SB4000 Support Thread

[Bowie]

hard to tell in that pic, but do you have the right jumpers jumped in the VCA section?

JMP 1, 4, & the two on the panel board are populated.
JMP 2, 3, 5, 6, 8 & 9 + non populated.

Thx for taking a look.

 

[Harpo]

It still has no audible compression even though it can be seen on the meter. What might cause the signal to come through uncompressed even though the compression meter is showing what looks to be normal compression behavior as I adjust the knobs?

Separat paths for the sidechain-VCAs logging loop and audio-VCAs control voltage. Your issue might be no audio-VCA control voltage arriving at the audio-VCAs EC- port pin3 that varies the hearable amount of compression or makeup gain for whatever reason.
Step by step.
For your THAT2181B audio-VCAs check jumpers JMP1 and 4 are fitted and JMP2, 3, 5, 6, 8 and 9 are not populated. Power down, pull both audio-VCAs, pull both attenuator buffers NE5534 U12 and U18 and power on again. With no audio signal present, measure the voltage at TL072-pin7. Should vary between 0V and about 12V*100000/620000*-1=-1.935VDC when varying the makeup gain pot from CCW to CW. Yes- moving on, else check for shorts at R118, C116, R77, R121, at the pcb traces to these parts or broken prementioned resistors (interrupting signal flow) or broken cap (shunting down signal) or broken U13 TL072.
Keep the makeup pot in its CW setting and measure control voltage arriving at pin3 of U18 and U12. Will be about -1.935VDC/(100R+1000R+120R)*120R=-0.190VDC. Yes- moving on, else signal still interrupted or shorted out from not fixed previous step.
Power down, repopulate the pulled NE5534s, power on and measure output of U12 and U18 pin6 as well as audio-VCAs pin3. Should be the same as previous readout. Yes- moving on, else replace broken U12 and U18.
Power down, repopulate the pulled audio-VCAs and power on again. With audio signal present, audio level should vary when varying the makeup pot, else replace broken audio-VCAs.
Good luck.

 

[sr1200]

Damn harpo, that was amazing.

 

[Harpo]

Damn harpo, that was amazing.

Why that? Only using more words than my previous reply (and only covering the most suspicious spots with still missing answers from previous questions for signal level and frequency or measured control voltage at U13-pin7).

 

[sr1200]

Just felt it was quite an awesome display of understanding of the circuit and the potential areas of issue. 

 

[Bowie]

It still has no audible compression even though it can be seen on the meter. What might cause the signal to come through uncompressed even though the compression meter is showing what looks to be normal compression behavior as I adjust the knobs?

Separat paths for the sidechain-VCAs logging loop and audio-VCAs control voltage. Your issue might be no audio-VCA control voltage arriving at the audio-VCAs EC- port pin3 that varies the hearable amount of compression or makeup gain for whatever reason.
Step by step.
For your THAT2181B audio-VCAs check jumpers JMP1 and 4 are fitted and JMP2, 3, 5, 6, 8 and 9 are not populated. Power down, pull both audio-VCAs, pull both attenuator buffers NE5534 U12 and U18 and power on again. With no audio signal present, measure the voltage at TL072-pin7. Should vary between 0V and about 12V*100000/620000*-1=-1.935VDC when varying the makeup gain pot from CCW to CW.

Thanks again, Harpo.  I'm at the first step and after pulling the parts you instructed, pin 7 on the TL072 (U13) is -0~.  Pin 8 is 12v, and pin 4 is -12 which looks correct as per the schematic.  The Gain pot itself reads between 0 and 12v as it's turned from CCW to CC.  Does this mean that the TL072 is bad or is there something else in the circuit I should be checking?
Thanks!

Just felt it was quite an awesome display of understanding of the circuit and the potential areas of issue. 

Indeed! 

 

[Harpo]

.. pin 7 on the TL072 (U13) is -0~.  Pin 8 is 12v, and pin 4 is -12 which looks correct as per the schematic.  The Gain pot itself reads between 0 and 12v as it's turned from CCW to CC.  Does this mean that the TL072 is bad or is there something else in the circuit I should be checking?

Maybe. Pull U13 and with makeup pot in CW position measure for positive voltage (<12VDC, too lazy to calculate the resistive network for 12V/(R123+(R135+VR16)||(R119+R118+((R121+R125)||(R77+R83)))*(R135+VR16)||(R119+R118+((R121+R125)||(R77+R83)))) at this spot to 0V reference voltage) arriving at U13-pin6 in order to exclude R123 not conducting. Double check R119 for parts value 100K (not 100R) and measure U13-pin7 for little lower value than previous measurement. If same voltage as previous measurement, remove the probably shorted (DOA or fried from too long soldering on) 100pF cap C104 that shorts out the 100K feedback resistor R119. Repopulate U13 and check pin7 for about -1.935VDC with makeup pot in CW position. Yes- done (maybe put a working 100pF cap C104 back in, but this is not essential), else exchange the broken TL072.
Good luck

 

[Bowie]

.. pin 7 on the TL072 (U13) is -0~.  Pin 8 is 12v, and pin 4 is -12 which looks correct as per the schematic.  The Gain pot itself reads between 0 and 12v as it's turned from CCW to CC.  Does this mean that the TL072 is bad or is there something else in the circuit I should be checking?

Maybe. Pull U13 and with makeup pot in CW position measure for positive voltage (<12VDC, too lazy to calculate the resistive network for 12V/(R123+(R135+VR16)||(R119+R118+((R121+R125)||(R77+R83)))*(R135+VR16)||(R119+R118+((R121+R125)||(R77+R83)))) at this spot to 0V reference voltage) arriving at U13-pin6 in order to exclude R123 not conducting. Double check R119 for parts value 100K (not 100R) and measure U13-pin7 for little lower value than previous measurement. If same voltage as previous measurement, remove the probably shorted (DOA or fried from too long soldering on) 100pF cap C104 that shorts out the 100K feedback resistor R119. Repopulate U13 and check pin7 for about -1.935VDC with makeup pot in CW position. Yes- done (maybe put a working 100pF cap C104 back in, but this is not essential), else exchange the broken TL072.
Good luck

I pulled U13, not sure if I understood correctly but I measured .04v at pin 6.  R123 is 12v at one end, .04 at the other when Gain is fully CW.
Pin 7 measures around .02 with or without U13 in place.
I pulled C104 and the voltage at pin 7 remained .02.
So, as I understand it, I should replace the TL072 unless there is a problem with the R123 that I didn't understand.  Thank you!

 

[Harpo]

Replacing the TL072 would be my next step, just in case there isn't a short between pins 6/7 (using a magnifying glass might help to either spot it or confirm separation). R123 seems to be working correct.

 

[Bowie]

Replacing the TL072 would be my next step, just in case there isn't a short between pins 6/7 (using a magnifying glass might help to either spot it or confirm separation). R123 seems to be working correct.

New TL702s arrived today but, unfortunately, still no audible compression and the gain knob has no affect.  There is no chance pins 6 and 7 shorting.  Any ideas would be greatly valued. 

 

[Harpo]

New TL702s arrived today but, unfortunately, still no audible compression and the gain knob has no affect.  There is no chance pins 6 and 7 shorting.  Any ideas would be greatly valued.

For no makeup gain, the only remaining fault condition that I could forsee might be a broken 620K R123 then (varying voltage between 0...+12V when varying the makeup pot at one side and 40mV (not zero) arriving at the resistors other side excludes a broken pcb trace between resistor and opamps inverting input. With U13 populated and unit powered on, this is a current node, not a voltage node.) 

 

[Bowie]

New TL702s arrived today but, unfortunately, still no audible compression and the gain knob has no affect.  There is no chance pins 6 and 7 shorting.  Any ideas would be greatly valued.

For no makeup gain, the only remaining fault condition that I could forsee might be a broken 620K R123 then (varying voltage between 0...+12V when varying the makeup pot at one side and 40mV (not zero) arriving at the resistors other side excludes a broken pcb trace between resistor and opamps inverting input. With U13 populated and unit powered on, this is a current node, not a voltage node.)

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.  The other side measures -.57mv.  Does that reveal an issue?
I re-soldered the U13 socket and the associated resistors and caps to eliminate a bad joint.  Didn't change the behavior.  Getting desperate for ideas.  Thanks.

 

[Harpo]

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.

Different from your answer to my 1st.reply. When the wiper of the pot shows varying voltage in range 0...+12V and this same varying voltage doesn't arrive at R123, there is something wrong on the way between these spots (pcb trace, ribbon connectors, ...).

 

[Bowie]

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.

Different from your answer to my 1st.reply. When the wiper of the pot shows varying voltage in range 0...+12V and this same varying voltage doesn't arrive at R123, there is something wrong on the way between these spots (pcb trace, ribbon connectors, ...).

You're right, I recall it sweeping from 0-12v before.  I'll check to see what happened there.  To determine a PCB trace issue, do I just check for continuity and is there a certain point or series of points you suggest I check?

 

[Bowie]

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.

Different from your answer to my 1st.reply. When the wiper of the pot shows varying voltage in range 0...+12V and this same varying voltage doesn't arrive at R123, there is something wrong on the way between these spots (pcb trace, ribbon connectors, ...).

I checked the ribbon cable for any shorts and it's fine.  The Gain pot is also spanning only 10.2v  to 12v.  It goes from 0 ohms to 57k though.  What could cause that at the pot itself?  I've never seen that before.  Also, should the shell of the pot be grounded against the chassis?  I touched a ground wire to it and it didn't seem to make a difference. Thanks again for the help.

 

[Harpo]

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.

Different from your answer to my 1st.reply. When the wiper of the pot shows varying voltage in range 0...+12V and this same varying voltage doesn't arrive at R123, there is something wrong on the way between these spots (pcb trace, ribbon connectors, ...).

I checked the ribbon cable for any shorts and it's fine.  The Gain pot is also spanning only 10.2v  to 12v.  It goes from 0 ohms to 57k though.  What could cause that at the pot itself?  I've never seen that before.  Also, should the shell of the pot be grounded against the chassis?  I touched a ground wire to it and it didn't seem to make a difference. Thanks again for the help.

The makeup gain pot is a simple voltage divider with the pots outer pins connected between 0V and +12V, so the wiper will give out any voltage in this range in respect to 0V reference voltage on paper and for this circuit. The +12V obviously arrives at the pots CW end, so the pots 0V connection at the opposite taper side is missing. (your measured 10.2V is the voltage divider in respect to the opamps virtual ground node with the 620K resistor in between and the pot with the missing/broken CCW outer connection to 0V reference voltage now operates as a rheostat).

 

[ruckus328]

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.

Different from your answer to my 1st.reply. When the wiper of the pot shows varying voltage in range 0...+12V and this same varying voltage doesn't arrive at R123, there is something wrong on the way between these spots (pcb trace, ribbon connectors, ...).

I checked the ribbon cable for any shorts and it's fine.  The Gain pot is also spanning only 10.2v  to 12v.  It goes from 0 ohms to 57k though.  What could cause that at the pot itself?  I've never seen that before.  Also, should the shell of the pot be grounded against the chassis?  I touched a ground wire to it and it didn't seem to make a difference. Thanks again for the help.

Bowie, this is simply "follow the breadcrumbs".  You need to open the schematic and follow the signal from the Gain Pot up to the VCA.  This is about 6 parts we're talking about, so should be pretty simple to track down.

Gain Knob > Ribbon cable > R123 > U13B > R118 > R85 > VCA's

Since both channels are not responding then problem is before R85, as signal splits at that point to both audio VCA's.

First your gain pot - One end is connected to +12V, other to Ground.  With Gain knob set to the middle the wiper should read about 6V.  If it's not, then your gain knob isn't soldered correctly, simple as that.  With unit off, check continuity of outer pins of Pots to verify actual connection to +12V & Gnd respectfully.  Measure on the pins of the pot themselves.  Test Points are available right where power comes into the main board.

Once you are getting around 6V at wiper with gain knob set about in the middle, then just follow the sinal with your voltmeter.  If you don't read 6V at R123 then your problem is before that (ie ribbon cable or J6-10 / J18-10 solder joints).

Check continuity of pins between J6 & J18 (with ribbon cable plugged in of course).  Flux/Reflow all solder joints at U13 (TL072) and associated resistors/caps (R123, R119, etc / see schematic).  Check and make sure ribbon cable connectors J6 & J18 are not backwards.  Check U13 is not installed backwards. 

Your problem is somewhere at U13 or bad termination on ribbon cable.  Since you are seeing meter responding with turn of threshold pot then it only further supports likely problem at U13, either bad solder joint on it or supporting part, R119 primary suspect.

 

[Bowie]

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.

Different from your answer to my 1st.reply. When the wiper of the pot shows varying voltage in range 0...+12V and this same varying voltage doesn't arrive at R123, there is something wrong on the way between these spots (pcb trace, ribbon connectors, ...).


The makeup gain pot is a simple voltage divider with the pots outer pins connected between 0V and +12V, so the wiper will give out any voltage in this range in respect to 0V reference voltage on paper and for this circuit. The +12V obviously arrives at the pots CW end, so the pots 0V connection at the opposite taper side is missing. (your measured 10.2V is the voltage divider in respect to the opamps virtual ground node with the 620K resistor in between and the pot with the missing/broken CCW outer connection to 0V reference voltage now operates as a rheostat).

R123 checks out at 620k but one side goes from 10.3v to 12v when going from fully CCW to foll open CW on the gain pot.

Different from your answer to my 1st.reply. When the wiper of the pot shows varying voltage in range 0...+12V and this same varying voltage doesn't arrive at R123, there is something wrong on the way between these spots (pcb trace, ribbon connectors, ...).

I checked the ribbon cable for any shorts and it's fine.  The Gain pot is also spanning only 10.2v  to 12v.  It goes from 0 ohms to 57k though.  What could cause that at the pot itself?  I've never seen that before.  Also, should the shell of the pot be grounded against the chassis?  I touched a ground wire to it and it didn't seem to make a difference. Thanks again for the help.

Bowie, this is simply "follow the breadcrumbs".  You need to open the schematic and follow the signal from the Gain Pot up to the VCA.  This is about 6 parts we're talking about, so should be pretty simple to track down.

Gain Knob > Ribbon cable > R123 > U13B > R118 > R85 > VCA's

Since both channels are not responding then problem is before R85, as signal splits at that point to both audio VCA's.

First your gain pot - One end is connected to +12V, other to Ground.  With Gain knob set to the middle the wiper should read about 6V.  If it's not, then your gain knob isn't soldered correctly, simple as that.  With unit off, check continuity of outer pins of Pots to verify actual connection to +12V & Gnd respectfully.  Measure on the pins of the pot themselves.  Test Points are available right where power comes into the main board.

Once you are getting around 6V at wiper with gain knob set about in the middle, then just follow the sinal with your voltmeter.  If you don't read 6V at R123 then your problem is before that (ie ribbon cable or J6-10 / J18-10 solder joints).

Check continuity of pins between J6 & J18 (with ribbon cable plugged in of course).  Flux/Reflow all solder joints at U13 (TL072) and associated resistors/caps (R123, R119, etc / see schematic).  Check and make sure ribbon cable connectors J6 & J18 are not backwards.  Check U13 is not installed backwards. 

Your problem is somewhere at U13 or bad termination on ribbon cable.  Since you are seeing meter responding with turn of threshold pot then it only further supports likely problem at U13, either bad solder joint on it or supporting part, R119 primary suspect.

I fixed the gain pot but there is still no audible compression or gain pot function.  It's as if the input signal is reduced by 6db and sent directly to the output, even though compressor shoys it's working in the meter.

I re-built the ribbon cable and othe Gain pot went back to a normal 0-12v from the center-pin as it's potted.  However, that 12v would not travel to R123 for some reason.  I soldered a wire directly from the center pin on the Gain pot to R123 to get the 0-12v there and got around -.32mv at the other end of R123.  The -.32mv arrives at pin 6 of U13 but does not change at all when the Gain pot is adjusted.  So, to re-phrase, after the jumper I get 0-12v on one end of R123 and an unchanging -.32mv at pin 6.  I checked pin 7 and it sweeps from -19.4mv to -.40mv.  I don't know if any of that is normal but please let me know where to go from there.
I've spent many hours checking connections and trying to understand the schematic better, I'm just not fining what is going wrong.  Thank you.

 

[friesdan]

Hi all!
Just a quick THANK YOU to Serpent and the awesome helpful people on this thread!
Thanks for keeping this product available and support alive for those of us who were late to the party.
The music coming through my studio is already sounding better!

 

[Harpo]

I soldered a wire directly from the center pin on the Gain pot to R123 to get the 0-12v there and got around -.32mv at the other end of R123.  The -.32mv arrives at pin 6 of U13 but does not change at all when the Gain pot is adjusted.
So, to re-phrase, after the jumper I get 0-12v on one end of R123 and an unchanging -.32mv at pin 6.

Already told you the opamps inverting input pin6 is a current node (not a voltage node). 

I checked pin 7 and it sweeps from -19.4mv to -.40mv.

Looking back on your pics from previous page, just exchange the (looks like 1K) R119 with its supposed to be 100K resistor value and pin7 will increase by factor 100 for an about -1.935VDC readout with makeup gain turned full CW.
This will fix the unit not compressing as well.
Good luck