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December 01, 2021, 11:33:17 PM

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Author Topic: Fender M80 Issues  (Read 2033 times)

Angelopz

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Fender M80 Issues
« on: September 24, 2021, 03:50:11 PM »
I have my original M80 combo amp with the speaker. I haven’t used it for some time and was fully operational when I stored it. I just got it out and hooked it up and found that the 2 inputs don’t work. Also there is no light on the overdrive and when I plug in the pedal no light on that. The power light goes on and if I plug into the power amp in with my guitar, I hear my guitar. Looks like I’m not getting power to the inputs or something. Anybody out with some ideas? Would really appreciate it

Enzo

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Re: Fender M80 Issues
« Reply #1 on: September 24, 2021, 08:41:41 PM »
Start with the low voltage supplies, +15v and -15v.

Angelopz

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Re: Fender M80 Issues
« Reply #2 on: September 25, 2021, 08:22:22 AM »
Ok I will check the transformer in the back. There are 3 wires on the low voltage. One is the center tap. Voltages according to schematic are 40v and 16v. I hope this is what you are talking about. I know you said 15v but I don’t see that on the print.

Enzo

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Re: Fender M80 Issues
« Reply #3 on: September 25, 2021, 09:17:15 AM »
Transformer is likely just fine, it makes AC.  I was concerned with the DC supplies.

15v 16v whatever.  It is the low voltage supply for the op amps.  SOme amps use 12v, I see 17v a lot on some mixers.   Your problem is that the preamp is just not working at all, and the lights don't even work.  This is not going to be because voltage is off by one or two.  I'd be concerned one or both supplies are MISSING.

Iy is just a place to start.  NO circuit works without good power supply.

Loudthud

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Re: Fender M80 Issues
« Reply #4 on: September 25, 2021, 10:06:31 AM »
Back in the days of fixing vacuum tube TV sets, the diagnosis was No Picture, No Sound.

Jazz P Bass

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Re: Fender M80 Issues
« Reply #5 on: September 27, 2021, 12:20:51 AM »
More likely than not there was a zener diode failure.
Cr 25 or CR26 on this drawing.