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Palmu85 22nd October 2008 09:10 PM

Guitar amp problem (marshall 8080 valvestate)
2 Attachment(s)

I got some problems with my marshall amp, I started to fix a reverb problem, the reverb pot made some crackling noise so I thought id change it and the sound of the boost channel was pit tame so I thought ill try to fix that too... but... Im not an expert and I messed up with the amp... now Im starting to get the hang of it but too late... meaby?javascript:smilie(':confused:')

I checked the net for an diagram and found one (two).
I bought and installed wrong resistors to places of R120, R118 and R96. It says 0ohm33 and I didnt really know what that ment, and I remembered when I was shopping that it was 33ohm0 and thought it to be a some kind of misspelling and because there was other 330ohm resistors I bought ones to these places.... After realising I changed the old ones back, and got hiss that gets louder-->growling (capacitors loading?) I know theres another thread on this, but it didnt answer my question... I just wanna know if the bigger if resistors might have caused some damage? (current going in the wrong way??) :cannotbe:

I readed the another thread and it sayed that the problem is in the power amp if turning the volume knob has no effect... and it doesnt.
I can`t get it figured out which component could have failed, and how to test all (transistors and op`s?)

I also changed all the pots, some capacitors, one is bit bigger than the original (C59, org=33uF and new =47uF...solen fast capacitor volts tested from the cap =400+v) and resistors, I inform here only the components that I know are bit wrong size, I know the other comps are correct, do these have any effect to other components?

I know people say that I started the repairing wrong and " you shouldnt change healthy components" but I just started to get in to these amps, and all that I learn comes from practice not from books.
Cant learn allmost anything from books, I allways learn things like this, first I do then Im screwd, then I get the solution somewhere... sometimes I figure it out by myself... sometimes not javascript:smilie(':D')
Big Grin
I also wanted to change some parts to better components, not just to fix the amp, but like I sayed I messed it up.

I get the hiss and growl but my guitar doesn`t play at all from the amp. Please help, I`m pretty screwd with this amp

I attached one of the amp drawings, the power amp part, but if you can`t get it its: pc0689p.pdf and the other part is the same but without p-letter. They can be easily found in the net.


rc0nn3ll 25th September 2011 09:14 PM

Hi mate, just reading your thread. Your resistors aren't would cause your current to flow in the worn way, that is diodes. They restrict the amount of voltage getting through, they dissipate it in heat... Say you have a 3.2V LED on a 24V circuit you need to lose 21.8V so you don't blow the LED. So you'd use a resistor to drop the voltage, you'd use Ohm's law to workout the size of the resistor needed to drop 21.8v normally a 2k2 (2200ohm resistor).
So that said, your resistors aren't responisble for the direction, however you said you weren't sure of the size this coukd have caused two problems.... excessive voltage drop which would mean not enough voltage getting to the device requiring it or not enough voltage drop causing too much voltage to reach the device and for a prolonged period damaging (more often than not irrepairable)

Printer2 26th September 2011 01:46 PM

Not to be too harsh, but if you can not learn from a book how can you learn in front of a monitor (after all, these are words also)? Could you have blown components in your amp? Sure thing. You changed other components but will not say which? How would anyone be able to help without knowing all the changes you made?

I am sorry to say but you are out of your league here. Best to go take the amp to a tech and start with learning the basics of electronics before you go improving amps.

Enzo 26th September 2011 09:33 PM

Well, since he posted this three years ago, one would hope it had been resolved one way or another by now.

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