Troubleshooting Tips-Output Failures - Mackie M2600 Service Manual

Troubleshooting Tips-output failures

After a catastrophic failure, it is likely that some of the supply fuses will be blown. Check and
replace any fuses on the 129, 130, 133, and 171 boards. Short across the Left most lead of R1 to
the right most lead of R2 on the 133 board to short out the two 15W inrush resistors (see the
diagram on the next page). 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. If excessive
current is not pulled, but a channel still stays in "protect" mode, the unit will still need repair.
Remove the main heatsink sub-assembly from the unit and check for shorted output
transistors. Once it has been determined which channel failed, remove it's respective V-amp/
Protection board (055-170-00) to make access to the channel board easier. If one bad output is
found, replace all 16 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 16 of the 3W 0.33Ω 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 16 base drive resistors (2.2Ω, 1/4W, fusible) are all OK. Verify that all
eight drivers and that the pre-drivers are not shorted. If one driver is shorted replace it's mates.
Do the same with the pre-drivers. Also check all the resistors surrounding the drivers and pre-
drivers. Measure the resistance between the "+I-sense" and "-I-sense" lines. The resistance should
be 82.5Ω +/-5%. If this resistance is higher, check for open 33Ω fusible resistors on these lines. Take
your time here looking to find all damaged parts: If one part is missed it still takes the same huge
amount of time to disassemble the main heatsink assembly. After replacing the outputs, drivers,
and pre-driver verify that the silicon transistor insulator wasn't damaged (No shorts between
collector legs and the heatsink).
On the 170 board, 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 Q24, Q25 or open R151 or
R146. 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 2 Ω.
All of the above trouble shooting (not including part replacement) takes perhaps 30 to 45
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.
Reassemble the main heatsink sub-assembly and plug the two power supply cables back
into the 171 board (This can be done without installing the assembly back into the unit). Slowly
bring up the supply and verify that line consumption isn't excessive and that the output is
centered (no DC offset). This can be measured on the "AMP-OUT" testpoint on the 170 board. It
might be desirable to defeat the amplifier muting temporarily (Short "MUTE-VA" to "-15V-AMP"
testpoints on the 170 board) so the channel is active even when the supplies are still very low. If
the amp stays centered, power down the assembly and discharge the two main +/-100V
supplies on both channels. Remove any temporary jumpers and reinstall the assembly into the
chassis. Power the unit and verify that the repaired channel will pass a nice clean rail to rail sine
wave. Next proceed to the "Reliability Verification" section on page 8.
WARNING: FUSIBLE RESISTORS MUST ONLY BE REPLACED BY THE EXACT REPLACEMENT PARTS.
ALWAYS CHECK THE PARTS LISTS TO VERIFY WHICH RESISTORS ARE THE FUSIBLE TYPE
BEFORE REPLACING ANY RESISTORS IN THIS AMPLIFIER.
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