SB4000 Support Thread

[mica]

So, im nearly there
BUT alas.. I got the right voltages on the PSU, but when i connect the main pcb: SMOKE from the 10R resistor at R182, and the powerrails read wrong values.. If i disconnect the main-pcb - everthing is fine, and perfect values on the powerrails..
I have checked the connection between the PSU and mainboard, and its wired ok (+15V goes to +15V, 0 to 0 etc.). I also cant seem to find any wrong wired components, but clearly theres an error somewhere!

Any thoughts would be appreciated..

A hi-res pic. of the unit:
(VR18-VR19 are wired on the other side of the psu-pcb.. They are not missing.. )

EDIT:
I had turned the three TL074 the wrong way
Now the 12V rails read ok, but im only getting 3V on the 15V rails..

 

[sr1200]

Those tl074s are probably shot... i did that once, had to get all new chips (and it wasnt just the TL's that blew, had to replace a couple of the THAT chips as well)  Check and see if those are toasted... sometimes you cant see any physical damage.

 

[mica]

Ill get some TL074's tomorrow and some new 10R resisters for the PSU..

How do i check if THAT chips are damaged? Which ones should be checked?

/Michael

 

[sr1200]

If someone else more qualified chime in, i think theres a way you can just bypass the IC's to see if the rest of the circuit is good.

 

[mica]

Right.. I got some new TL074's and some fresh 10R and i have good voltage readings on the rails

IF i damaged the THAT's how will i notice it during calibration/tests?

 

[sr1200]

If im not mistaken if the "THAT" chips are blown you wont be passing audio when the unit is engaged (you will pass audio when bypassed)

 

[ruckus328]

On the audio path VCA's (U11/U17), remove them and jumper pins 1 & 8 (use a piece of bus wire or cut off a lead from a diode/resistor).  It should now pass audio when engaged.  Make sure your input pots to the VCA's (VR1/VR2) are trimmed to the correct value (27K between the square pad and wiper/center pin - they should be somewhere in the ballpark of being centered).  Do this before you jumper the VCA's to get an accurate reading with your multimeter.

 

[ruckus328]

So, im nearly there
BUT alas.. I got the right voltages on the PSU, but when i connect the main pcb: SMOKE from the 10R resistor at R182, and the powerrails read wrong values.. If i disconnect the main-pcb - everthing is fine, and perfect values on the powerrails..
I have checked the connection between the PSU and mainboard, and its wired ok (+15V goes to +15V, 0 to 0 etc.). I also cant seem to find any wrong wired components, but clearly theres an error somewhere!

Any thoughts would be appreciated..

A hi-res pic. of the unit:
(VR18-VR19 are wired on the other side of the psu-pcb.. They are not missing.. )

EDIT:
I had turned the three TL074 the wrong way
Now the 12V rails read ok, but im only getting 3V on the 15V rails..

If you're not reading correct 15V on the main board, then you need to remove ALL IC's (excluding the two TL074's and TL072 in the sidechain since these run off of 12V and you've already replaced them).  You've blown something, first thing you need to determine is if it's an IC or a short somewhere else.  You shouldn't install any IC's until you have correct voltages, otherwise it's just a guessing game and you'll just keep blowing things and chaising your tail.  If you get correct voltages after removing them, then jumper the audio VCA's as indicated above and then systematically install one IC at a time (triple check value/orientation & then triple check again).  Power the unit up between each IC install until you find the culprit.  Best of luck.

 

[miketeachesclass]

I finished my unit over this last weekend, and it already sounds great.  It's pretty easy to calibrate if you've got a true RMS multimeter.  Even without the distortion adjustments done, this thing kills.

Thank you for such a well documented build.  That really goes a long way to making things achievable.

Mike

 

[wimmy7986]

I was wondering if anyone would lend me some thoughts on my problem.  For some reason, everything is working beautifully except for the makeup gain.  I can't get it to work at all.  I've read through all of the posts and tried all the rcommendations, but I havent had any luck.  After going through the schematic and checking voltages, I have a clear signal up until the TL072 for the makeup gain.  I seem to be getting signal into the IC but pin 6 which is the output shows me no signal.  Could this be a bad IC?  Even though the threshhold is working correctly?  I know pins 1-3 are for the threshhold... I also checked the voltages at the switch and I am getting the necessary 12V. 

Any thoughts?  Thanks in advance. 

 

[sr1200]

i had a similar issue only with my threshold pot.  Check that your ribbon cable has complete continuity.  Mine looked like it was all together correctly, but it was off like a half of a micron which caused the little knife edge things in the ribbon cable end to not make connection to ONE wire in the cable.... everything else worked fine, passed audio, signal in/out, LED VU worked... drove me bananas for the better part of a day (and i wound up completely dis-assembling one of my other working units to swap parts out to see what was "wrong".

 

[wimmy7986]

Thanks sr1200!  I checked continuity on the ribbon cable and everything seems to have good connection.  What do you think of the tl072?  Shouldn't I be getting signal out of pin6?  I'm getting no AC or DC voltage...

 

[ruckus328]

Thanks sr1200!  I checked continuity on the ribbon cable and everything seems to have good connection.  What do you think of the tl072?  Shouldn't I be getting signal out of pin6?  I'm getting no AC or DC voltage...

Measure DC at the input to the TL072.  Move the threshold pot from one end to the other and see if the DC voltage changes (it should).

 

[wimmy7986]

I seem to be getting voltages going into the TL072.  I've tried reseating the ribbon connectors and reheating the connectors at the PCB, but I am still getting nothing from my makeup gain pot...  I am completely out of ideas.

 

[mica]

I got to Calibration step 3 now.. Voltages from PSU + Unity gain are good. Unit is passing audio both bypassed and engaged..

Step 3:
TP8 reads 0.586v
TP10 reads 1.013v
TP11 read 0v
TP12 reads 0v

So: no reading at TP11+12.. Adjusting VR7+8 does nothing..

Can anyone pinpoint an area to check?

 

[sr1200]

U sure youre in the right mode mica?  did you try switching the stereo/mono switch?

 

[David R.]

Got a newbie question.  I have seached for the answer because is seems so basic, but have not been able to find it.  I would rather ask than risk a shock, so here goes.  When measuring the voltages of the rails (these are the 4 smaller wires coming out of the transformer - right?), I understand the test points on the board, but where does the other probe go?  I am guessing to ground, but I wanted to know and not guess.

Thanks for your help.

 

[sr1200]

the wires coming off the transformer arent the rails (well, not yet atleast) Your rails are the +/- 12v +/- 15v and your 0v is your reference.  So if your meter is standard... black probe on the 0v and red probe on the rail youre testing.... and DONT cross the streams!

 

[David R.]

the wires coming off the transformer arent the rails (well, not yet atleast) Your rails are the +/- 12v +/- 15v and your 0v is your reference.  So if your meter is standard... black probe on the 0v and red probe on the rail youre testing.... and DONT cross the streams!

Thanks sr1200.  I think I understand the rails now, still not sure where to place the black probe.  I feel like I am so close! 0v is where?

 

[mica]

U sure youre in the right mode mica?  did you try switching the stereo/mono switch?

Im sure im in the right mode.. If I wasn't tp10 wouldn't have any voltage on it right? anyway I have tried turning buttons on and off but still no readings.. I'll try replacing TL072 next..