Early Mackie SRM 450 bias adjustment.

[oc disorder]

Have a Mackie early Model SRM450 (not V2 as doesn't have LM chip for horn)
which blew a 12 inch driver.

The local distributor no longer has spares for it (only the recent ones V3)
so bought a new Celestion TF 1230.

LF Pressed Chassis / Ferrite - TF1230 - Celestion - Guitar, Bass & Pro Audio Speakers

Celestion TF1230S Speaker (SRM450 v2/3 Replacement) | Sweetwater.com
"Replacement Speaker for Mackie SRM450 v2/3"

I find it a bit odd it suits the 2 and 3 as the 3 is now rated at 1000watts.

However by deduction the 450 here its not the new 1000 watt (?) version nor the version 2
with a chip,so it must be the first version , made from 1999 to aprox 2003.

Reading the service manual the big purple button draws one to the service bulletins.

They stress the bias must be checked which I did and I found them both to be 0.3mA
at the test points instead of 4 to 4.5mv.

I have just been reading the gospel according to DJ's and it seem heaps of them
overheated and caused problems that is until the version 3.

Methinks this has done good service and doesn't seem to have ever been serviced before.

So 13 years service wasn't too bad from the original speaker considering the radical
bands that have played through it !

When pushed the mid range seems cluttered and muddy with a hint of distortion on sibilants.

My question is... should I break the seals and adjust the bias which should improve the distortion
or am I then moving the amp into "overheating territory".

The manual was written as I understand it, 4 years before the SRM450 I have here was made.

Hoping someone has some direct experience with this.

[Jazz P Bass]

My first thought is :" are you seeing, hearing xover distortion?"

If Not, I would leave the bias alone.

[The Dude]

^^^^^ Agreed. If you have a scope, run a test tone into the amp and see what the output looks like. That will tell you if it's crossover distortion or something else.

[oc disorder]

Ok Thanks.

Dunno what came over me. Obviously the Harmon Kardon amp I was working on previously whose bias would not stabilize until I warmed the workshop (it's winter here and I got ticked off for having too many electric heaters on) then accidentally dropped a screwdriver and blew one of the outputs..
I really did have to wait 20 mins !!

Back to page 1... so I just should check for DC and distortion that I do every other time but I'm going to make a plug to fit the connectors
that provide the test points for the bias. That should reduce the risk of a probe shorting.

I am aware that I could be over fussy but am curious. Must admit couldn't get it very warm even after deafening myself for half an hour.

Our air con just packed it in again I told the warranty agent the fault code but he wouldn't accept it and had to wait until I could video the flashing lights and send it too him. I suspect it's more to do with the number of call outs he can claim on. How could an elderly couple without a mobile possible do this????

Faulty output thermocouple or something like that.

Looks like I'll have to drag out more heaters and warm the room again.... and re-test the Mackie.
That point 3mA seems like an error to me but I did check them twice yesterday.

Hopefully I'll be able to see crossover distortion if not it's probably time for a holiday.

Thanks again JP and The Dude!

[Jazz P Bass]

Please try to keep in mind that this is a Class A/B output section.

The less bias you have applied to the output transistors, the better.

Even if you can see a slight amount of crossover notch, it will probably be most noticeable at low volume.

Which will be swamped in the overall scheme of things.

To boot, it uses rail switching.

If anything, that in itself leads to a slight notch when the rails switch.

Mackie-SRM450 V1.zip

[oc disorder]

Well , connected a dummy load and couldn't detect any crossover distortion.
Drove it with a sine wave at about 3/4 output and measured "woofer" section at 2.5ma with the heat sink warm not hot.
When approaching clipping a flurry of squiggles (highly technical term there!) and the output pulled back ..compression in action.
Measured the o/p power at that point at 225watts RMS.
Not sure why the cro displayed what looked like other harmonics for a split second but seems to happen when compression cut in.
Perhaps the bit I read from the DJ page which said that "vinyl LP's sounded better than CD's" .. relates to dynamic range?
Just my curiosity showing.
So considering everything I chose to leave it alone.... it had been working fine until the speaker blew.
There were no complaints about distortion.
So I guess out of the thousands that have been made we only seem to read about the faulty ones ..
I shall keep an eye out to see if the new MK 3 has as much press... as now Mackie have supposedly
refined and improved it ..
Thanks for the reminder JP I would have liked to be able to explore it further but it also had to fit a budget.