MAC Rack kit Build Thread from October 2023 with slow attack led

[Joe Malone]

Check with VU/OFF/GR in off that you have identical output levels ? Y/N
I have both channels of the opto receiving 1.23 VAC and the outputs of the opto are the same but they have dropped to 1.197 VAC.

This is hard relay bypass so the level difference is your interface sagging slightly under the load. So you need to turn up your interface to get 1.23vac on the MAC output which will also be the level on the mac input.

Channel A has 1.246 VAC for output and Channel B has 1.190VAC

4.13dBu - 3.73dBu = only 0.4dB difference which is normal in all analog gear. The makeup pots have 20dB range so this is easily corrected.

Channel A makeup is 9.26K Channel B Makeup is 9.64k a difference of .38k,

You can add a resistor across channel B makeup pot outside pads to balance the outputs exactly. Somewhere between 240k and 330k should balance it perfectly at the minimum makeup setting. When you have it balanced adjust the interface so both MAC outputs are 1.23vac and adjust the VU meters to read 0VU and do the alignment twice after 30 minutes on with lid sitting on top to warm up.

[saxmonster]

Thanks again for all you help Joe.

You can add a resistor across channel B makeup pot outside pads to balance the outputs exactly. Somewhere between 240k and 330k should balance it perfectly at the minimum makeup setting. When you have it balanced adjust the interface so both MAC outputs are 1.23vac and adjust the VU meters to read 0VU and do the alignment twice after 30 minutes on with lid sitting on top to warm up.

Can I use a trimmer resistor to find the exact resistance needed to get this side equal to the other? Also why does the B channel get the resistor and not the A channel. My thinking is if B already has a higher resistance and a lower output why add more resistance to it, wouldn't that drop the output even more? I need to study electronics more, errrr.

Thanks again.
-Scott

[Joe Malone]

Thanks again for all you help Joe.

You can add a resistor across channel B makeup pot outside pads to balance the outputs exactly. Somewhere between 240k and 330k should balance it perfectly at the minimum makeup setting. When you have it balanced adjust the interface so both MAC outputs are 1.23vac and adjust the VU meters to read 0VU and do the alignment twice after 30 minutes on with lid sitting on top to warm up.

Can I use a trimmer resistor to find the exact resistance needed to get this side equal to the other? Also why does the B channel get the resistor and not the A channel. My thinking is if B already has a higher resistance and a lower output why add more resistance to it, wouldn't that drop the output even more? I need to study electronics more, errrr.

Thanks again.
-Scott

The makeup pot is a gain resistor so the smaller it gets the more gain you will have. You can fit a 500k 25 turn trimpot as a variable resistor (centre leg shorted to one side leg) across the outside makeup pot pads.

[saxmonster]

Thanks for the explanation Joe, I really appreciate it. I will try to get this done soon, can I solder the resistors to the outside pads that are closer to the middle of the pcb since they are connected to the ones closer to the edge that are under and covered by the case’s edge.
Thanks
-Scott

[saxmonster]

Hi Joe,
I have started to work with figuring out what exact resistor size I would need. I have not made any changes or soldered anything yet. In doing that I decided to setup my Apogee duet interface to use for the REW Signal generator because both outputs are equal on that.

With that said I sent 1.23VAC to both channels with the meter switch to off and both knobs CCW and got 1.23VAC on the output of both channels on the OPTO. Last time I did this I had a voltage drop ( it was the other interface too). You said that was probable the interface sagging on the load. Now it seems like just a straight signal pass through which is great.

With the meter turn to VU the voltage on Channel A goes jumps up to 1.279Vac and Channel B goes down to 1.221VAC which are different from what I said before especially Channel A went up a lot. Obviously channel B is sagging on the signal load like you said before. Should I add a resistor to channel B that will give me the same voltage that Channel A is outputting or should I add resistors to both channels so they both output 1.23VAC each? Then re-calibrate?

Thanks
-Scott

[Joe Malone]

Hi Joe,
I have started to work with figuring out what exact resistor size I would need. I have not made any changes or soldered anything yet. In doing that I decided to setup my Apogee duet interface to use for the REW Signal generator because both outputs are equal on that.

With that said I sent 1.23VAC to both channels with the meter switch to off and both knobs CCW and got 1.23VAC on the output of both channels on the OPTO. Last time I did this I had a voltage drop ( it was the other interface too). You said that was probable the interface sagging on the load. Now it seems like just a straight signal pass through which is great.

With the meter turn to VU the voltage on Channel A goes jumps up to 1.279Vac and Channel B goes down to 1.221VAC which are different from what I said before especially Channel A went up a lot. Obviously channel B is sagging on the signal load like you said before. Should I add a resistor to channel B that will give me the same voltage that Channel A is outputting or should I add resistors to both channels so they both output 1.23VAC each? Then re-calibrate?

Thanks
-Scott

Yes you only need to bring Channel B up to match Channel A.

Channel B is not sagging its makeup pot value is just slightly higher than the one on channel A at minimum makeup.

[saxmonster]

Thanks Joe,
Once I get them equal should I drop the output of my daw so that I get 1.23vac on both opto outputs so I can zero out the meter when I recalibrate? So is channel A already adding some gain even though it is already fully CCW?

[Joe Malone]

Thanks Joe,
Once I get them equal should I drop the output of my daw so that I get 1.23vac on both opto outputs so I can zero out the meter when I recalibrate?

Yes

[saxmonster]

Ok, I just got done adding the trimmer pot to balance the two output pots. It wound up being 98.6K and I got the outputs VAC to be equal now when you switch on the VU meter. Thanks so much.

I have done the calibration again and made some slight tweeks to adjustments trimmers but now have something weird going on.

When I have both channels set to -5db on the VU and flip to GR I still get the -5db on both channels which is great but when I turn on the link switch, both meters go to 0db. Both channels then read 1.23VAC on their outputs. Almost like the compressor section of the unit has been shut off yet when you do adjust the threshold pots it does change the VAC on the outputs and the meters so they do seem to be connected. It did not do this prior to adding the trimmer pot. So I took the extra output trimmer pot out and the unit is still doing go to 0db when the link switch gets tuned on. I am not sure what is going on here. Should I take out the trimmer pot and let it sit for awhile in the off state? Before adding the trim pot it used to stay on -5db with only the faintest of a variation between the meters when the link switch was turn on after i did the calibration, so I know it did work right before. Not sure how anything could have changed? If it were a loose cable the threshold and output pots wouldn't work right? I double checked all the cables and they are nice and tight. Weird
-Scott

[Joe Malone]

Ok, I just got done adding the trimmer pot to balance the two output pots. It wound up being 98.6K and I got the outputs VAC to be equal now when you switch on the VU meter. Thanks so much.

I have done the calibration again and made some slight tweaks to adjustments trimmers but now have something weird going on.

When I have both channels set to -5db on the VU and flip to GR I still get the -5db on both channels which is great but when I turn on the link switch, both meters go to 0db. Both channels then read 1.23VAC on their outputs. Almost like the compressor section of the unit has been shut off yet when you do adjust the threshold pots it does change the VAC on the outputs and the meters so they do seem to be connected. It did not do this prior to adding the trimmer pot. So I took the extra output trimmer pot out and the unit is still doing go to 0db when the link switch gets tuned on. I am not sure what is going on here. Should I take out the trimmer pot and let it sit for awhile in the off state? Before adding the trim pot it used to stay on -5db with only the faintest of a variation between the meters when the link switch was turn on after i did the calibration, so I know it did work right before. Not sure how anything could have changed? If it were a loose cable the threshold and output pots wouldn't work right? I double checked all the cables and they are nice and tight. Weird
-Scott

Ok never seen this before and the trimmer if done correctly will not affect this.

Check the mac link pcb has the 10 pin headers and cables the correct way around and no shorts to 0v.
With link switched on unplug one ribbon cable from the link pcb at a time to see if both comps come back to -5dB with either cable unplugged. If one side stays on 0 then there is a short most likely on the link pcb.

[saxmonster]

Ok never seen this before and the trimmer if done correctly will not affect this.

Check the mac link pcb has the 10 pin headers and cables the correct way around and no shorts to 0v.
With link switched on unplug one ribbon cable from the link pcb at a time to see if both comps come back to -5dB with either cable unplugged. If one side stays on 0 then there is a short most likely on the link pcb.

When I unplug just one cable at a time off of the link pcb both meters return to to -5db. Each cable does the same thing. When they are both plugged into the pcb the meters go to zero and when either one is out both meters return to -5db. Did I hook up the trimmer pot correct? See pics below. I don't see any bad soldering joints or bridges on the link PCB and all cables are in correct and fully seated. How should I check for shorts on the link PCB? When I do unplug channel B the LED goes out, it stays on when I unplug Channel A, I guess this is normal?

IMG_6953.png

IMG_6954.png

-Scott

[Joe Malone]

Ok never seen this before and the trimmer if done correctly will not affect this.

Check the mac link pcb has the 10 pin headers and cables the correct way around and no shorts to 0v.
With link switched on unplug one ribbon cable from the link pcb at a time to see if both comps come back to -5dB with either cable unplugged. If one side stays on 0 then there is a short most likely on the link pcb.

When I unplug just one cable at a time off of the link pcb both meters return to to -5db. Each cable does the same thing. When they are both plugged into the pcb the meters go to zero and when either one is out both meters return to -5db. Did I hook up the trimmer pot correct? See pics below. I don't see any bad soldering joints or bridges on the link PCB and all cables are in correct and fully seated. How should I check for shorts on the link PCB? When I do unplug channel B the LED goes out, it stays on when I unplug Channel A, I guess this is normal?
IMG_6953.png
IMG_6954.png
-Scott

Yes trimpot spot is correct across makeup pot. This is very seperate from the link area of the circuit.
Did the link switch work before? Do you have a photo of the link pcb?

[saxmonster]

Yes trimpot spot is correct across makeup pot. This is very seperate from the link area of the circuit.
Did the link switch work before? Do you have a photo of the link pcb?

Yes the link switch used to work just fine only a slight difference between the two -5db readings on the meters when engaged. So weird for this to happen when I wasn’t even working in that area? Could the switch gone bad? Here are some pictures of the link pcb.

IMG_6962.png

IMG_6961.png

IMG_6957.png

[Joe Malone]

Yes trimpot spot is correct across makeup pot. This is very seperate from the link area of the circuit.
Did the link switch work before? Do you have a photo of the link pcb?

Yes the link switch used to work just fine only a slight difference between the two -5db readings on the meters when engaged. So weird for this to happen when I wasn’t even working in that area? Could the switch gone bad? Here are some pictures of the link pcb.

IMG_6962.pngIMG_6961.pngIMG_6957.png

OK that looks ok. The clue will be what ac volts you are getting at pin 6 on each IDC 10 pin header on the mac link pcb when separate and when linked measured to 0v while -5dB GR is on each channel. Make sure the switch and threshold settings on both comps are the same.

[saxmonster]

OK that looks ok. The clue will be what ac volts you are getting at pin 6 on each IDC 10 pin header on the mac link pcb when separate and when linked measured to 0v while -5dB GR is on each channel. Make sure the switch and threshold settings on both comps are the same.

Ok,
So just and FYI that I was gonna try to fix with you after I got the outputs to match closer. When I have -5 db on both meters the threshold knobs are about a 1/4 of a mark off from each other using the marks on the front panels as a guide. So from one white line to the next they are about a 1/4 of that distance off from each other. Channel A threshold always had to be turned up CW a little more than channel B. This is next on my list to fix but not sure if it can be done.

Here are my measurements for pin 6 on the link pcb. I used the middle pins closest to the switch on each side. Settings are 1:3, GR, and Flat

When both meters say -5DB with the link switch off I get 1.832VAC on Ch A pin 6 and 1.800 on Ch B pin6 . When the link switch is on I get .416vac on ch A pon 6 and .415vac on ch B pin 6.

When I adjust the the thresholds to have equal .855vac on the xlr outputs (same amount of compression on both sides) these are the measurements I get on pin 6. Same settings as before.
Link switch off I get 1.833vac on ch A pin 6 and 1.832vac on ch B pin 6. Link switch on I get .124vac on ch A pin 6and .128vac on ch B pin 6.