Members denimboy Posted January 15, 2009 Members Share Posted January 15, 2009 I got this 1986 Fender The Twin (red knob) which has been really noisy lately. I get this issshh coming from the amp as soon as I switch the Bypass off. There is only a Fender Telecaster plugged into it. I get the noise whatever input I use and at any volume level. Impedance match is well set and speakers seem fine. The circuit is very clean and no visible breaks. I'm thinking it might be a leaky capacitor. I would start by replacing the filter caps in power supply. Any suggestions? Here is the schematic diagram:http://www.schematicheaven.com/fenderamps/the_twin.pdf I can record the amp for the need. Can take guts pictures too. Your help is appreciated. I'm into second year of audio electronics repairing school. I can do it myself with some help. Link to comment Share on other sites More sharing options...
Members denimboy Posted January 15, 2009 Author Members Share Posted January 15, 2009 bump! Link to comment Share on other sites More sharing options...
Members knucklefux Posted January 15, 2009 Members Share Posted January 15, 2009 have you tried new tubes? is the hiss volume dependent? have you thought about taking it to school and fixing it for a grade? Link to comment Share on other sites More sharing options...
Members denimboy Posted January 15, 2009 Author Members Share Posted January 15, 2009 There are lots of tubes on this amp. Not sure where to start. Honestly I'm more used to solid state repairing. I wish I could bring it to school but it weights a ton and I don't have a car hehe. The hiss is not volume dependent and it is constant. Link to comment Share on other sites More sharing options...
Members knucklefux Posted January 15, 2009 Members Share Posted January 15, 2009 i know there are lots of tubes. power tubes need to be replaced pretty frequently, typically every year or two depending on use. you should replace the phase inverter when you replace the power tubes as well. tubes behave a lot like transistors in terms of function, just at typically much higher voltages. the fact that the hiss is not volume dependent means that it's source is after the master volume control. so find that, and then replace all the tubes between the MV and the output transformer. make sure you bias the power tubes properly as well. you could also scope the B+ to see if you do, in fact, have a failing filter cap. of course, unless you have an O scope at home, you're stuck with taking it to school. basically, until you actually take the time to troubleshoot it, you're just shot gunning. that's ok to do with your money, not so cool if it's a customer's. Link to comment Share on other sites More sharing options...
Members denimboy Posted January 15, 2009 Author Members Share Posted January 15, 2009 No worries it ain't a customer's. Got all the time I need. Thank you for the useful infos. I actually do not own a oscilloscope. Therefore I will go with replacing tubes after MV and see. Didn't know the power tubes could cause hiss. There is a 12AX7 tube not lit when turned on. I check for the part number and tell you later. Thanks. Link to comment Share on other sites More sharing options...
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