Mackie M-800 Repair Tips

 

Schematics: https://supportloudtech.netx.net/loud-public/#category/11444

 If your M-800 HOT LED is always on and no signals pass and +16V rail is at 0V then read on:

https://cdn.gearsource.com/catalog/stockitem/normal/832abdc2-92d3-4786-8c5f-a2e52d609903.jpg

I could not find much online for this M-800 problem but it also seems to have affected the M-1400 big brother.

Symptoms:
- HOT LED on the front panel remains lit, even after a cold start.
- No audio passes through the amp as the HOT Status activates the Muting Circuit
- All DC power rails can be read by multimeter except the +16V (it is at 0V)
- R10 (A 10ohm through hole resistor that feeds Q15) is open (+64V on one side and 0V on the other).

Solution:
- Replace R10 10Ohm resistor
- Replace Q15 2SD1264A power transistor

Notes:
- You need a small Torx screw driver to remove screw holding Q15 to heat sink
- Screw the replacement Q15 to the heat sink before you solder it
- R10 replacement can be soldered to underside if you do not have the tools to remove it.
- De-soldering R10 from +64V rail is difficult due to heat absorption. There are two convenient larger unused through-holes next to R10 that you can use.
- Sometimes the ribbon cable causes strange problems and simply unplugging and reconnecting fixes these intermittent problems.
- There is a line of filled through-holes on the circuit board that are test points for all rail voltages (and more).
- Bonus Points: The 2 pin header test points for biasing should be set to 30mv using the trimmer pot for each channel.
- You can get Schematic and more at Mackie's Support Site.

 

Front Face Plate with Red HOT LED on:

Problem: Q15 (2SD1264A) and R10 (10Ω) have most likely failed and loss of +16V rail occurs.

 

Here they are on the M-800 Schematic:

 

Here is a top view of the circuit board art. Q15 and R10 are between the two big capacitors and the heat sink. Q15 is screwed to the heat sink and R10 is very near.  There is a big bridge rectifier above these components so look carefully.

 

 

 

 

Here is the underside artwork. The R10 is difficult to de-solder as the +64V rail absorbs a lot of heat. You can use the empty through holes beside it to make life easier. There is also enough room on the underside for R10.  Just cut the old one out and leave the legs in situ if you wish to replace it from the underside.

 

The M-800 has a nice section of test points on the circuit board. They are simply filled through-holes all in a row. This is the easy place to check the rails with a DVM and see if +16V is dead.

 

 

 

When finished, you can turn the bias trimmers to obtain 30mV on each channel's bias test points