Troubleshooting amp gain.

2liveis2die

2liveis2die

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Mar 22, 2014
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Ireland
#1
I have a trace elliot ah200 Gp 12 series 6. the gain keeps changing as i play. not by much but it usually goes up and causes the clip light to come on faintly. also the head sounds a bit darker than my other ah200.

Ive recently replaced all the electrolytic caps on the preamp board and the gain pot also. (pot was broken, caps were old) this problem existed before i did any work, I was hoping the new caps would have solved the problem. But it didnt.

Can anybody offer a solution or a place to start troubleshooting? I have a schematic and service manual that has a basic circuit description of the input.
 
#3
here it is
 

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  • gp12 schematic.png
    gp12 schematic.png
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#4
That schematic is tough on old eyes. Have you looped the effects send and return jacks with a pedal cable or instrument cable? You do have a serial effects loop and those switching contacts in the return jack could well be a problem. Other than that time to start checking voltages make sure they are clean DC and stable not drifting up and down. A signal generator set to a high frequency 5KHz to 10KHz (since you say it is dark sounding) can be used to check for loss from a coupling cap (scope at input and output of each opamp).
 
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#5
I havnt tried the fx loop. It seems to drift upwards after a few minutes of playing. ill check voltages of the ics and the points marked in the schematic with refrence voltages. if all looks normal ill send it to a tech as im not really compfortable working on amps. im more into pedals and the extra volts scare me

this probably a better schematic.
 

Attachments

  • trace-elliot-GP12-serie6-200w schematic.pdf
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#6
Try the effects loop first. Unused micro-corrosion (fretting) builds up in the switch contacts inside the effects return jack. This is common to any unit with a serial effects loop and the list of possible troubles it can cause are very very long (including fade in/out, squeals, loss of frequency response, noise.....).
 
#7
Ill try the effects jacks. There never used and the guy i got the amp from didn't use them either so it could be a likely cause. I cant believe i doesn't think of doing that. I must have read that 100s of times on here since i joined.
 
#10
Well its not the fx loop. I bridged it and played for a while. Same problem. What i did notice is that if i push the gain up slightly it seems to brighten up the sound. Makes it sound like my main head. But the clipping light will come in even though it dosnt sound distorted. Maybe its just a problem with the gain indication
 
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