Mackie SRM450 Loud Hum

[sweatyk]

I received this unit not working. When I disassembled it I noticed immediately that the four 6800uf @ 50V caps were dome topped. In the schematic they are C168, 170, 171, 173. After I replaced them I plugged it into a source and powered it up. It produced a very loud HUM! I have looked over this with a magnifier several times to ensure that there was accidental bridging when I was soldering. Per the service manual the voltage readings are the following:

Should be Mine were
D56 = +45vdc and –45vdc D56 = +45vdc and –45vdc
D55 = +80vdc and –80vdc D55 = +95vdc and –95vdc
These are the 2 bridge rectifiers


Should be Mine were
Q34 = -15vdc Q34 = -15vdc
Q35 = +15vdc Q35 = +15vdc

There are 53vdc on the positive side of the output for the low frequency speaker. For schematic purposes this is one of the earlier ones from 1999/2000.

I’m not the best troubleshooter so I’m hoping to get some direction on what to look for. Nothing on the PCB appears to be overheated or has physical damage. Thanks in advance. If I’m leaving out any significant information please let me know. Attached are what I have been going by.............



Also wanted to wish everyone a Happy New Year!

[Enzo]

So you have power supplies OK.

Now look at the outputs. You have 50v on the LF out, so check the output transistors, the MOSFET commutators, the drivers, ALL resistors associated with them.

Q18 is the bias transistor, and whatever voltage is on it will appear at the output, so is there 50v on the legs of that? If so, look further left.

[Mark Black]

As I recall the R&R of those main filter caps is extremely difficult without accidentally also removing the plating in the mounting holes, some of which serve as a connection between circuit traces above and below the PCB.

Just something else to consider...

[sweatyk]

So you have power supplies OK.


Q18 is the bias transistor, and whatever voltage is on it will appear at the output, so is there 50v on the legs of that? If so, look further left.

So I do have the same voltage on all legs of Q18. So looking to the left............

Also Mark Black pointed out what I had not thought of. The fact that this has traces on both sides and the possibility that the solder may not have made the connection to the other side. If that is the case would that create the dc voltage on the output?