mackie 1604 vlz-pro malefunction

When I was working with my small desk it suddenly started to output periodical square-wave sounds and all the channel solo leds lid up. Because I use the desk in a basement and the permanent on-off switching might affect the power-supply (which gets quite hot) I checked all the solder points on it (some of them looked pretty bad), but after that ALL the leds on the desk went on. no fuse blows, "just" logic error on the board I suspect.

any suggestions or hints?
thanks in advance
christoph

Was the 'squarewave' 120Hz or 100Hz by some chance... and can you put a location in your profile?

Keith

Didn't measure the frequency, but it was indeed very low. What I should add is, that the 12V supply for the BNC lamp is working, as well as phantom power. Before I checked the soldering points on the psu the "power" led wasn't working, now, after my check, it does.


and yes, my profile is updated as well (why, if I may ask?)

thanks
christoph

update: I wasn't very accurate when checking the desk - it's still a similar problem as in the beginning: all solo/pfl leds are on, the rude solo led is flashing and the meters show full output. the mute leds are off (now that I depressed all the mute buttons, which I activated when flipped the desk upside down - silly me) and there is no signal on the headphone out whatsoever - didn't check the main outs, as I didn't want to blow my speaker, but will try tomorrow.

thanks
christoph

location... welll the mains frequency is different in differnt countries, so 120Hz in Europe would be VERY unusual, but 100Hz would be exactly what I'd expect... the reverse would be true in America however. (Bad 'reservoir' capacitors in the power supply means that you get a LOUD noise at double the mains frequency.)

I reckon you should look at the reservoir capacitors first. They'll be right after the bridge rectifier in the power supply... -Unless you have a switch-mode power supply...

12V and 48V will very likely be run from different power sources. -Do you have an oscilloscope to look at the waveform on the power pins of the IC's? -If not, a DMM will usually also show a big difference if there's a lot of ripple.

That's where I'd look first.

Keith

Check the power supply voltages - I would suspect that one of the rails is very low, or absent.

This is usually the case when all the LED's come on.....

Look for +15 and -15 volts on pin 8 and pin 4 respectively of any dual IC.

If you find that one rail is missing - then check the voltages on the input side of the voltage regulator.

Come back with your findings - or the answer, if you find it!

This is a common fault with 1604VLZ and I have seen many other people experience it.

I bought a 1604vlz that was almost new for a silly price because it had the same fault.

The fault was fixed by replacing the LM317 / 337 regulators, and - crucially - the insulating gaskets around them.

I do not think you need to replace the regulators, but they are cheap, so you might as well if you have the PSU open.

My theory is that the gaskets that insulate the regulators are poorly installed / poor quality. After a few minutes use, the insulation resistance breaks down due to heat, and you have a direct short between the T0220 case and chassis...

This is how I repaired my VLZ.

Justin

edit: Note that one of my regulators had turned black...

Note also that my fault was identical to yours.

hey again,

I checked both LMs and found out that the insulation for the 337 was indeed very week. But replacing both power transistors didn't solve the problem.

I checked the voltage supplies on all 8-pin ics and they receive 4V on pin 4 and 8. Now that I completely dissasembled the unit, the problem is different again. Only the rude solo led is flashing and a periodic, pulsating sound is on the output, that is in time with the flashing of the led.

thanks so much so far - hope you guys have some more ideas
christoph

a small push....
has no one another idea?

thanks
christoph

Right - you have found out that the voltages are low on the output side of the regulators.
What do you measure on the INPUT side of the regulators? (or across each of the big electrolytic smothing capacitors). You should get at least 18 volts - possibly a few volts higher.

Are you able to disconnect the power supply from the mixer circuitry? If so - what do the voltages measure now?

Report back with your findings......

The voltage on the smoothing caps are about 29V each, they drop down to 26V when I connect the mixing board. Looks quite fine to me.

As I said, phantom power is there as well. don't know if it's a power supply fault.

thanks so much
christoph

small push again...

cheers
christoph

Clearly some current is being drawn when the mixer is connected - but you have good voltages across the main smoothing capacitors.

Now you need to check that the regulators are operating correctly - by connecting a dummy load resistor across the output (to draw some current) - and see what happens to the voltages.

If you have a 100 ohm 10 watt resistor - connect it across the -15 and +15 supply (30 volts) - this will draw about 300mA.

Does the voltage hold up?

You can check the +15 and -15 voltage independently, by loading them separately with the resistor to 0v or earth.

I just connected a 1Ohm 3W to each 15V rail - I measured really quickly 2.2V across the resistor - I'll have to get a bigger one for that purpose, cause it already started to smell.

don't know if the voltage drop is good or bad?

thanks
christoph

You are attempting to draw 1.5 amps - which may be too much for the regulator and the rest of the supply.

Try it with 100 ohms - this will draw a reasonable current that the PSU ought to be able to supply and will be enough to check the voltage regulators are operating correctly......

Also, while you're at it, replace the ribbon cable. Many of the VLZ Pro's had problems with defective ribbon cables, so you might as well change it while you're in there.

Cheers,
Zach

as far as I've understood the desk now, the ribbon cables just connect all the audio in/outputs, and I never had any problems with them. at the moment I just connect the desk with the power supply and nothing else, and there is still a malefunction going on.

as soon as I've bought a 100Ohm resistor, I'll post my results.

thanks so much
christoph

I've finally managed to get an 100Ohm 10Watt resistor.
The power supply has seven outlets (3 is ground, 6 is blank, 1 is 48V)

Unloaded: 7=-15.72, 5=15.44, 4=-15.37, 2=15.13
Loaded: 7=-15.69, 5=15.44, 4=-14.08, 2=13.86

Thanks in advance
Christoph

You have proved that the PSU is OK - so now you have to find the short circuit that is loading down the PSU voltages in the mixer section.

This will not be easy - but my first suggestion is to look for a blackened or discoloured bypass capacitor (often 100n and fitted close to the IC's). The next fairly non technical (but powerful) technique - connect the mixer to the PSU and sniff around, literally, for overheating components.......

Before you do this - perhaps - disconnect any ribbons that might carry power between different PCB's in the mixer - this might isolate the problem to one particular board.

John

sounds like one of the MANY 0.1uF caps that sits on the supply line is shorted...you usually get a little "tan" on them when they go bad, and that is how you can spot them.
also, ground to each chanel is through the sleeve connector/chassis mounting on the line inputs, so if you disassemble the unit, it'll be really hard to track down..use a piece of solder to wrap around the phoneconnectors. this way you can operate it "naked"...
you can also try disconnecting the flat cables going to the pod one at the time to see if it is in the main pcb or in the input/output pcb

johan