SB4000 Support Thread

[sheadstedford]

The tubes i have are 6n2 though not the ones stated in the advert. Schematic is the same though.
Is unbalancing the input a major problem then?

 

[jandoste]

No, -12V rail should be -12V.  Don't plug the PS board into main board until you figure out the cause.  Not much to go wrong here, just a simple 7912 regulator.  Are you sure you're probing the correct pins?  Correct part installed?  How much voltage do you have on the negative rail (TP15)?  Maybe you have a bad regulator perhaps.

Thanks Mike,
yes, I'm sure the pins are correct, and parts too.
- TP 15 = 28.6V
Is it correct?
cheers,

 

[ruckus328]

No, -12V rail should be -12V.  Don't plug the PS board into main board until you figure out the cause.  Not much to go wrong here, just a simple 7912 regulator.  Are you sure you're probing the correct pins?  Correct part installed?  How much voltage do you have on the negative rail (TP15)?  Maybe you have a bad regulator perhaps.

Thanks Mike,
yes, I'm sure the pins are correct, and parts too.
- TP 15 = 28.6V
Is it correct?
cheers,

Oops, I meant TP13, TP15 is the Positive rail test point, but voltage at both should be about the same (negative reading at TP13, positive at TP15).

Voltage at TP13/TP15 is completely dependant on the size transformer used and current draw.  With PS not connected to anything you won't be drawing any current so will be much higher than when connected to the main board and drawing current.  Does seem a little on the higher side though, would expect it to be more like 24V-25V (18V x 1.4 = 25V), but 28V might be ok since no load present.  What VA transformer are you using?  Do your other rails all have correct voltage? 

Again, look on the schematic, negative rail feeds to Pin 2 of the 7912 (U22) through a 10R resistor (R182) - R182 acts as a fuse if anything goes wrong.  If you're not getting -12V at the output of the 7912 (Pin 3) then something's wrong with either the 7912 or you've blown the 10R resistor, or bad solder joints, nothing else it could be.  First thing I would do is replace both. 

What voltage rating caps are you using on the PS?

 

[jandoste]

Thanks Mike,

What VA transformer are you using?  Do your other rails all have correct voltage?  

25VA
18V-0,69A

Again, look on the schematic, negative rail feeds to Pin 2 of the 7912 (U22) through a 10R resistor (R182) - R182 acts as a fuse if anything goes wrong.  If you're not getting -12V at the output of the 7912 (Pin 3) then something's wrong with either the 7912 or you've blown the 10R resistor, or bad solder joints, nothing else it could be.  First thing I would do is replace both.  

What voltage rating caps are you using on the PS?

I changed  the 7912 and I got the same Voltage(28V)
I used,

- 10uf 50V
- 22uf 35V
- 47uf 50V
- 100uf 35V
- 1000uf 35V
and everything is ok ! not wrong resistors or diods or caps !
Strange thing is I see the other voltages correctly ! I mean , I got 0V+12V/     +15/ +12/ 0V/ -15/ just got -28V

Ok, what do you think that if I send -28V to the mother board?
Thanks,

 

[ruckus328]

Ok, what do you think that if I send -28V to the mother board?
Thanks,

I think you will blow all the IC's in the sidechain.  Do not connect it to the main board until you've fixed the problem, it won't work, and you'll be causing even more problems.  Just be patient, you'll fixure out where the problem is.

If you don't have -12V at the output of the 7912 then it is not regulating.  If you're seeing (-28V?) on your -12V rail now then this is like the regulator isn't even there and a dead short.  Check all part orientations in this area, and check with a multimeter for continuity and bad solder joints on all the parts in the -12V regulator circuit (there's only like 7 parts here).  Somewhere one of these 7 parts is either in backwards, blown, or you have bad/cracked solder joint.  Did you inspect/replace R182?

Measure DC volts across the two pads of C151 and also C158, let me know what it says.

Check correct orientation of D29 - the cathode should be pointing towards D30.

What brand of 7912 are you using? Is it the same as your 7812?

Some high res pics of both sides of the board might help.

 

[jandoste]

Ok, what do you think that if I send -28V to the mother board?
Thanks,

I think you will blow all the IC's in the sidechain.  Do not connect it to the main board until you've fixed the problem, it won't work, and you'll be causing even more problems.  Just be patient, you'll fixure out where the problem is.

If you don't have -12V at the output of the 7912 then it is not regulating.  If you're seeing (-28V?) on your -12V rail now then this is like the regulator isn't even there and a dead short.  Check all part orientations in this area, and check with a multimeter for continuity and bad solder joints on all the parts in the -12V regulator circuit (there's only like 7 parts here).  Somewhere one of these 7 parts is either in backwards, blown, or you have bad/cracked solder joint.  Did you inspect/replace R182?

Measure DC volts across the two pads of C151 and also C158, let me know what it says.

Check correct orientation of D29 - the cathode should be pointing towards D30.

What brand of 7912 are you using? Is it the same as your 7812?

Some high res pics of both sides of the board might help.

I used this one ...
http://fr.mouser.com/ProductDetail/STMicroelectronics/L7912CV/?qs=hprEbj%252bdINrTD2aXjPyteQ%3d%3d
it's right D29 and D30 orientations...

Measure DC volts across the two pads of C151 and also C158, let me know what it says.

-29V

Did you inspect/replace R182?

No because I don't have second one ! and it says 10R and nothing wrong(?) with it ( I guess/hope )

Some high res pics of both sides of the board might help.

Thanks,

 

[sheadstedford]

Hello rukus any verdict on the balanced/unbalanced post from yesterday?
I posted a link to the board and schematic.
Think its just xlr pin 3 and 1 tied together and pin 2 to audio input of srpp. This makes the input of the sb4000 unbalanced inside the box.
I read some info about the connection options for balanced to unbalanced and it stated to either tie pin 1 and 3 together or just dont connect pin 3....

 

[ruckus328]

Umit, at advice of Harpo, put a temporary load on the -12V rail just to verify that there is actually a problem, it's possible the 7912 is giving false readings because there's no load.

Just temporarily connect a 1K or 1K2 resistor (1/4W) between the -12V rail and ground.  You can do this at the power output connector (J14).  This will put approx 10mA load on the 7912.  Let us know what happens after you try this.

 

[ruckus328]

Shead, I'll try and look later tonight.  Don't think you'll hurt anything grounding the negative input of the debalancing stage of the sb4k (aka tying pins 1 and 3 of xlr together).

Peronally though would annoy me though to have unbalanced input and balanced output though, might want to consider xfmr balancing the input of your preamp.

Curious what you're trying to get out of adding this though?  Just color?

 

[jandoste]

Umit, at advice of Harpo, put a temporary load on the -12V rail just to verify that there is actually a problem, it's possible the 7912 is giving false readings because there's no load.

Just temporarily connect a 1K or 1K2 resistor (1/4W) between the -12V rail and ground.  You can do this at the power output connector (J14).  This will put approx 10mA load on the 7912.  Let us know what happens after you try this.

Tried (with 1k) it and got -20V ???

 

[sheadstedford]

Shead, I'll try and look later tonight.  Don't think you'll hurt anything grounding the negative input of the debalancing stage of the sb4k (aka tying pins 1 and 3 of xlr together).

Peronally though would annoy me though to have unbalanced input and balanced output though, might want to consider xfmr balancing the input of your preamp.

Curious what you're trying to get out of adding this though?  Just color?

Yeah and a drive with a 100k log dual gang pot.
Thanks though mate i appreciate the help

 

[ruckus328]

Umit, at advice of Harpo, put a temporary load on the -12V rail just to verify that there is actually a problem, it's possible the 7912 is giving false readings because there's no load.

Just temporarily connect a 1K or 1K2 resistor (1/4W) between the -12V rail and ground.  You can do this at the power output connector (J14).  This will put approx 10mA load on the 7912.  Let us know what happens after you try this.

Tried (with 1k) it and got -20V ???

OK, with that resistor still installed how much voltage do you have between the - rail (TP13) and ground (TP14)?

 

[jandoste]

Umit, at advice of Harpo, put a temporary load on the -12V rail just to verify that there is actually a problem, it's possible the 7912 is giving false readings because there's no load.

Just temporarily connect a 1K or 1K2 resistor (1/4W) between the -12V rail and ground.  You can do this at the power output connector (J14).  This will put approx 10mA load on the 7912.  Let us know what happens after you try this.

Tried (with 1k) it and got -20V ???

OK, with that resistor still installed how much voltage do you have between the - rail (TP13) and ground (TP14)?

+\-29V

 

[ruckus328]

Umit, put your meter on continuity aka "beep" mode, check you have good connection between Pin 1 of the 7912 and ground (TP14).

Also, check your wire going from the Power Supply Board ground to your Mains Ground/IEC Connector : 

If you don't have a good connection here your whole PS will be floating.  Make sure you unplug you mains cord first before messing around with your mains/IEC connector.

Please read a couple pages back to my post to gandalf3's problem.

On a sidenote, ironically, while googling I stumbled upon this (sounds familiar):

http://www.electro-music.com/forum/post-134222.html

 

[Neil]

Jandoste, from the same thread Mike quoted there were some problems with St Micro brand regulators. If everything else checks out you may want to try swapping in a different manufacturer's 7912.

 

[Niketouille]

Does a 67x34mm toroidal transformer will fit ? I'm not sure about it so I prefer to check...
This is this one :
http://www.electronique-diffusion.fr/product_info.php?products_id=1077

 

[jandoste]

Jandoste, from the same thread Mike quoted there were some problems with St Micro brand regulators. If everything else checks out you may want to try swapping in a different manufacturer's 7912.

Thanks Neil,
Yeah, I am sure ! because I tried three 7912 and all had the same Voltage !

@ Mike, what did you use ST 7912 or NATIONAL SEMICONDUCTOR 7912?

Thanks,

 

[Ptownkid]

I think Mike used the On Semi part that is listed in the BOM.

 

[ruckus328]

Hold the phone guys.......

Umit, did you ground your power supply like I instructed?  Please read post above.

 

[jandoste]

Hold the phone guys.......

Umit, did you ground your power supply like I instructed?  Please read post above.

Yep, I did it... But nothing has changed... and I will try the semi LM7912

Also, check your wire going from the Power Supply Board ground to your Mains Ground/IEC Connector Roll Eyes

If you don't have a good connection here your whole PS will be floating.  Make sure you unplug you mains cord first before messing around with your mains/IEC connector.

bad news... I got a bit problem with the PSU board! it's not clean no more... three times I have changed the 7912
cheers,