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Woody's
Fender M80 Combo Bass Guitar Amplifier page

QUICK HISTORY OF THE FENDER M80 SERIES BASS AMPLIFIER
Manufactured between 1991 and 1994, the US-made Fender M-80 160-watt solid-state bass guitar
combo amplifier unit was great for smaller sized venues such as churches, small clubs and/or practice facilities.
The amplifier chassis slides and screws into the space in the upper bout above the speaker.
The cabinet is built to house a single 15-inch bass woofer.

TWO CONSIDERATIONS ABOUT BASS AMPLIFIER SPEAKERS:
PRIMARY AND UPPER HARMONICS

Because I play five string basses (the low-B string rings out at about 31 Hz) and 
because the upper harmonics of the bass note fundamentals could run up to around 5000 Hz,
I desired to update the default Fender speaker to better cover my desired spectrum.

A great tutorial on bass guitar frequency information can be found at:
studybass.com



A NEW SPEAKER
In 2016 I upgraded the original Fender-supplied speaker with a new one. 
After spending most of a day shopping on line, reading reviews and comparing specifications
I determined to upgrade the original Fender-provided speaker with a new
Made-In-Spain BEYMA Model 15P80Fe loudspeaker.

The original Fender speaker 
weighed a total of 12-pounds with a posted frequency range of 47 to 3000 Hz. 

The Beyma 15P80Fe replacement speaker 
that I had chosen weighed 27-pounds
and has a posted frequency range of 30 to 5000 Hz.
Due to the additional weight of this enhanced speaker, 
I added handles on the sides of the amplifier cabinet to 
better (and more safely) maneuver it.

This speaker is so good I may eventually upgrade the 15-woofers in my 
Ampeg Full-Stack systems using this same speaker.
Click on a picture below to enlarge for closer viewing

  ORIGINAL SPEAKER DETAIL  
   
     
  REPLACEMENT SPEAKER INFO  

Click on a picture below to enlarge for closer viewing
Speaker replacement was easy as I just laid the cabinet over on its back,
removed the screws with a power screwdriver and lifted out the original.
Inside the cabinet is, as would be expected, unfinished and unremarkable.

TROUBLE SHOOTING AND RESOLUTION

SQUEALING. CRACKLING. REDUCED OUTPUT

During a 2016 band practice the amplifier started experiencing loud squealing and crackling noises 
as well as a somewhat reduced audio output. 
It was terribly annoying to the point that practice was ended.

At home on my test bench I replaced the two electrolytic filter capacitors in the Power Supply (PS).
As filter caps age they can introduce a hum into the output of the amplifier.
While this was not the source of the piercing crackling noises, it did quiet down the background "hum" from the PS.

The originally installed caps had the expected short lead lengths soldered to the board as per normal installation.
The two caps had a glob of silicon glue between them to reduce vibration that they would experience during high volume bass guitar work.
When I replaced them I chose to leave the leads longer and rubber band them together.
The theory here might be that the separation up and away from the circuit board along with rubber banding may provide some vibrative isolation.
Time will tell on that theory.
I do see other components that also had a padding of factory added silicon glue to soften their vibration.

Since, in the case of this Combo Amp, the boards are literally sitting atop the 15-inch bass speaker,
at higher volumes vibration would have to be hard on circuit board components.

During the troubleshooting effort it was determined that several solder connections on the system board were defective.
Using a low-wattage soldering pencil and touching all the solder connections 
on the left half of the PS board fixed the squealing and crackling problems.
Click on a picture below to enlarge for closer viewing

BOARD REMOVAL
Above: Note the five screws holding the heat sink against the chassis.
Right: The are two Phillips screws holding the board to the chassis.
POWER SUPPLY CAP REPLACE
Above: The original Fender-installed PS filter caps with silicon glue providing some padding from vibration.  Above: Resistor with padding silicon glue
Below: All PS connections left of the pencil we touched with a small soldering pencil which cured the squealing and crackling problems
Below: When the two PS filter caps were replaced I left the leads longer to provide for some insulation from vibration when playing bass at louder volumes.
Below:  Original Fender-installed caps Below:  Caps ordered from DigiKey.com  (3-days) Below: Caps ordered off eBay arriving from China (3-weeks)
CURING THE SQUEALING & CRACKLES
Below:  The two caps were rubber banded together to provide some vibration protection during practices or performances. Above:  All solder connections to the left of the pencil were touched using a low-wattage soldering pencil to cure the crackling and squealing problems. Below:  There are four screws that hold the amplifier section into the Combo cabinet. 

Click on a picture below to enlarge for closer viewing

FENDER M-80 160-WATT BASS GUITAR AMPLIFIER
SCHEMATIC DIAGRAM

Click on a picture below to enlarge for closer viewing


ADDITIONAL INFORMATION




  Click on a picture above to enlarge for closer viewing

LO-386609  1993  eB    350

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