Mackie 406m Full Schematic

[lowell]

I've found a few posts with the "service manual" which is only 20pgs long. The full schematic is not in there. I have one here that has an issue between the mixer line out and the PA in. There's no signal reaching the PA.

Mixer line out works
PA-in jacks do not work
Voltages all look good in the PA, fuse is good, nothing looks out of the ordinary so I'm guessing an open coupling cap or broken lead

[Jazz P Bass]

Does the unit function without inserting anything into the jacks.

If it does, then you have a Pa In jack issue.
408M_808M.pdf

Note: the 406, 408 & 808 all use the same schematic.
Mono Mixer PCB Assembly 408M, 808M (055-205-04, Rev F).zip

[lowell]

It does not function normally. Putting signal into any channel does not produce output at the speaker jacks. Only at the mixer output jack.

I jumpered the effects send/return. Did not help.

I put signal into the PA 1/2 jacks, nothing out the speaker jacks.

Thanks I'll take a peek!

Oh and after my OP I found 0v at the bias test points. Soooooo... the PA has a bias issue.

[lowell]

Also..do you have the full PA schematic? It doesn't seem to be in your zip.

[Jazz P Bass]

As far as I know, none exists.

This is what is available.
Mackie 406M_204 amplifier.pdf

The preamps are identical.
See that you have a good signal after U42 (left) & U53 (right).
I have seen issues with U45 (compressor IC) loading down the signal.
For whatever reason, that IC does not like a +15Vdc supply fault.

[lowell]

Well I sure am loving the new LOUDtech portal!

Just found this!

Also here's the portal link
https://supportloudtech.netx.net/lou...c/#/asset/9389

[Jazz P Bass]

Nice.
Thanks, Lowell.

[lowell]

So the thing is muted. There is indeed -15v at E65 MUTE test point. I had found Q30 in the mute circuitry to be shorted C/E and replaced it. I discovered this when I had -10.4v on it's collector and emitter. Now there is Vc -15v and Ve +7.6v.

My main question at this point is how does the MUTE work? I've mulled this over a bunch staring at this schematic and just cannot for the life of me figure out WHERE the -15v is coming from.

I'd like to know this, and if possible give me all the options of where it could be from. I am posting my notations here on the schematic.

Voltages in parentheses are AFTER replacing the shorted Q30 mute transistor. Feel I'm missing something obvious.

[lowell]

Anyone? Even a basic explanation of the mute circuitry is much appreciated.

I realize that the +15v is provided by the mute circuit transistors, and that if they don't "allow this voltage to pass" then the +15 will not reach Q43's base to turn it on. Now, where does the -15v come from? And what tells it to arrive at Q43's base?

Maybe Q43 has an E/B short, haven't checked that....oops.

[MarkusBass]

Lowell,

The circuit is very simple but you made it very complex by providing voltages for circuit with shorted transistor (which is already fixed by the way). Those voltages are not needed and they make troubleshooting of the circuit more difficult.
The circuit works in the following way:
- D59 provides positive voltage on C74 (most probably it's about +20V).The voltage divider made up of R133 and R125 resistors causes that there is a small positive voltage on the base of the Q29 transistor. The voltage causes that the transistor conducts and pulls down the voltage on base of the Q30 transistor, which starts conducting and provide positive voltage to the MUTE rail.
In situation when the amp is muted (for example when positive voltage on C74 disappears when the amp is switched off), Q29 is not conducting and Q30 is not conducting. -15V on the MUTE rail is provided by the power amp itself. If you look at the schematic of the power amp and look at the R134 resistor (which is connected to -60V), you will understand it. The other side of the resistor is connected to -15V (through diode) to make sure that the voltage is not lower than -15V.
Looking at your voltages, I see that you have -10V on collector of Q36 transistor in the mute circuit. But there should be at least +10V there. So it's not conducting. Maybe it's open, or you made a mistake and wrote "-10V" instead of "+10V". Please check it and provide schematic with actual voltages.

Mark

[lowell]

Thanks MarkusBass. I shoulda looked at Q36 when replacing Q30, being that they both share equal current. If one is bad, it's very possible that the other is too.

Sure enough Q36 measured MORE than the usual .6v from base to emitter. Pulled it and replaced it, the MUTE is now +15v and the unit is functioning great now.

Thanks again!

[MarkusBass]

Nice. Did you move from Chicago to Pensacola? Nice move

[lowell]

Yes sir

[nosaj]

Nice. Did you move from Chicago to Pensacola? Nice move

It much more humid down here. heat an rain an mosquitos and cockroaches..

nosaj

[lowell]

Mosquitos are absolutely insane. Cockroaches too...super grody!!!!

Gotta get me a few of these!
https://www.amazon.com/gp/aw/d/B071V...epellent&psc=1