Console Power Issue

[narwhal]

Hello all. I have a problem that is probably boring and easy to all of you, but I am honestly stuck. I have an old Dynamix 3000 mixing console. I had a new power supply built for it that supplies the +15 and -15 volts required for operation, as well as the +48 volts for phantom power. I just recapped the entire board with Panasonic FC series capacitors. I replaced voltage regulators. I rewired faulty connectors. Every module works perfectly independently, yet when I connect more than 17 (out of 27 - 16 input, 8 sub, 2 master, 1 monitor) the negative voltage drops from -15 to about -.71 volts. The positive voltage drops just noticably, but nothing to worry about.

I don't understand how adding 'one too many' modules can bring the negative voltage so unusably low. Any thoughts would be GREATLY appreciated.

Thanks,
narwhal

[Steve Conner]

Either your new power supply isn't powerful enough, or you messed something up in one of the modules that makes it draw a lot of current. If it drew 10 times as much as it should, you might not notice, because the power supply could still power it when you tested it by itself.

Are you sure you put all those new capacitors in the right way round?

[narwhal]

All capacitor polarities are correct. It doesn't seem to matter which modules are connected. I have tried several different combinations, trying to find a 'problem module' but there doesn't seem to be one. I knew how important it was to observe polarity and kept good notes, double checked my work, etc. so I don't think the issue lies with my work (not to sound conceited). Is there a reason that just one module, with all good components, could possibly cause such a voltage drop? The tech that built the power supply for me is out of town right now, but he made sure to build it a little bit more powerful than the original since I was thinking about auditioning other opamps.

Thanks for the quick reply.

[Enzo]

Or your negative regulator is faulty.

[narwhal]

Enzo, are you referring to all of the negative regulators I replaced, or should there be one in the power supply? I've never cracked it open, so I'm not sure what is all inside.

Thanks.

[Enzo]

How many negative Vregs are there? Yes, I mean the one in the power supply.

Can you insert an ammeter in series with the negative rail to check the current draw on that supply?

[narwhal]

There are both a positive and negative voltage regulator in each module.

I tried measuring with an ammeter, but I must be doing something wrong since I'm getting a reading of 0.

[jrfrond]

All console power supplies are built with internal regulation. What it sounds like to me is that you've exceeded the current draw of the negative rail and the regulator has gone into protect, either thermal or overcurrent, with the last module being the proverbial "straw that broke the camel's back". As long as we are on the subject of thermal shutdown, it could be a mechanical issue with the regulators not properly bolted to the heat sink and/or lack of thermal compound. Hopefully, there ARE heat sinks, because if not, that is an improperly-built supply. Whether the regulators are self-contained TO-3 or TO-220 IC's, or they are series-pass transistors, they MUST be mounted to heat sinks.

Then again, reading over your post, you stated that each module has it's own regulation, and perhaps the original supply was meant to deliver bipolar ±V unregulated, brute-force style, perhaps because the current draw is such that external regulation would have been to costly to implement. I don't know that console, and certainly not the power supply, but there's no doubt in my mind that, somehow, it's current overload and not a module-specific short.

[Steve Conner]

Sorry, I didn't mean to dis your work, all I know is that if it was me, I'd have got at least three backwards.

In the light of the other information you gave, it really sounds as if the guy who built your power supply got it wrong. Maybe it just can't output enough current for your desk, and one of the regulators is going into current limit. Or maybe it can, but there isn't enough heatsinking, so one of the regulators is going into thermal shutdown.

[Enzo]

I'd think the loads were pretty similar, so if the design was faulty, I'd think both rails would collapse. Then again if they run the LEDs off the same rail as the op amps, it might be added draw. I'm still betting on a bad Vreg.

[jrfrond]

There ARE many console (and other equipment) designs which draw current asymmetrically from the bipolar rails, e.g, LED's (or bulbs, in some cases) as Enzo pointed out. A bad regulator? Perhaps. I'm betting more on a design flaw.

[narwhal]

Wow, great input!

Steve, I too usually make silly mistakes. No offense was taken. I was just trying to convey how thorough I was.

The regulators on each module are TO-92 style. There is no heat sink in any module.

In the supply, there are 2 transformers: the 'big' one, supplying +15 and -15, and the 'little' one, which supplies the +48 volts. I see 2 TO-3 style regulators on the big transformer side, and one on the small side. I also see one TO-220 with the usual heatsink on top, and another TO-220 with a long, ribbed 'extra' heatsink attached to the usual one. This one is lying horizontally, as opposed to the usual vertical stance, and the heatsink is perpendicular. There looks to be a TO-92 on the 48 volts side.

From looking at schematics and following traces, I can tell you that both the positive and negative voltages go to the opamps. This probably doesn't help much, and I'm still a but unfamiliar with all the pins on the opamps. There are 2 TLO72CP, one 5532, and one 5534. I haven't even switched out these 'stock' opamps for the more current hungry BB ones.

The power supply looks kind of prefabricated. I think it may have just been a kit of some kind, and I'm not sure if my guy designed much of it. The supply is rated ± 24 volts dc at 2.4 amps. There are two potentiometers inside, I'm guessing to control the voltage and current you want the unit to output. Perhaps this whole issue is just a quick turn of a pot away?

Thanks!

[Enzo]

AHA!!!

So the power supply provides +/-24VDC raw rails to the mixer, whose modules then reregulate down to their operating voltages individually. I bet the current foldback is mis-adjusted on the -24v.

Commercial linear power supplies sometimes have not only voltage adjust, but also current limit adjust, so in overcurrent situations the thing doesn't sit there cranking 20 amps into a burning resistor somewhere.

If the power supply looks commercial, is there a label on it? Like a brand and model?

Look at the two controls. Is there anything silk screened by them to identify their function? POssibly "V" and "I"?

When the neg rail collapses, if you leave it on and remove a module or two does the rail come back up on its own? Or do you have to power off and on to "reset" the thing? We are checking to see if it is a crowbar circuit or just current foldback limiting.

Monitor the voltage while it is not loaded with modules. Pick one of the controls and adjust it slightly up and down. Does the output move with it? If not, try the other one. Looking to identify the voltage adjust.

Once the V pot is IDed, then connect just enough modules that it still works - one or two less than what shuts it down. Now the power supply is well loaded but not collapsing. Now turn the current pot - and I am now assuming that is what we have - and if adjusting one way causes the boltage to collapse, that direction is lower current. SO adjust it the other way. Now add more modules and adjust it up.

But it still could be faulty.

[jrfrond]

The sad part about all of this is that you could take almost any analog console PSU and adapt it to use it with just about anything, as long as the current specs are correct, and you change the cable connector. This ain't rocket science! You need the same voltages: 48V, ±15, sometimes a +5V (which is also occasionally derived internally in the console), and in the case of some Behringer mixers +10V. A good example would be the Mackie 400W 8-bus PSU. Over the years, we saw MAYBE a couple of failures, but usually it was always in the mixer. We've adapted these in our shop to power up multiple consoles for service.

[Enzo]

I'd agree, it shouldn't be major surgery to make a power supply for a mix desk. The individual Vregs on the cards call for a couple exta volts on the raw supply, but even so, pretty simple.