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delcarmen 24th August 2012 06:25 PM

Help me bring back to life an old peavey rage 108 guitar amp!
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This is going to be a long one

Well my friend was about to throw this amp to the garbage when I said "give it to me, I have a technicall degree on electronics, maybe I can fix it"

So I plugged it and turn it ON without the guitar connected to it. And this high volumen whooom noise started coming out of the speaker. I turned the knobs (volumen, gain for the distorted channel, bass and middle) and made no diference to the noise; only when I turned the knob of the high pass filter the noise seemed to change slightly but rather than that its just noise. Then I plug the guitar and tried to do the same experiments and the noise was the same and it seemed like the guitar signal it is not doing anything, because when I played the guitar the noise did not change and there is no guitar sound coming out of the speaker, just the same noise.

I took out the circuit (unplug the speaker and unscrew the cicuit board) and it looked really disgusting, it was like a roach cementery. So I cleaned it with an electronic cleaning spray and the only thing I could see damaged was a 2200F 35V cap that was swollen. I took it out and did the dgital multimeter test and it seemed to be ok but I replaced it any way. And then put all the things back together and turned it ON and the noise was the same. I took out againg the circuit and started making additional tests, all the resistors are ok and the other caps seems ok.

So the FAQ begins:

1 About the power supply:

It looks ok, It has an AC 120v to 18v and 18v transformer that I could check and is fine. The AC/DC circuit seems ok for what I Could gather it rectifies to 25V DC.

2 About the TDA2040:

This is the main amplifier, so I dont really know if here is where the problem radicates. I mean, if you turn the amp ON and the only thing you can hear is noise regardless of any guitar o knob movement maybe is the amp that is blown. Acording the datashit the maximun voltage (Vs) is 20V but I am meassuring 25V between legs 3 (-VS) and 5 (+Vs). Is this the problem? Is it the power supply? How can I test this amp?

3 About the RC4558p:

This is the pre am. According to the datashit is a dual preamp, does it means that one of the outputs is used for the clean channel and the other output is used for the supersat (disorted) channel? Could it be that is here where the problems is? because if this pre amp is blown it will mess up the input of the main amplifier (TD2040)?


About the images:

- Rage 108 Circuit. Looking around the internet I found this image, it is the circuit for a rage amp but I can really tell if it is the same exact circuit but it looks really similar, my guest is like 85% exactly. Also you can see in the picture that this circuit was dated 10-Aug-92 but the circuit of the amplifier I have is dated 29-Sep-89. In the picture you can see that de voltage for the RC4558p and TD2040 is 27V and 28V and it makes me wonder about the voltage espesification in the datasheet because it spesifies 20V max for both models.

- Actual Amp. This is the picture I took of the amp itself.


I was thinking on replacing the TD2040 but I prefer to wait and see what responses I get about this, and to understand what is happening before making any changes. I would like to know how can I test the amps. I dont know if I will find the replacements in the local electronic parts stores because down here (Venezuela) is not that easy to find some electronic components. Never the less, I will try to find it. Fixing this amp is great oportunity to learn about audio amplifier circuit.

Mooly 24th August 2012 06:42 PM

You need to determine if your amp runs on a "split" or dual supply. The one in your picture is a single 28 volt rail.

So first step is measure from signal ground to pins 5 pin 3 ? (can't make a pin number out). See if it is like in your circuit or whether instead there is a negative voltage on pin 3 ?.

If single rail then the output pin should be around 14 volts DC and if a dual supply it should be at or near zero volts.

Experience says the TDA will be faulty possily caused by the faulty cap you found.

Usually the

delcarmen 24th August 2012 07:41 PM

Thank you very much Mooly for your fast answer.

I think it is a split supply because it has a transformer with two secundary coils and in each one I meassure 18V AC.

Hear is what I meassure:

Signal Ground (0v) with TDA2040 pin:

pin 3 (-Vs)= 0.7V DC
pin 4 (output) = 12,7V DC
pin 5 (+Vs) = 24,7V DC

Would you think is necesary to replace the TDA2040?

Mooly 24th August 2012 08:15 PM

I'm not convinced yet :)

And sorry, I should have asked at the start. If that photo is of your amp then it is single supply.

24 volts on pin 5 and half that on pin 4. That fits with a single rail configuration but doesn't explain why pin 3 is at 0.7volts.

Why do I suspect the TDA ? Because as a service engineer I used to change these type of chips on a daily basis in a lot of audio gear many years ago. There fragile and easily zapped by overloading or shorts etc.

johnr66 25th August 2012 02:13 PM

Short the wiper of the "high" pot to signal ground. This will prevent any of the signal from the pre stage getting to the power amp IC. Hum may change a little even if the power IC is damaged, but it should get quiet if it is okay. Also check the supply ripple (check supply caps)

delcarmen 27th August 2012 08:31 PM

Hey thak you!
1 Attachment(s)

Originally Posted by johnr66 (
Short the wiper of the "high" pot to signal ground.

What you are meaning by this is that I could make a bridge between the Pin 1 (in) and the Pin 3 (-VS, Ground) so this way the signal comming from the pre amp will go to ground. Am I right?

delcarmen 27th August 2012 08:38 PM

Greetings Mooly

Pin 3 is, for what I could gather, is grounded maybe that is why there is 0,7v (some sligthly potential difference??)

Yes the this is th epicture of my amp!

Thnaks againg

gatorgate 16th January 2013 11:22 AM

He you guys is it possible to create from my peavey 108 rage a 110v tranformer unit to a 220 unit. I live in the Netherlands where they have 220v. I bought the Rage 108 in Canada.

JMFahey 16th January 2013 10:00 PM

You need to replace the original 120Vac to 20Vac transformer with a generic one , bought at some Europesn supplier, rated 220 Vac to 20Vac, 1.5Ampere or more.
It may be rated in VA , in which you'll need a 30VA or larger one.
Be careful with 220V mains voltage, work with the amplifier unplugged (turning it off is not enough), doubly so because as seen on that schematic there is no mains power switch, the transformer is always connected to mains and there is no mains fuse.:confused::eek::mad::mad::mad:

gatorgate 18th January 2013 10:28 AM

thanks for replay
Thank god there is a main switch with a fuse (220v 5w) :):).
My qestion is: how do you measure the transformer wires:confused: I tryed to but I do not get eny proper measurements. (By the way it is a working amp) I am curious for I am trying to understand the Transformer.
And thank you for responding.:D:D

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