Mackie 1604 charming problems

[nylicens]

Hi. I am working on a Mackie 1604. Complaint was loud hum in studio. Pulled the mixer and hum went and then opened it up and discovered the LM337 had 1/2v noise on it - so changed. Then had no output from LM317, or rather it was shorted (very hot PS). Put in new 317 and still no +15. Pulled things apart and isolated the main board from the input/output panel and hooked up PS. Now all green -20 leds lighted plus main meters leds all on. But +15 and -15 now OK and not shorted. Am I doing something wrong by pulling all the ribbon cables apart? Is that why the leds are lighted?

[The Dude]

I'd isolate your short first. If there's no +15 with the cable(s) hooked up and there is voltage with something unhooked, you obviously have a short on the +15 on one of the unhooked boards. Find the problem and go from there.

[Jazz P Bass]

Usually, when all of the LEDs illuminate, that part of the circuit, at least, is missing either the +15 or the -15 supply.

[nylicens]

...you obviously have a short on the +15 on one of the unhooked boards. Find the problem and go from there.

OK, I plugged it all back together and pulled ribbon cables apart one by one. I disconnected the cable from POD board (next to tp138) and power came back on the +15 line. So something is shorted on that cable on the pod board. This is the 40 pin cable that connects to the board behind Mic 12. Not looking forward to pulling that apart. Does anyone know which plug this is? I have the schematics, but no diagram of the plug locations and numbers.

[g1]

The power is carried to the pod section via J101. The lower left of pg.1 of pod schematic shows the IC power connections. To the right of that shows the J101 pins carrying power.
With the J101 disconnected, at the pod you should find low resistance from +16 of IC pins to ground. (pin 9 of IC's).
Where the IC power connections are shown, see that R179 separates IC's U1,2, and 3 from the rest.
Disconnect R179. Is the low resistance now with U1 or U4 ?
That will show you which group of IC's has the short.
From there, you will need to desolder pin 9 of each IC in whichever group, until you find the one with the short.

Edit: If the board is stuffed with 8 pin IC's, the +power pin for each IC will be pin 8 instead of 9.

[nylicens]

The power is carried to the pod section via J101. The lower left of pg.1 of pod schematic shows the IC power connections. To the right of that shows the J101 pins carrying power.
With the J101 disconnected, at the pod you should find low resistance from +16 of IC pins to ground. (pin 9 of IC's).

Wow G1 that was a fast response, thanks. I pulled the pod all apart, and the 4560's are easy to see, but it is all surface mount stuff in there. I'm having a devil of time getting anything on the ohmmeter. Finally got a low reading from pin 4 on the one of the ICs marked (2068 1004U JRC) 8 pin surface mounts to pin 1 of J101. But that doesn't seem to jibe with the schematic. I am at sea on this one. Tried all over to find info on the 8pin IC that's not a 4560, but no luck.

[Jazz P Bass]

Most opamps are opamps.

Try NJM2068.
http://www.njr.com/semicon/PDF/NJM2068_E.pdf

[g1]

Yes, the 2068 pin out is the same, anyway the only concern right now is pin 8, as that is the pin connected to the +16 supply for either type, 4560 or 2068.
From any pin8 you will probably find a low resistance to ground. This will be the fault. The trick is finding out which one. If you can run the unit taken apart, chances are the bad one will get hot. There is also the possibility it's not an IC but a decoupling cap., however this is less likely.

Sorry I thought this was an old unit, I see now in your pic it is a VLZ Pro. Ignore my reference to J101 or R179.
If you don't have the right service manual, it's available here:
MACKIE 1604-VLZ-PRO MIXER Service Manual free download, schematics, eeprom, repair info for electronics
Wait til you can see "Get Manual" under the preview pic, then you can download it.

[oc disorder]

If you can run the unit taken apart, chances are the bad one will get hot.

This may or may not be helpful

Had an earlier version of a mackie with similar problems.
From memory I ran the mixer via a light bulb limiter which lit very dimly .
After it had been on for a minute or two I simply ran my finger over the chips
'till I found a hot one !
To double check used a thermometer with a blob of transistor mounting heat paste on the bulb
and rechecked !

Cleaned the chip off being careful not to destroy the tracks then using my outback bush method
held the new chip on it's back and tinned it then super-glued a tooth pick to the top of the body
which allowed me to hold it in position while I quickly stroked the pins outward with a fine soldering iron tip!
When suitably attached , using a fine screwdriver .. held down the chip while wiggling and breaking the
toothpick off !
If any one else can suggest an alternative method that is not too expensive please let me know !

Of course in some instances the "next" serviceman will find it impossible to read the number on the chip!

[nylicens]

This may or may not be helpful...

Fantastic idea. I would love to try it, but my fingers are way too big to fit down to most of the chips. I may try and use a digital thermometer I have for cooking.

Looking at the schematic there are only grounds for ch11-16 circuits, trims and ins, there. I was unable to find any shorts from gnd to +16 anywhere. But P3 does bring the -16 to pod board. Hmmm. No shorts there either. Still the +16 goes away when P3 is connected. I will try the thermometer tomorrow.

[g1]

You did replace both regulators with exact same type and not possibly a lower current model?

[nylicens]

You did replace both regulators with exact same type and not possibly a lower current model?

Yup, Fairchild's 337 & 317, 1.5amp, both TO220's and really lathered on the thermo goop and clamped them good.

[g1]

It would not necessarily be a dead short, but I take it you measured similar resistance from +16 to ground as from -16 to ground.
A short/low resistance that only appears with voltage applied may mean it is a decoupling cap rather than an IC.
There is a "smoke test" for this type of failure where you apply an external source of the +16V that is capable of more current.
The faulty part will short and smoke/burn, thus revealing itself as the source of the problem.
But before that, try your digital thermometer, even for the decouplers. I count 10 decoupling caps on the pod board, so 5 would be on the +16 side. They are shown at the bottom of each of the 3 pages of pod schematics where power to the IC's are shown.
Also, if you can limit your line voltage with a variac or limiter lamp like OC disorder mentioned, it may prevent the regulator from going into current limit shutdown. That way the bad part is more likely to heat up.

Edit: Still more decouplers, at the phones circuit, and near the input to each mic channel.

[nylicens]

It would not necessarily be a dead short, but I take it you measured similar resistance from +16 to ground as from -16 to ground.
A short/low resistance that only appears with voltage applied may mean it is a decoupling cap rather than an IC.
There is a "smoke test" for this type of failure where you apply an external source of the +16V that is capable of more current.
The faulty part will short and smoke/burn, thus revealing itself as the source of the problem.
But before that, try your digital thermometer, even for the decouplers. I count 10 decoupling caps on the pod board, so 5 would be on the +16 side. They are shown at the bottom of each of the 3 pages of pod schematics where power to the IC's are shown.
Also, if you can limit your line voltage with a variac or limiter lamp like OC disorder mentioned, it may prevent the regulator from going into current limit shutdown. That way the bad part is more likely to heat up. Edit: Still more decouplers, at the phones circuit, and near the input to each mic channel.

Smoke test tomorrow. I swear the schematic I have is not the pod I have. Similar but not quite the same. I have inductors on each channel, plus 3ea 2068 op amps, 2ea TO92 pks marked A1316 and three 3 terminal SOT devices marked K2tm3.

[Jazz P Bass]

Mackie CR1604VLZ schematic

Attached you will find the CR1604VLZ schematic.
Complete with inductors & NJM2068 opamps.