Frustrating mixer troubles

[RawDepth]

I have a Soundtracs Topaz Project 8 mixer that quits working when it gets warm, (like after 10 or 15 minutes of use.) Just before it quits, a wild surge of power goes through the mixer for an instant, lighting up every meter and peak light and sending a loud blast of noise to my monitors at full volume, (nearly blowing them.) Then the entire mixer goes silent and will not function. Afterwards I have noticed that the little light labeled -17 volts is off while the +17 and +48 stays on. (All three lights are normally on.) It has done this every time I have tried to use it recently. It does it whether other gear is connected to it or not.

What a pisser!

It sits on a table top at room temperature and has no apparent reason to overheat.

I took it to a repair center where they ran it for an entire month and could not get it to do this trick even once. It worked flawlessly for them for hours on end. Of course they charged me an arm and a leg for diagnostics, cleaning, a few solder repairs, and a do-dad or two. I brought it back home and plugged it in and 15 minutes later, the friggin thing blows its top again. Needless to say, I am pretty damn angry with it.

Thoughts? Ideas? Anyone? Why only at my house?

[The Ghost of FM]

Something you are plugging into it is shorting something out.

Check all your cables for shorts and after that, plug in one item at a time until you find the offending piece.

Also disconnect your monitors...use a pair of headphones instead.

Good luck.

Cheers!

[RawDepth]

Originally Posted by The Ghost of FM

Something you are plugging into it is shorting something out.

Check all your cables for shorts and after that, plug in one item at a time until you find the offending piece.

Also disconnect your monitors...use a pair of headphones instead.

Good luck.

Cheers!

It does the same thing with no gear hooked up to it. I tried running it with nothing in the mixer but the AC cord, (zero gear, no cables attached to any jacks,) and after a while, it lights up like a Christmas tree, then goes down.

[RawDepth]

I am assuming it is heat related. Although, the AC voltage here at my house is always around 125 to 127 volts. Hmm, I wonder.

I go through a lot of light bulbs but, none of my other recording gear or computers have ever acted up.

[Beck]

A couple quick thoughts:

How is the unit mounted? Are the rubber feet on the bottom supporting it, or is it on a table or stand that makes contact with the frame?

Is the way you have it positioned the same as the repair shop had it positioned? There could be stress or twisting of the frame that is revealing a short.

Also, have you checked your room for proper grounding and polarization of the AC outlets? AC receptacles should be checked with a tester you can get from Radio Shack or a hardware store.

[The Ghost of FM]

Originally Posted by RawDepth

It does the same thing with no gear hooked up to it.

You didn't mention that in your initial post.

What Beck said about twisting the frame would have been my next guess as there's got to be something loose in there like a bad power rail or ground that causing this issue and the movement of the board back and forth to the shop kind of bears that out or at least strongly points to that.

Check your site wiring and if that's OK, bring it back to the repair shop and explain how the problem resurfaced...perhaps they offer a 30 day warranty on their work or lack there-of? If so, take advantage of it.

Cheers!

[Tim Gillett]

I wouldnt even be turning the mixer on if it's showing those symptoms. Power supply faults can cook the transistors, op amps etc and make a real mess.

Have the power supply totally checked out and fixed before it's reconnected to the rest of the circuit.

My 2 cent's worth.

Tim G

[RawDepth]

Originally Posted by Beck

A couple quick thoughts:

How is the unit mounted? Are the rubber feet on the bottom supporting it, or is it on a table or stand that makes contact with the frame?

All 6 rubber feet are present and in good condition. They are quite large and raise the frame up off the table nearly an inch.

Originally Posted by Beck

Is the way you have it positioned the same as the repair shop had it positioned? There could be stress or twisting of the frame that is revealing a short.

I don't know how the shop set it up when they ran it. I can go back and ask them. I have had this mixer on two different tables and it behaved the same on both.

I laid it upside-down on a large piece of foam and opened it up like a clam shell, (because cables connect the top half to the bottom.) I ran it until it quit and then tested the leads from the power supply with my multimeter. All still had correct voltages. I also bypassed (removed) the meter bridge before I began. Now I know the problem must be in the master channel main board and not the power supply. That -17 volts must be getting stonewalled somewhere in that mainboard causing the mixer to become crippled.

Originally Posted by Beck

Also, have you checked your room for proper grounding and polarization of the AC outlets? AC receptacles should be checked with a tester you can get from Radio Shack or a hardware store.

That was the first thing I did last year when I bought this house. All my recording gear is on the same circuit.

[RawDepth]

BTW, I didn't expect everyone to just up and solve my problem for me.

I only started this thread hoping that by chance, maybe someone once had a similar problem with their Topaz and could clue me in about a problem component or something. If my problem is unique, I will work it out somehow.

Any and all help is appreciated.

[Farview]

Your power supply is taking a dump. Send that in and have it fixed. This same thing happened to my Ghost a while back.

[RawDepth]

Originally Posted by Farview

Your power supply is taking a dump. Send that in and have it fixed. This same thing happened to my Ghost a while back.

Is that possible? All power supply leads still have the correct voltages after the crash yet, it's bad? I don't understand but, okay, I'll get it checked.

[Farview]

Originally Posted by RawDepth

Is that possible? All power supply leads still have the correct voltages after the crash yet, it's bad? I don't understand but, okay, I'll get it checked.

Now that you mention it, I don't think it's possible. Once you uplug it from the board, the -17 light comes back on? If it does, it's something in the board shorting out.

[RawDepth]

Originally Posted by Farview

Now that you mention it, I don't think it's possible. Once you uplug it from the board, the -17 light comes back on? If it does, it's something in the board shorting out.

Well, let me try to clear up what's happening.

There are four conductors running from the power supply to the main board.

+17
0
-17
+48

The three lights I mentioned are on top of the mixer, not on the power supply. Those lights are labeled +17, -17, and +48 and are normally always on. When the surge goes through the mixer, the -17 light goes out. The other two stay on.

While it was in this state, (power supply still on but mixer not working,) I pulled the four leads from the main board and tested for voltage. (That is, voltage coming out of the PS.) All three voltages were present when shorted to the ground lead.

The only way I can make the mixer work again is to power it down, allow it to cool down for a few minutes, and then power it back up again. If I power it back up too soon, it will still be in this odd state with no -17.

[Beck]

The PS will behave differently when the board is drawing power from it. You really have to put a load on it to see that it not only has the right voltage but can also provide the rated current under load.

Based on the experience of the repair shop it just sounds like something is shorting or loose. Maybe voltage regulator to heat sink contact. And if a V-reg has become that hot during your tests it’s no good anymore and should be replaced.

The more you operate it at this point, the greater risk you have of loosing other components through the domino effect.

I would call the repair shop’s findings a fluke and start from the beginning – take it somewhere else… or if you are able replace the voltage regs and rail capacitors on the PS.

Tim

[MCI2424]

Originally Posted by RawDepth

Well, let me try to clear up what's happening.

There are four conductors running from the power supply to the main board.

+17
0
-17
+48

The three lights I mentioned are on top of the mixer, not on the power supply. Those lights are labeled +17, -17, and +48 and are normally always on. When the surge goes through the mixer, the -17 light goes out. The other two stay on.

While it was in this state, (power supply still on but mixer not working,) I pulled the four leads from the main board and tested for voltage. (That is, voltage coming out of the PS.) All three voltages were present when shorted to the ground lead.

The only way I can make the mixer work again is to power it down, allow it to cool down for a few minutes, and then power it back up again. If I power it back up too soon, it will still be in this odd state with no -17.

Open the unit up. Expose the power supply. Unhook the entire supply to the rest of the board before going any further as damage to many channels could happen. Load the supply one at a time and draw 1/2 nominal current by soldering is a load resistor from the -17 Vdc output to analog ground. It will become hot, so watch out.
Example: 17Vdc/100 Ohm = 170mA 170mA X 17V = ~3watts
This would be a 100 Ohm, 3 Watt load resistor.
Read the current draw per supply output (maybe on back panel) and use a resistor matching 1/2 of the total current of that supply output using the above equation (Ohm's Law if you want to look it up). Put a meter (DC volt setting) on the power supply (start with the offending -17Vdc output). Measure the output. Take a hairdryer (low heat) and heat up the power supply until it fails. Don't overheat. When it fails, you have to have a can of freeze spray handy (electronics store). Now, beforehand, find and know where the input filter caps are, the bridge rectifier and voltage regulators (can or IC). Know where all active components and caps are.

When it fails (the meter will show lower than -17Vdc) start with squirt of freeze spry on the filter caps, if it does not come back, got to the bridge rectifier, then the voltage regulators etc.Work your way through the supply fronm input to output. Don't go nuts with the spray, just a small amount concentrated on 1 component at a time. When you find the component that is failing (the -17Vdc will come back on), you will know what to replace.

.
If you can't do this, go to a pro audio repair shop as a standard TV type place will not have schematics etc.

Good Luck and DON"T SHOCK YOURSELF.

[dementedchord]

it seems everyone wants you to rebuild the supply... so go ahead and when that doesnt work get back to us... it's not the supply as you said the voltages are up when the led goes out... are there wiring harnesses that go between boards for the channels or is it one piece??? if there's a wiring harness try pulling a few channels loose to see if it stops... then rehook till you find the offending channel.... alternatively... if you can get to the main connector there should be a series of resistors in parralell to feed the different sections but isolate them from each other... (called build out resistors) try to see which area maybe fed the voltages and which are down... if they are all down somewhat then the lowest is in all likelyhood the culprit... also check for bad solderjoints around the coonectors and that sort of thing... after 15 yrs or so doing repair ican assure you most of the time it comes down to physical problems not electrical...

[MCI2424]

Originally Posted by dementedchord

it seems everyone wants you to rebuild the supply... so go ahead and when that doesnt work get back to us... it's not the supply as you said the voltages are up when the led goes out... are there wiring harnesses that go between boards for the channels or is it one piece??? if there's a wiring harness try pulling a few channels loose to see if it stops... then rehook till you find the offending channel.... alternatively... if you can get to the main connector there should be a series of resistors in parralell to feed the different sections but isolate them from each other... (called build out resistors) try to see which area maybe fed the voltages and which are down... if they are all down somewhat then the lowest is in all likelyhood the culprit... also check for bad solderjoints around the coonectors and that sort of thing... after 15 yrs or so doing repair ican assure you most of the time it comes down to physical problems not electrical...

You want to either force the supply to fail OR eliminate it as the bad part. Troubleshooting always starts at the power supply and works its way out to the end. I agree that physical problems are the most likely causes.