Mesa Boogie Mark I Handbook

MARK I

Maintenance and repair information.

1

Malfunctions can be cured easily when a proper diagnosis of the fault is made. Usually
only one part (most often a tube) is all that has gone bad, and the Boogie has been
designed and constructed to make any maintenance simple and straight forward.
Following is an outline covering just about any possible problem which might occur with
suggestions on how to proceed in determining what is wrong and curing it.

AMP IS DEAD

Check fuse, A.C. cord and wall socket -Check On-Off switch, power transformer.

TUBES & PILOT LIGHT UP BUT NO SOUND

• Set all front panel controls at 5.

• Make sure speaker is plugged into correct jack.

• Check speaker wire connections.

• Plug external speaker into amplifier.

• Make sure all tubes are in sockets and that none are broken.

• Broken tube will appear white and cloudy inside.

AMP WORKS BUT TONE IS WEAK, MAY HUM OR BLOW FUSES

Set MASTER to 10, Volume 2 to 3-4, plug instrument into #2

• Check 6L6's: Dead tube will only be luke warm, properly running they will be too hot to
handle after a minute. NOTE: only outside (end) 6L6's run when 100/60 switch is in 60
watt position. Shorted 6L6 glows red hot. NOTE: when 100/60 switch is in 100 watt
position (all 4 on) one shorted tube will cause the one next to it to also become red hot:
turn switch to 60 watt position and see if the end tube remains red - if so, it is the bad one.

If both cool down then probably (but not always) the inside 6L6 is at fault. A shorted 6L6
will often correct itself and un-short when momentarily interrupted by the STANDBY
switch or even a pulse from the 100/60 switch, but should be replaced by a reliable tube.
Whenever replacing 6L6 tubes, allow a few minutes of warm-up operation, then with the
power fully on (but not volume controls) tap each tube 5-6 times while watching for
shorting, arcing, or noise. Remember: 10% to 25% or more of all brand new tubes aren't
good enough. Weed out the bad ones before your gig. Recommended MESA 6L6 (STR

427).

• Check the pre-amp tubes. The best way is to replace them one at a time with a tube you
know is good. Tapping on the pre-amp tubes will often reveal the one causing noise or
detiroration of tone.

Almost all are one type: 12AX7. Those numbered 7025 or 6681 are selected but they are
still 12AX7. Recommend MESA 12AX7. 12AT7 is different and is used in the REVERB
Driver circuit. It belongs in the left hand most socket (viewed from rear) in the front row;
that is the one nearest the ON-OFF switch.
A bad pre-amp tube can also be the cause of scratchiness in the amp's volume controls.

• Check Fetron. It is the metal can type tube device located in the right hand most socket ­the one nearest the INPUT jacks. To check, merely replace it with a type 12AX7.
Occasionally a Fetron will deteriorate, drift or break down altogether, overall though, the
Fetron's performance will be hotter (amplify more) than the 12AX7 and much more
dependable, particularly where noise, vibration or microphonics are concerned.

2