the 15 ohm 5 watt (R241 on the schematic) resistor on the tweeter positive feed with a 18 ohm 5 watt resistor - it did not help... The decade capacitor box just arrived today.
> What you do is remove the original cap from the circuit and clip the leads to the connecting points... Then you can scroll through different values (START WITH HIGH VALUES!) and play the unit so you can hear if the distortion gets better at different values.
I played with this yesterday. With the box set on 9999nF it had practically no effect. As I dropped the nF, it began to filter out the highs and the piercing, sizzling tone went away, but so did the highs.
Is this box basically an adjustable notch filter?
> Pretty much it is, it just selects different values and if yours has a resistor part as well as caps, you can get close to what you need. Then you will still need to play with res/cap values using off the shelf parts...
In my opinion there are still failed caps in your system that will need replacing.
Basically the distortion was completely filtered out at around 99nF, but as I said so was most of the tweeter.
I ordered a decade box for resistance but don't have it yet...
> When you have the resistance box as well we can do more testing as well. You really will need that piece of test equipment to complete this correctly.
и т.д., и т.п.
PS: Схема кроссовера, о котором шла речь в переписке:
![Изображение](https://s13.postimg.org/6uqew9t3b/KC-500_xover.png)