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Post subject: M-80 Bass Amp w/chorus No Power
Posted: Tue Sep 25, 2012 1:09 pm
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My 1990 M-80 bass combo with chorus pooped out completely. Zero power! It may have happened during transit when the amp bumped the curb. I popped it open and everything looked good. I traced the ac lines from a healthy looking power switch to the little round thingy on the heat sync. All lines and the one glass fuse appeared fine. I even sniffed around for burned out components. I found nothing. What could it be? I used the amp the night before and she was perfect. Anybody familiar with these symptoms?


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Post subject: Re: M-80 Bass Amp w/chorus No Power
Posted: Tue Sep 25, 2012 6:13 pm
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Could be a number of things. Solid state rectifier circuit, bad solder joint somewhere. Bad switch. It isn't blowing the fuse which is about the only good thing here. I'd check that fuse with a multi-meter to make sure it is good. Just because it looks good doesn't mean it is good.


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Post subject: Re: M-80 Bass Amp w/chorus No Power
Posted: Tue Sep 25, 2012 7:13 pm
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Thanks brotherdave!


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Post subject: Re: M-80 Bass Amp w/chorus No Power
Posted: Tue Sep 25, 2012 9:23 pm
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If it isn't the fuse, get it to someone who has the right tools to fix it right. It might help to have the schematic. It is archived by Fender and they will email it to you. Also there were some mods recommended in the power supply diodes by Fender in a tech bulletin back in the early 90's. The diodes were causing heat to build up and then the amp would go into thermal shutdown until it cooled when it would restart again. This was on the models with zener diodes.

Here's the link: http://www.fender.com/support/articles/fender-guitar-and-bass-amplifiers-owners-manuals-and-schematics-hard-copy-archives


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Post subject: Re: M-80 Bass Amp w/chorus No Power
Posted: Wed Sep 26, 2012 5:21 am
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Wow thank you again! I do have a fair collection of tools in my arsenal. I modify Mic pre amps and build ribbon microphones when possible. I was hoping someone would have had the same experience as I and say ahh yes that's the xy&z. Truthfully I was hoping not to have to break out the millimeter.


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Post subject: Re: M-80 Bass Amp w/chorus No Power
Posted: Wed Sep 26, 2012 4:38 pm
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I'd start with the fuse, then go to the power supply. If it isn't doing anything at all it is power supply related.


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Post subject: Re: M-80 Bass Amp w/chorus No Power
Posted: Wed Sep 26, 2012 8:18 pm
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I've been thinking about modifying it. I wonder if there is a kit. Wonder if it could be converted into a tube amp?????? Hmmm. Do you have any ideas?


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Post subject: Re: M-80 Bass Amp w/chorus No Power
Posted: Wed Sep 26, 2012 11:52 pm
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I'd absolutely fix it and do the diode change recommended in the service bulletin if required on the model you have. I would not do anything else to it. If you want a tube amp you are probably better served to buy a working tube amp instead of Frankensteining one.

Fix this one, sell it and then use that money toward a tube amp. There are pretty respectable solid state amps and I enjoyed the one I had. I just had too much stuff.


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