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Disassembly Procedure
1) Remove the bottom cover first by removing the six # 6 screws on the bottom and the three # 6 screws on each side. One side has an extra
screw which may require a # 4 screwdriver. Note that the center screw on each side is a machine screw and will need to be returned to the
tapped hole in the side of the heat sink when reassembling the unit. On a combo unit, removing only the side screws will allow the
electronics to be lifted away from the speaker compartment. The wires connecting the speaker to the electronics should then be detached
from the power amp.
2a) ML-S: The ML-S preamp removes easily by removing the three # 6 flat head screws attaching the front panel to the heat sink.
2b) ML-E: The ML-E preamp requires the same three screws to be removed. Also, at the rear panel, remove the two # 2 screws that
secure the RFG4 jack and the two # 6 screws holding the balanced outs in place. Using a 15mm nut driver, remove the plastic nuts from the
Stereo Aux. In, Return, and Send jacks. Pull the preamp board out and away from the rear panel being careful not to lose the fiber washers
from the ¼" jacks.
3) The two separate parts- the power amp/power supply attached to the heat sink, and the preamp board- can be set on a flat surface with the
component sides facing up and the wires remaining connected. The required troubleshooting can be done.
Problems with the Amp- Where to Look for Help
This short troubleshooting guide will attempt to analyze problems to quickly determine whether preamp, power amp, or power supply related.
Use the power supply/amp troubleshooting guide or the preamp circuit descriptions wherever applicable to fix the problem.
1) Front panel LED's do not light (no power): Possible blown fuse or other power supply problem; see power supply troubleshooting
procedure.
2) LED's work but no output from power amp: Could be preamp or power amp problem. First, turn power switch off for 3 seconds, then
back on without input signal to clear shutdown feature. Set all knobs to 10 and put in a signal at 5mVrms. If there is still no output, look
at either end of the red and white wires going between the preamp and power amp with a scope probe. If there is a signal there but no
output, then there is most likely a problem with the power amp. If there is no signal, the problem is in the preamp. Check the wires to
make sure there is a good connection at either end.
3) Distortion at the Output: Follow procedure from #2 above (may want to start with output level on 0 and gradually increase it). This
time, note whether the signal is a clean sine wave like the input, or if it is clipping off part of the signal or has any distortion. If the signal
is clean, the problem is most likely from the power amp. If the signal is cut in half, check the 15V supplies (any LF353- pins 8 & 4 should
have +/- 15VDC). If those supply voltages are not there, or are not clean DC levels, refer to the power supply/amp troubleshooting
procedure. If there isn't a problem with the 15V supplies but the signal is still distorted, the problem is most likely preamp related.
4) Excessive noise or humming: If the problem goes away when the output level is set to 0, it is probably a preamp problem- often a loose
part or bad jack. If it doesn't, the power amp/supply has the problem.
5) Amp works OK except for some individual features: Problem is usually in the preamp, but check the footswitch as well.
Scope
This document lists some steps to follow to quickly troubleshoot the microamplifier series of power supplies and power amps. Only common
failures for Rev. E power amps / supplies will be addressed. Previous power supplies used TIP50 transistors, which have different failure
modes. For more detail, this procedure should be used along with the assembly line power supply / amp start-up procedure, GK Document
#420-0071.
NOTE: The following instructions are given with the heat sink on the bench and the rear panel away from the technician.
Microamplifier Series- Lead
Disassembly and Preliminary Troubleshooting Procedure
GK Document # 426-0063-A
Model #'s: ML260E, ML260S, ML120E, ML120S- all options
2/15/91 Rev. 9/10/99 SW
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