Mackie M1400 poweramp problem

Assuming a constant hf then a matter of divining, by literally floating your hand/finger over, until zeroing in.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Yes, you haven't found all the broken stuff yet. Don't start guessing, trying to link symptoms with components, you'll be there all day and get nowhere. Go back and check very carefully all the components from the output transistors back as far as it is possible for the damage to have gone.

One technique I use is to use the diode check function on my multimeter, giving a reading of forward voltage drop. This gives an indication of resistance and semiconductor junction integrity at the same time. As this is a stereo amp you can directly compare the working channel with the faulty one.

Check each transistor junction and all the components using BOTH test lead polarities, starting from the power devices backwards. (You may be measuring a base emitter junction with a 100K resistor between the two. In one direction you will read 100k, the other you will read a 0.6V diode).

Investigate any discrepancies between channels, allowing for changing readings due to charged/charging capacitors. Both sides should read and behave the same. This may seem at first to be a very labour intensive procedure, but with practice can be carried out surprisingly quickly, and discrepancies easily located.

Gareth.

At all power levels ?

Graham

Actually, I run it without the dummy load for initial testings, in order to operate the amp more safely.. until I get a decent output signal. Then I go for power tests. I think I drove the amp up to somewhere 30-40V rms and the high frequency oscillation is riding on top of my sinewave test signal. The OK channel doesn't behave like this, if anyone may think it's just because I've no dummyload connected. Besides, there seems to be different version of the M1400, since my schematic isn't exactly like the amp... Maybe anyone has a few schematics for this unit in PDF?

Regards Steve

Have you compared the components, waveforms, etc in the good channel with the "bad" one?

It might be less trouble to simply shotgun the components you haven't already replaced.

There is also the remote possibility that the amp was behaving this way before it failed.

Note that if you do add a dummy load you well may find that you get very different waveforms than your nice sine wave with a little hash on top - this could be very misleading at this time. You could even destroy some more transistors, so I wouldn't bother since you know the amp is still faulty anyway.

Yep.

Very true. Make sure you don't just repair the symptoms!

Gareth.

But at lower levels ?

Ooooh, I did at one time I think. I'll look.

But anyway, do you know what I mean by the 'output zobel network' ? An RC series combination. The R should be of several watts rating. Do make sure that the R isn't burnt out (open).

Graham

A Zobel network is used for impedance compensation -- usually to make a circuit look more resistive.

An amp with such a network at its output might very well be unusually load-sensitive.

ng

that

The Zobel network is all fine.

The Zobel network is all fine.

Can you clarify whether you have tested and got these waveforms with a dummy load or not?

You said earlier: "The OK channel doesn't behave like this, if anyone may think it's just because I've no dummyload connected".

Gareth.

I doubt it if as advertised it can "handle 2 ohms all night long".

I wouldn't repair this amp for a "customer" and probably wouldn't waste my time on it for myself having been burned by Mackie in the past. Besides, I can buy a 2000 watt amp brand new also made in China for around $300 US. Someone brought me a used Crown Micro-Tech 1200 they wanted to sell for $400.00 US and even though I am a big fan of Crown especially that series I offered half that and they refused. I told them I can get 2000 watts into 4 ohms bridged for 300 bucks. I used a Micro-Tech 1000 for my bass guitar rig along with a BBE 383 bass preamp for 10 years worth of regular club and party gigs without ever a problem. Here's a picture of it along with 6 and a half feet of other gear I have in a rack.

Which it needs to be to counter the effect of the output inductor or you'll wreck the phase stability of the amplifier.

Graham

Every amp has one prior tot he output inductor.

Graham

I wouldn't expect it to work for very long.

Graham

No schematics though

Here we go

Graham

It's the other way around, I believe. The inductor is there for a good reason.

I will ask JC about this.

We've been using one for our 2x18 subs for several months with no problems. I pulled it apart to have a look and it's as well constructed as any other amp besides the high dollar brands.

They both are.

I design power amps btw.

Graham