Mackie 1604 VLZ Pro Help

Hi,

I'm trying to fix a Mackie 1604 VLZ Pro for a client.

When turned on, all the solo LEDS are lit, The rude solo light is flashing and the Meter LEDs are all lit. No signal passes to the main outs but you can get it off of the direct outputs.

I've disconnected the power supply and tested it with a 10ohm load and the voltages seem alright.

When I connected it back to the main board and I/o board, the negative rails went screwy. One jumped to +5v and the other to -3v or so. I didn't write it down at the time (stupid).

I disconnected the ribbon cables connecting the Main board to I/O one-by-one. Removing one of them caused the rails to return to normal, I later confirmed that this cable carried -15 to the I/O board.

I removed the I/O board from the chassis (so many washers), and now I'm getting different (but still worrisome) readings from the rails. Now when referenced to earth  the + rails are at 8v, and the negative rails are at -20v. Removing the ribbon connecting the negative rail to the I/O still causes the power rails to return to normal.

Before it seemed clear that there was a short of the negative rail of the I/O board but now with these new readings I'm not so sure.

Ideas? I haven't been able to smell or spot any burnt components so my next thought was to try to isolate the short by cutting the negative rail. Which I don't wont to do if I don't need to.

Thanks,

Sam

not sure if this helps, it's an earlier model i think
http://bmamps.com/Schematics/Mackie/Mackie_1604-VLZ_16_channel_mixer.pdf

Regulators seem to be working. If the solo relay relies on logic levels referenced to ground, then it makes sense that a floating ground or other PSU problem might send it into SOLO. I've seen some strange rail readings from a shorted zener in an amplifier PSU cause digital mayhem. Same thing happens sometimes when clipping indicators ref to GND, one of the rail goes whack, and you get lights when there shouldn't be. Might something along those lines. In my limited experience, screwy PSUs have been mostly failed regulators, blown electro caps, blown rectifiers, or shorted zeners or tantalums. These are all pretty easy to test for, it just takes time.

In that schematic i linked, D1752 (page 15) is a 1n5230B zener in the solo section, but I'm not sure how that would affect things if it died short. That's the sort of thing I'm suspicious of.

Probe for continuity along the circuit ground? There should be very low impedance everywhere to everywhere. Just a guess really. I don't do this for a living

Edited to Add: cross linking with a similar thread on an older model
http://groupdiy.com/index.php?topic=2767.0

gave up on a Eurosound PMP 3000 today, undid the lid and it is one huge motherboard, in order to do service work you have to remove 5 million knobs and 3 million jack nuts, probably screw something else up in the process,

not made to be serviced, terrible support, no schematics online, nobody answering a phone, if they did answer they would say "no parts available"

complete opposite of say, Peavey,

in other words, if it breaks, get  on evilbay and buy a used one for 200 bucks, because that is what it is going to cost you to have it fixed,

kind of like lap tops and sell phones, it breaks, throw it away,

now if a Neve board came in, we might have a little motivation, eh?

Those cheap mixers are terrible to fix!
Once I had a Mackie 1604 that produced a loud hum.
One of the supply rails was low, probably because of a defective chip. The voltage was too low, because the internal power supply could not deliver enough current. And a massive ripple was the result.
But how to find the defective chip? There are hundreds (SMD!) of them.
So I connected my big lab power supply that could deliver 5 Amps and waited...
Soon I saw some smoke and the current dropped!
I literally blew up the defective chip, that was now easy to find.
The chip was replaced and the mixer has been working perfectly until today!

RuudNL, that sounds fun. And possibly less destructive then cutting traces.

Yes they are terrible to fix.  If it was mine, I might not bother, but someone is paying me to work on it. I kind of regret taking the gig now. I was hoping it was a PS problem which would have been fairly easy.

I haven't had to remove the motherboard yet thank god.

Sam

I've never spent any time inside a mackie (ewww) mixer... ribbon cables from mackie are suspect. The symptom of + and - rails adding up to near correct total but being imbalanced WRT ground, makes ground connections suspect (or VOM ground connection suspect).

All solo lights acting up sounds like PS weirdness.

Maybe check the center tap of the power transformer. an open CT might cause PS rails to shift.

JR

RuudNL said:

Those cheap mixers are terrible to fix!
Once I had a Mackie 1604 that produced a loud hum.
One of the supply rails was low, probably because of a defective chip. The voltage was too low, because the internal power supply could not deliver enough current. And a massive ripple was the result.
But how to find the defective chip? There are hundreds (SMD!) of them.
So I connected my big lab power supply that could deliver 5 Amps and waited...
Soon I saw some smoke and the current dropped!
I literally blew up the defective chip, that was now easy to find.
The chip was replaced and the mixer has been working perfectly until today!

Click to expand...

Never did that to clear a chip, but IIRC that was a technique to troubleshoot PCB copper shorts. With a many amp into short circuit supply, a small sliver could vaporize, otherwise feel for the hot spot... to locate the short.

JR

samgraysound said:

Hi,

I've disconnected the power supply and tested it with a 10ohm load and the voltages seem alright.

When I connected it back to the main board and I/o board, the negative rails went screwy. One jumped to +5v and the other to -3v or so. I didn't write it down at the time (stupid).

Thanks,

Sam

Click to expand...

While the power supply is removed, the housing/back panel needs to be connected to the PSU ground otherwise all LEDS will light. Its not that bad to remove the main board if you need to.

RuudNL said:

Those cheap mixers are terrible to fix!
...
So I connected my big lab power supply that could deliver 5 Amps and waited...
Soon I saw some smoke and the current dropped!
I literally blew up the defective chip, that was now easy to find.
The chip was replaced and the mixer has been working perfectly until today!

Click to expand...

That was brilliant, Dr. House!

Do you mean connect the housing to the Power supply chassis, or to the neutral pin off of the power supply?

I tried connecting my bench supply to to find a shorted chip. I connected the + to the  +16 input pin, -  to -16, and ground to 0v. It draws about 500ma of current and nothing has exploded or smoked. I left it on for about a minute. My bench PS can only provide about 20v max though so it is 6v short on either rail. Is they voltage too low to work? Did I not leave it on long enough? Or does this mean I don't have a short after all?

Sam

Don't follow the veer.... 

Look for the mixer power supply ground reference. If the CT of the mixer power supply is not anchored at 0V the rails could drift around like you measured.

JR

Okay. Checked, and the center tap is anchored to 0v. The drifting reading we're due to the grounds for the PS and the two boards not being connected. I have them all clipped together now and here's what's happening.

Solo LEDS are lit.
-16v rail is at -3v
-16v LED rail is at .5v
other rails are okay.

When I disconnect ribbon cable #3 (which brings -15 to the I/O board), those rails return to normal.

Board is drawing about 750ma

And when I tried to blow a chip before I'm not confidant my bench supply was working properly. It seems to only be putting out 4v.

Sam

samgraysound said:

And when I tried to blow a chip before I'm not confidant my bench supply was working properly. It seems to only be putting out 4v.

Click to expand...

Your bench supply is likely current-limited so its output drops. If you want it to put out 15 V and it's only doing 4 V, you've got something shorted or at least seriously loading it down.

-a

CJ said:

...

kind of like lap tops and sell phones, it breaks, throw it away,

...

Click to expand...

Somehow I think that was not a typo

As for anything throwaway, SMD, lead-free, this is why I love my old Soundcraft 200. I know if anything ever goes wrong I can probably fix it. Besides, it just sounds so much sweeter than my Mackie 1642 VLZ Pro, which just sounds "boring" in comparison.

RuudNL said:

Those cheap mixers are terrible to fix!
Once I had a Mackie 1604 that produced a loud hum.
One of the supply rails was low, probably because of a defective chip. The voltage was too low, because the internal power supply could not deliver enough current. And a massive ripple was the result.
But how to find the defective chip? There are hundreds (SMD!) of them.
So I connected my big lab power supply that could deliver 5 Amps and waited...
Soon I saw some smoke and the current dropped!
I literally blew up the defective chip, that was now easy to find.
The chip was replaced and the mixer has been working perfectly until today!

Click to expand...

Haha! I've read this 3 times now! Lovely! It's stories like this that keep life interesting.

how much you want for the Soundcraft?

hymentoptera said:

As for anything throwaway, SMD, lead-free, this is why I love my old Soundcraft 200. I know if anything ever goes wrong I can probably fix it. Besides, it just sounds so much sweeter than my Mackie 1642 VLZ Pro, which just sounds "boring" in comparison.

Click to expand...

What's so hard about repairing things built with SMD parts and lead-free solder?

The Soundcraft 200B I had at the club back in the day was always breaking down.