Mackie 1604 charming problems

[bkahuna]

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?

When you changed the regulator, did you perhaps forget to reinstall the mica insulator behind it, or somehow otherwise affect the isolation of the regulator?

When running these mixers disassembled, all the lights will come on unless you provide a ground connection (typically from power supply chassis to the jacks of the free floating pcb)

[nylicens]

When you changed the regulator, did you perhaps forget to reinstall the mica insulator behind it, or somehow otherwise affect the isolation of the regulator?

Don't think so. The unit has gray thermal pads. But when I changed the regulators I noticed that the pads suffered some pull apart when I removed them, so when I gooped them back I also slipped in a couple of mica pads back there, just in case.

When running these mixers disassembled, all the lights will come on unless you provide a ground connection (typically from power supply chassis to the jacks of the free floating pcb)

I ground alligator clipped everything to death just to deal with that problem.

[nylicens]

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

Thanks a whole, whole bunch! Now it's making more sense.

[nylicens]

Smoke test tomorrow.

Well, I tried putting a big supply on the +16 (protecting it with a 3Ω resistor) and all it did was promise to melt the alligator clips. Let it run while checking for hot spots on the POD board, but nada. I might go for 1.5Ω next.

[g1]

If the gator clips are getting hot, there must be some excess current there, so at least you are on the right track.

[nylicens]

If the gator clips are getting hot, there must be some excess current there, so at least you are on the right track.

Tried it with 1.5Ω. Nothing seemed to get hot anywhere except for one of the alligator wires which melted itself into the plastic insulation I had on the bench. The other clip simply fell apart. Why I can find nothing else seeming to get hot anywhere (main board or POD) is a mystery. Doubley mysterious in that the lowest Ω I can find between GND +16 and -16 in all combos is 59.9kΩ.

[tedmich]

mica is good on 317s but you also (naturally) have to be sure the mounting fastener (if present) is not grounded out. Are they TO-220 versions? Their fastening is usually done with a precision nylon collar which assures the 4/40 tiny bolt sits dead center and doesn't contact the live tab. Heated nylon does shift sometimes and I've had grounded tabs that looked AOK. Aavid 4880MG mounting kit works well on these although it costs 3 times the chip...

If they are SMD nevermind

[nylicens]

mica is good on 317s but you also (naturally) have to be sure the mounting fastener (if present) is not grounded out. Are they TO-220 versions? Their fastening is usually done with a precision nylon collar which assures the 4/40 tiny bolt sits dead center and doesn't contact the live tab. Heated nylon does shift sometimes and I've had grounded tabs that looked AOK. Aavid 4880MG mounting kit works well on these although it coasts 3 times the chip...

They are TO220's and they are not thru screwed but rather clamped on the plastic front of the package by a large piece of aluminum that fastens to the heatsink.

[Jazz P Bass]

I don't see how 16 volts through a resistance of 59.8K can melt a plastic test lead.

[nylicens]

I don't see how 16 volts through a resistance of 59.8K can melt a plastic test lead.
[ATTACH=CONFIG]30904[/ATTACH]

Nor do I! That's the mystery :-)

[g1]

Just for clarification, you have the pod completely disconnected from anything else, you have your external +16V supply connected to the pods 16V line and ground. Which 16V and ground points of the pod are you connected to?

[bkahuna]

If your supply has variable constant current, it may be time to just crank it up without the limiting resistor and see what smokes. My supply is rated up to 3A, and I've been able to find all kinds of "mystery shorts" this way. It usually is a .1uf decoupling cap, or an opamp. If it turns out to be a surface mount opamp, don't let it cook too long or you could damage the traces beneath it.

[Jazz P Bass]

A question that I have is: did you use both rail voltages. (ie: + & - 15Vdc)?

If you only used the +15, that may not be such a good idea.

Opamps designed to run on two different polarity rails may do odd things given only one supply.

[nylicens]

Just for clarification, you have the pod completely disconnected from anything else, you have your external +16V supply connected to the pods 16V line and ground. Which 16V and ground points of the pod are you connected to?

I was thinking about this over the weekend. I have everything connected when doing the burn test. But when checking the resistance on the pod board I have it disconnected. So maybe I will have to try allie clipping all the grounds together or checking each ground to the + supply one at a time. There are a lot of them connecting to the pod via the ribbon cables.

[nylicens]

If your supply has variable constant current, it may be time to just crank it up without the limiting resistor and see what smokes. My supply is rated up to 3A, and I've been able to find all kinds of "mystery shorts" this way. It usually is a .1uf decoupling cap, or an opamp. If it turns out to be a surface mount opamp, don't let it cook too long or you could damage the traces beneath it.

I have a car battery charger as my burn supply. So it's running at about 8/9 amps near as I figure. Max continuous is 10a with peak at 13a. No wonder the wires are melting.