Troubleshooting Tips - Output Failures - Mackie FR Series M-1400 Service Manual

TROUBLESHOOTING TIPS - OUTPUT FAILURES

After a catastrophic failure, it is likely that the four +/-80V supply fuses will be blown.
Replace fuses and very slowly bring up the Variac while monitoring line consumption. It
is likely that substantial line current will be pulled due to shorted output parts. At the
same time, look for DC offset at the output. Offset of greater than +/- 2.8V will trigger
the protection circuits and re-blow the four fuses. Keep the output offset below +/- 2V,
or temporarily defeat the DC-offset detector (Lift one side of D56).
Remove the main board from the unit and check for shorted output transistors. If one
bad output is found, replace all eight in the channel. When an output device shorts it
can place high current stresses on the other output parts. These output parts can fail
over time. Since long term reliability is paramount, please replace all the outputs.
All eight of the 3W 0.22Ω emitter resistors must be verified for proper value. Any off-
tolerance, or open parts, need to be replaced. An off tolerance (higher resistance)
emitter resistor will prevent it's related output transistor from "doing it's share" and will
place more stress on it's mates in the output section. Also verify the 8 base drive resistors
(2.2Ω, 1/4W, fusible) are all OK. Verify that the drivers and pre-drivers are not shorted. If
one driver is shorted replace it's mate. Do the same with the pre-drivers. Also check all
the resistors surrounding the drivers and pre-drivers.
Check the VI limiters and detectors. It is not uncommon to damage these parts when
the amplifier fails in a spectacular way! Look for shorts on Q76, Q77, Q94, Q95, Q83,
Q84, Q96, and Q97. Verify proper value on the resistors that go to the bases of these
devices. It is critical that these sections are working correctly. Shorted transistors can
cause some odd asymmetrical clipping problems. Open parts will not allow the current
limiting to operate effectively. If problems exist in these sections the amplifier might fail
into a short, or might clip prematurely when loaded to 2Ω.
All of the above trouble shooting (Not including part replacement) takes perhaps 15 to
30 minutes to do. If you take the time, and do all that is indicated above, it will allow
99% of the amplifiers to come up the first time! Trying to hurry, and skipping what is
suggested, can lead to a frustrating and time consuming repair.
Slowly bring up the supply and verify that line consumption isn't excessive and that the
output is centered (no DC offset). It might be desirable to defeat the amplifier muting
(Connect U5-12, 13, & 14 together) temporarily so the amplifiers are active even when
the supplies are still very low. If the amp stays centered, verify that it will pass a nice
clean sine wave. Remove any test jumpers (I.E. DC-fault defeat & muting defeat) and
proceed to the "Reliability Verification" section.
12 SERVICE PROCEDURES
M•1200/M•1400 SERVICE MANUAL