Help me bring back to life an old Peavey Rage 108 guitar amp!

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 2200µF 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)?

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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.

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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.


 

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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
 
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?
 
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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.
 
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)
 
Yes.
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:
 
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
 
Thank god there is a main switch with a fuse (220v 5w) .
Are you sure?
The schematic shows a Power switch and a fuse, but it's on the scondary :(

Besides that, post both a closeup picture and a hand drawn schematic of how the original transformer is connected, showing wire colors.
Yes, Mooly's method is fine to measure :)
Anyway I ask picture and schematic because Rage has been produced for many years, with some obvious variations, so yours might easily *not* match the posted schematic.
 
oeps

Yes
your right:eek: wrong fuse I have'nt looked that close.
I will change it right away:D
And the date:rolleyes: I changed it also:eek:.
Here's the right date.
And thanks for the info.
Just one stupid qestion for the pro's.
Could you explain the (18+18V , 30 to 40 VA .)
I am just starting to be an electronic mech.
Greetz GatoPS: another pickture with the right date
 

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Could you explain the (18+18V , 30 to 40 VA .)
You mean, besides being what Peavey originally used ? :p :D
I don't have one here, of course, but post #3 claims:
it has a transformer with two secundary coils and in each one I meassure 18V AC.
Since you "can't measure" it :confused:, I'll have to settle for second best, which is what somebody else posted.
Your picture matches the one on post # 3 , and both show 3 secondary wires: two red ones (18 + 18V AC) and a red/yellow center tap.
Neither yours nor # 3 matches *exactly* the schematic #3, which shows a single (red red) secondary with no center tap, and rectifies with a bridge (4 diodes connected in a "diamond" pattern).
Yours used a Red/RY/R secondary and 2 rectifiers (the glass 1n4002 shown).
Same end result by different means.
And the 30 to 40 VA size comes from the circuitrequirements.
By the way, Peavey itself prints "40W" (really 40 VA) as "power consumption", so, who am I to disagree? :(
If you want to measure your transformer (recommended), set your multimeter to the 200VAC scale , touch ground (chassis) with black probe, measure with red probe touching red wires.
With the amp on, of course.
I guess you are now using a 220/110V transformer to make the amp work.
Post results.
 
correct

Yes thats correct I use a 220 to 110v to use the amp.
I want to thank you for the patience and the info that you gave me and I will go and try to get a proper transformer for my peavey rage 108.
I will keep you up to dat.
Thanks again and keep up the static (haha just joking):D:cool:
And :hohoho:
 
Hi, I know that this is a super necro-bump of this 10-year-old thread, but I'm working on a Peavey Rage 108 and I'm getting exactly the same symptoms as the OP in this thread. Unfortunately, it looks like they never returned to this thread to report if anything worked to fix it. When I turn on the amp I get a really harsh, square-wave hum (you can hear harmonics in the hum). I followed the advice in this thread and tested it by jumpering the treble control, which didn't fix the hum but now made the bass control interact with it. I ordered a new power amp chip and just finished replacing it and sadly the amp is still humming exactly the same way.

Thoughts?