Mackie 1202 Mixer Fault

[Cones]

Hi,

I have an older Mackie 1202 mixing desk that developed a fault a few years ago and I just shoved it to the back of the cupboard.

But now I have started to buy a few diagnostic tools I think I am finally better equipped to try and tackle the problem.

The mixer works fine on all channels when it is first switched on. The problem arises after it has been on for a few minutes and has got warm to the touch.

This is the scope trace I get from the main outputs at switch on with no sources connected.



This is what I get once the desk has been on for a while.


There is also a tremendous amount of noise on the outputs with this as well.

I am wondering if something is failing on the DC supply to the Op amps?

I have not yet tried to isolate the onboard supply and replace it with my bench supplies, but perhaps this would be a good first step.

Circuit here.


Input circuit (Tape input)


Main output circuit


VU meter display



I thought a Cap failure would show up straight away at power on? Perhaps it is the supply regulation LM7815 LM7915 that are failing once up to temperature?

Any suggestions are gratefully received.

Many Thanks

Mark

[SeanB]

Check the -15V rail, probably it had dead caps or a dead or dying regulator. Replace the caps with new ones anyway in the power supply for both 15V and 48v sections and use the scope to look for noise on the supply rails. Opamps are rather famous for oscillating if they lose the negative rail in mixers.

[David_AVD]

Mackie mixers are also infamous for intermittent ribbon cable assemblies.  This can cause all sorts of weirdness.  Something (else) to check while you have it open.

[SteigsdB]

Funny timing on this thread, I've got my 1202 up on the bench currently for a constant low hum it's developed.

Watching with interest.

[Cones]

Mackie mixers are also infamous for intermittent ribbon cable assemblies.  This can cause all sorts of weirdness.  Something (else) to check while you have it open.

Hi,

Yes, I have already replaced the ribbon between the PSU regulator board and the main audio PCB. Just 5 wires. It was frayed and corroded on the ends. Impossible to restrip and reuse as the isolation had gone brittle so I used solid core hook up wire instead.

I am now waiting for it to fail again after warming up so I can probe the -15v rail.

Thanks

Mark

[David_AVD]

If you want a lasting repair, it's often better to replace the ribbon that goes across the channel strips too. (going from memory)

[Cones]

Hi,

When I open the case the fault goes away after a short while? I suspect the cooling is better on the LM7815 and LM7915.

Voltage measured on the supply rails is

LM7815  15.445v
LM7915 -16.617v

That seems a bit out of spec as well.

Thanks for the help so far.

Mark

[Cones]

If you want a lasting repair, it's often better to replace the ribbon that goes across the channel strips too. (going from memory)

The only other ribbons (two) are to the 4 channel inserts on the back. Everything else is directly soldered to the main PCB. I suspect more ribbons are used on the larger 1604 and such.

Thanks

Mark

[Cones]

The power supply board and regulators.

[David_AVD]

I'd probably replace both regulators and filter capacitors.  Caps going low capacitance when warm can cause loss of regulation.

EDIT: Replace the 2 x 22uF in the regulator section as well I reckon.

[Cones]

Thanks

I've ordered the parts. Amazon seem to sell everything!
 
Otherwise it would be through RScomponents through work. But they often have minimum quantities.

I won an TTI TG215 Signal gen today as well on ebay. So hopefully I can test each channel once the power supply is fixed.

I've been using a PC based audio signal generator through the headphone jack that works pretty well.

Thanks

Mark

[Monkeh]

Thanks

I've ordered the parts. Amazon seem to sell everything!
 
Otherwise it would be through RScomponents through work. But they often have minimum quantities.

Spare parts are always good.

For small quantities, http://www.bitsbox.co.uk/ are often good. Don't recall what (if any specific..) brand caps they supply, though.

Loving the high-tech heatsink design in there. "Oh, hey, our heatsinks aren't big enough. Should we just bolt some spare bits of metal on the end?"

[Psi]

Voltage measured on the supply rails is

LM7815  15.445v
LM7915 -16.617v

What do the two voltages look like on the scope ?

[Sm58]

Loving the high-tech heatsink design in there. "Oh, hey, our heatsinks aren't big enough. Should we just bolt some spare bits of metal on the end?"

it more looks like a poor design to me... 78xx 79xx are not realy that good in therms of noise... but for a mackie it's probably ok

[Cones]

Hi, I should have mentioned that the bolted on bars on the heatsink were my first attempt at resolving the problem several years ago. 

The original heatsinks are pretty small.

Thanks

Mark

[Cones]

Voltage measured on the supply rails is

LM7815  15.445v
LM7915 -16.617v

What do the two voltages look like on the scope ?

Not great, there looks to be a lot of ripple on one of them.

Would that indicated a bad 2200uf input cap?





Measured on the regulated side.



Input to the regulator



Thanks

Mark

[maor]

the input caps are fine, my money is on a faulty regulator, that 4Mhz oscillation is quite the red light for you.


replace it, I'm sure it will solve the problem.

[SeanB]

4MHz says regulator or very dead 22uF capacitors on the output. Replace the regulators anyway, as they have been well toasted by now. Use some 47uF 35v low ESR  caps on the output as well, especially as the negative regulators are more prone to oscillate. See if you can get a better heatsink in there as well, instead of the 2 strips see about putting a u shaped piece between the 2 parts, or move the board to where you can connect the heatsinks to a metal chassis, as the regulators are insulated tab types.

[Cones]

Hi,

Thanks for all the advice. It seems that replacing both the regulators and the 22uf caps has fixed the problems. 



One of the removed bad caps


The other one


A new one


Very low ripple now




Voltage Rails


New regulators installed


I have some 2200uf replacements on order as well. But not sure if I really need them at all now? 
It won't hurt to change them anyway.

I have a nice Yamaha DSP amp to look at next, but that has more of a socket problem than anything else.

Thanks

Mark

[Felix]

Hi there!

Please if somebody can tell me, what kind of mains transformer is used in this mixer? Is it dual primary transformer or single primary? Reason why I'm asking is because I'm planning to get USA version of this mixer, so I need to know will it be possible to use it on different mains voltage, by changing the primary tap settings.

Thanks in advance.

[calexanian]

Caps and regulators, CAPS AND REGULATORS!!!!!!! If you put 25 volt caps back in I would actually pre form them at 25 volts for a few hours, likewise if you go up to 35, etc etc. just because  O0 also check that 28 volts input to the regulator. 16 to 18 volts comes to mind for what that should actually be.

[BluesCam]

I have a 1402 doing the same thing, but it has way more PS caps and a TIP29C.