BSS AR-416 power problems

[b0b]

Hello all,

being quite the novice when it comes to electronics I one again turn to you for some guidance. Some time ago I bought a secondhand BSS AR-416 that I knew was not 100%. I have worked on some DIs before and because they are quite simple to their design I was sure I could figure out what's wrong. I've tried all I can think of and I'm almost done but there's some really strange behavior still...

I live in Germany so we use the Schuko power plug. One of the weird things this unit is doing is that depending on how I plug in the power plug it turns on or not. Plug it in one way, it works, turn it 180° and it doesn't. So the main problem has to do with power. I kind of figured that from the start because there is a fairly lour buzz on all channels. I don't have a schematic so I'm a bit lost.

What I can't figure out is how the earth should be connected... There's a built in power supply and the earth of the power cord is connected straight to the main board. After the power transformer there is another earth that is connected to the chassis which is also used by all the ICs. Then there's separate isolated audio signal earth for each of the 4 channels.

The other thing that I notice is that when there is nothing plugged in to the input of a channel the buzz is really loud but as soon as I plug something in (or activate the pad) then it's much lower, so there is some sort of open circuit then, like when having just a cable connected to the input of a guitar amp without plugging it in to a guitar.

Any insight would be greatly appreciated!

Cheers,
Andy

 

[kilmister]

Broken rectifier bridge/diode would be my first place to check.

-Paavo

 

[b0b]

Niinpä tietysti

Cool. Thanks for the tip!

Have to go out and get some diodes then...

//andy

 

[kilmister]

Onnea vianetsintään 
Good luck!

-Paavo

 

[b0b]

Ok, so initially this was a major user FAIL. I had hooked up the mains incorrectly, no wonder it didn't work as it should...

Now the mains is hooked up correctly and that corrected most of the problems. What remains is that the transformer is now quite noisy, causing a buzz on all channel and it's heating up like crazy. Had to unplug it after 1 or two minutes. It sounded like stuff was starting to almost melt and there was a definite smell of warm/hot components...

What could cause the power transformer to buzz and heat up? It's a TALEMA ta60E. It's dual primary for 115V/230V operation and looks to have 4 secondaries because each of the 4 channels on the DI has it's own power feed with separate diode bridge rectifier (all seem to be in order there, measured them all). 

 

[Walrus]

Primary set for 115V and not 230V??

 

[kilmister]

Maybe trafo has failed. I have two Focusrite Green series compressors on my bench at the moment which have very same kind of problem.
It's quite easy to test main trafos for failures like this. Disconnect secondary side from circuit and hook up 1ohm resistor in series at the primary side. Measure voltage over the resistor to see current 1mV = 1 mA. There should not be current flowing at all not be very much current flowing. I made this measurement for trafos mentioned above and they ate several hundreds of milliamps -> trash.

Isolating transformer is very recommended for your own safety before playing with main voltages. Also Variac is very usefull tool.

-Paavo

 

[Walrus]

Only a "perfect transformer" will have zero current flowing in the primary with the secondary open circuit!

 

[kilmister]

Yes of course. Dunno how much current is flowing in healthy trafo while secondarys are open but definitely it's not hundreds of mA's.

-Paavo

 

[CJ]

that is a toroid so you will not hear transformer noise due to lams vibrating.

lams vibrate when you have no load on the secondary due to transformer being saturated due to a  no load condition. this runs counter intuitive but thats the way it is,

a toroid that is connected wrong will maybe vibrate the chassis due to all the current creating a mechanical force,

220 opertion means hook up the pri's in Series,

if  you get zero volts on sec, then you need to flip one pri,

running it out of phase will not damage the transformer, the coils will just cancel each other, you might see a few mv on the sec due to imbalance, but should be very low,

 

[b0b]

It's quite easy to test main trafos for failures like this. Disconnect secondary side from circuit and hook up 1ohm resistor in series at the primary side. Measure voltage over the resistor to see current 1mV = 1 mA. There should not be current flowing at all not be very much current flowing. I made this measurement for trafos mentioned above and they ate several hundreds of milliamps -> trash.

Isolating transformer is very recommended for your own safety before playing with main voltages. Also Variac is very usefull tool.

-Paavo

Terve Paavo,
thanks for the tip! Before I electrocute myself I just want to clarify one thing. After I connect the 1 Ohm resistor, do I plug the unit in to 230V? Or am I supposed to only feed it 1 mV which means I would have to find a variable AC power supply? And now I think I figured it out
So use a Variac and send 1 mV AC trough the trafo primary... The thing is, I don't have a Variac. Any way I could use a 9V AC guitar pedal PSU?

I have now completely disconnected the secondaries. I measured the resistance of the primarys and it's definitely in series when switched to 230V operation. Both of the two primaries also come up nearly identical (+-0.5%) when measured separately.

And thank you CJ and Walrus for the additional info.

//andy

 

[CJ]

give it the full monty=230,

call 911 and plug er in, by the time you hit the floor the EMT's ill be a knockin, 




hey jus kiddin, we like to joke around a bit here in the snake pit, 

230 VAC and look for 5 ma mag current,

use a 10 ohm and look for .005 amps  x 10 ohms = err,,,,

50 mv ac across the resistor,

wire it up and put a cardboard box over it just for kicks,

5 bonus points if you can start the cardboard on fire,  ???

 

[kilmister]

Yeah give it full juice. Do not electrocute yourself or I'll come and kill ya!

You know R=U/I thing called Ohm's law?
Now you have 1 ohm resistor and let say 1V over it, makes current flow at 1A.
Similary if you measure 5mV across the resistor there's 5mA current flow.

-Paavo

 

[b0b]

Thanks again for the info. I was trying to use Ohm's law and Joule's law (I think) to figure out what kind of 1 Ohm resistor I would need if I use 230V and the way I calculated it I'm not sure if there is a resistor like that...

With R being 1 wouldn't P = U^2 for the resistor? My resistors are normal 1/4 W...

 

[CJ]

either volts times amps or current squared times resistance,

50 mv at .005 = 0.25 milli watts

or .005^2 x 10 ohms  = same thing, so watts are not an issue unless there is a dead short across the primaries,
'check the DCR of the primary first just to make sure,