Soundcraft 400b meters slamming over

[Ike Zimbel]

Yes, it could be a PS issue. Which PS do you have and what do the lights on the front of it tell you? There may also be corresponding lights on the desk (can't remember) like "+/-17", 48v etc.
Can you get audio out of the desk?
Best,
Ike

[Darenzo]

The power supply is the standard 200b/400b unit that comes with the desk ...i rebuilt it with new caps and voltage regs etc (i did not change the rectifiers though) before i recapped my desk.




I have checked the voltages on the +17 (+17.8v) and the -17 (-17.5v) and the 48v is 48v current draw is around 8 amps
I tested it in its lock up state (meters hard over/ meter lights dim/peek leds on hum though the monitors ) and when its working fine ??
Thanks for the help bro
BTW there are no leds for the power anywhere

[Ike Zimbel]

And this problem just cropped up by itself, or, after some of the above mentioned work was done? It sounds like something is shorting the supply rails, or at the very least drawing more current than the supply wants to put out. Did you do any other upgrades that might be causing the desk to oscillate?
Best,
Ike

[Darenzo]

Hey ike thanks for your help the fault surfaced after the desk had been recapped , some of the values have been increased and the transistors on the mike pre, s have been changed also the master channel has 2 new opamps , like I said in my first post if I remove any one channel the desk is fine but if connect them all it freaks out when powered up but ...saying that if the desk is left switched off for a day or so it seems to power up ok ??

[Ike Zimbel]

Well it sounds like it's right on the edge, either of oscillation or too much current draw. Do you know if anyone else was ever into the desk? Maybe somebody changed a bunch of TL072's to 5532's on a previous "upgrade". I saw that once in a D&R desk, and the power supply was NOT happy.
Best
Ike

[Darenzo]

Yeh the desk is drawing 8 amps sounds quite a lot for a16 channel desk to me? What would cause oscillation in the desk is that AC ripple..?
Cheers

[Ike Zimbel]

8 amps at 220 volts? That's way too much. I would double check that reading as I would think your PS would be a smoking ruin by now.
As for oscillation, a microphone feeding back is an oscillation....just picture that at, say, 3Mhz instead and you'll get the picture of what happens when an electronic circuit is oscillating (these are gross generalizations for the purpose of simplification...).
Typically, oscillation after the kind of work you have done is caused by "upgrade" IC's that need added components to keep them stable in the older circuit that they have been dropped into. This, at the very least, is typically local power supply bypass caps (aka "de-coupling" caps). You haven't said what new IC's you put in, and you haven't said if you took any steps to keep them stable.
Best,
Ike

[Darenzo]

Hey ike I have only replaced 2 opamps in the master strip and adding local bypass by soldering a ceramic 5zu cap from pins 4&8 to ground I will check the amps it's drawing again after the week end and let you know .. Thanks for your help it is very much appreciated

[Ike Zimbel]

You could try putting the original op amps back and see if the fault goes away.
Best
Ike

[Darenzo]

Hi ike yeh I tried that and it did not fix it .. Interestingly though if I unplug one set of meters the fault goes away so it's looking more like the power supply is right on the edge of its limit..
Is there anyway I can squeez a bit more headroom out of the power supply I have by modding it ??
Cheers

[tinn]

I would recommend to lower the power consumption to normal.

The regulators switch off at 2.5 A. How did you measure the 8 amps?

measuring the current of the boards is easy, cause they are all fed
via resistors, where you can measure the voltage drop across.

The meter board has to feed the lamps as well. maybe a lamp is bad.
Maybe one of the swapped caps is reversed and shorts the rail.

Check the Voltage(current) across the 6.8 Ohm resistor of the meter board.
Do both boards draw the same current?

Check the voltages on the boards, not only on the supply rails.
In case your schematics are incomplete, I put it up on elektrotanya.com

good luck,

Tinn

[Darenzo]

Hey Tinn thanks for the reply bro ..
OK so i ran some more tests with a new digital meter..
My old meter was faulty lol
I am pulling 800ma (not 8A) the current draw is the same for +17v and -17v
also the current draw is around 100ma less when the desk is freaking out..
this is measured at the power supply on the output socket with the desk connected and switched on for the load..
The voltage is +17.5v when running the desk in working and in freakout mode..
but...the -17v rail is only reading -2.4v when its freaking out (its -17v when all is ok)

The voltage regs and all caps were replaced 3 months ago so would you think it might be the rectifier ??
The fault is intermittent

BTW thanks for the schematics they are much better than the copy i have here ..

[Ike Zimbel]

That points to a short on the -17 volt rail. Perhaps it's in the power supply input wiring, or something that gets jostled around in the course of removing a module. You could try flexing the PS cable while the board is on, and/or banging the board around a bit to see if you can trigger the fault, or get it to stop when it's happening.
Perhaps you have left some leads in there from the re-cap, or even neglected to trim them somewhere. We've all done that (although I always try really hard not to!).
Best,
Ike

[tinn]

When the current is lower in "freak mode" i would suspect the power supply.

Try to isolate the problem (Mixer/powersupply)
check the power supply with two resistors which draw ca. 1 amp .
10...15 Ohms/15 watt Types in a glass of water might survive the test, or 1kW
220V or 500W/110V Tungsten lamps which has around 10 Ohms when cold.

Tinn

[tinn]

A note to dual power supplies:

in rare cases the following is happening in dual power supplies with
a voltage regulator of the 79 or 337 Type:

during start up the negative output is drawn to the positive side by
the load and the regulator latches then in a "no start up" mode.

You can try to short the negative output (which has reversed polarity in that case) for a moment . You can disconnct the load when powering up,
an connect it, when the Powersupply has stabilized.

Some Types of regulators are more prone to this effect,
Sometimes Diodes at the output can save the output from
reversing the polarity.

If you put a bigger cap (like 1000µF) from V+ to V- you might
test if your powersupply survives this. When the time constants are
very unbalanced, the regulator has to work outside it's parameters and blows off.

At the 400B: you can short R4 for a second (output goes down to -1.2V)
and see if the power is then coming up correctly

Tinn

[Darenzo]

Thanks for your help Tinn..
I have decided to build a new power supply using a JLM Audio power station kit..
Rather than fix and ageing supply with its 26 year old transformer....
The kit will provide me with 5 rails ! +48v (phantom) at 300ma and 2x v+ and 2x v- rails at 1.5 amps each ... which should work out nicely
BTW the kit can be modified easily to provide me with 48@300ma and 1x +v and 1x -v rails at 3 amps each should i need it.. :D
Here is the built kit..the transformer should be here tomorrow so more to follow ..

[Darenzo]

Hey guys
Big Thanks to Tinn and ike for helping me on this ....
OK so for anyone in a similar situation ,the fault i had was in the power supply (the -17volt rail giving out under stress)
The symptom was the meters slamming over and the desk freaking out..!
so i decided to build a new power supply using the original Sound craft shoe box with a new power supply inside...
Here's what i did....


The old power supply (-the transformer) on the right



The JLM power station kit i used gives out 5 rails at 1.5 amps
since i only needed 3 rails (+17v -17v +48v) I joined 2 rails together via 3 amp diodes to provide me with 2x 3 amp rails should i need the extra power.. Big thanks to JOE from JLM AUDIO for helping me out


The heat sink i used was taken out of an old computer power supply and fitted nicely but the temperature of the heat sink got to around 96C on initial testing , this seemed way to much for my liking so i added a 96mm computer fan inside ...it sits about 6 mm above the heat-sink and draws air over the heat-sink and out of the vents in the top lowering the temperature to a more reasonable 36c....The fan BTW has a speed controller on it and is set to its slowest speed so you can hardly hear it ..

OK now back to making music ....

[Ike Zimbel]

Nice work! Really well done...and, I was just thinking nice thoughts about JLM and the great building blocks and problem solvers he has made available to our industry this very morning.
Cheers,
Ike

[error_trials]

Hey guys
Very helpful post. I know it's been a while since this last post but I'm having a similar problem. I'm in the middle of chopping my 400b in half and have had similar problems now that I'm putting it back together. Of course I decided to do this right in the middle of a recording project...

I took my 24x4x2 desk completely apart and chopped it down to be 14x4x2, that's all I'll need and it's easier for me to work with it that way, for the applications I'm using it for. Now that it's back together I'm getting some problems. With only the meters connected, powering on gives the expected blink from all the peak LEDs, then after a second or two the meter for group 2 slams and stays slammed with the LED lit for about 45 seconds, then goes back down to zero. If I plug in the master module and hook up the channel outputs, then the L R meters also peak.

I figure something is up (loose, broken, shorted) on the meter board for that meter only? During the fault, I can measure the voltage at the power supply, and also at the power to the modules, and it's +/- 17 V like it should be. so I don't suspect a power supply issue. I think next step is to look around on the meter PCB during the fault...

My question is what kind of problem could cause this, especially only for a minute or less when powered on? What should I look for?

Thanks.

[error_trials]

A couple more clues: when it's powering on, the voltage to meter 2 is about 16 V. All other meters are 0 V. after 30-45 seconds the meter 2 voltage is back to 0 V.

I'm trying to figure out what would cause it to stop after a short bit of time. A cap charging too slow? There are transistors, variable resistors, and opamps in each meter circuit, could any of these failing cause this kind of thing?

Meter bridge PCB schematic link:

http://bptechnical.com/BPFTP/meter_schem.pdf

[Darenzo]

Hi sorry for the late reply ..
sounds to me like a power supply fault somewhere..
When the desk is first powered on you get an in rush of current into the desk which can stress the power supply enough to freak it out .. (the fault on my 400b)
do you have one or two meter boards ? 6 or 10 vu .??
Darenzo