SB4001 uneven left and right levels - 5db out?

[canidoit]

I just noticed the Left and Right of my SB4001 is 5db out. It seems to be the unit and not the rack, as this issue only occurs once I put Comp In on. Could this still be a rack issue, even if it only occurs when the comp is on? I did buy this new 500 series rack used.

I am thinking of fixing this through the calibration process:
Unity Gain (VR1L/R) - Toggle between bypass and comp in. Adjust VR1L/R for unity gain (ie - same levels when the compressor is engaged as it is when it is in bypass)

It's been a while since I worked on this project, so I cannot recall if I need to dismantle the unit to access the Left and Right trimpots?

Has anyone ever had similar issues as this, and how did you solve it?

Could this issue be more than the trimpots adjustments, as I rarely used this unit. Could something else be causing this issue - capacitor, resistors?

Is it normal for the left and right to eventually drift and just need a trimpot adjustment?

Any tips regarding troubleshooting this issue and confirming it is just a trimpot adjustment?

Thank you.

 

[sr1200]

You need to calibrate the two sides. Try and see if you can locate the manual. It has a detailed calibration procedure. There are trim pots, thats pretty much it. The unit can and does drift over time (the trim pots can drift). Mine was about 3db off after 6 years of daily use.

 

[pucho812]

At 5dB I would expect a failed component like an opamp or something.

 

[canidoit]

Is there a way of checking this without pulling it a part yet to see if it is an opamp issue, maybe using test tones and settings?

Is it possible that the used lunchbox could have damaged it? Maybe I knocked the trimpot when I moved the SB4001 to the new used lunchbox.

@sr1200, the only manual I have is a user guide for his commercial versions.

I have his instructions in the support thread but it has been a while since I looked at this build, so it all looks foreign to me.

 

[sr1200]

Not familiar with the commercial versions (if theyre any different at all). Not sure if he put a metal chassis over the circuit. Either way, you're gonna have to get in there. I'll take a look and see if i still have the build manual somewhere tonight. As @pucho812 said, you may still have a bad IC in there, but try the trim first. I had thought the same thing on mine, and swapped the chips but the unit was still way off... the trimmer just was off for me.

This is on page one of the build thread. Will tell you which pot is which and what the ideal value "should" be. Not sure if you have a couple of test jig's for 500 series... i needed to use them for mine so i could run signal thru the unit to set unity properly. (that was what was off on mine)

Pretrimming Pots:
You have to pretrim the Pots BEFORE you install them. There will be very little calibration required if you do. Some get trimmed between Pin 1 and the wiper, some are trimmed between pin 3 and the wiper. Pin 1 is the pin that goes in the square pad. Most of the Pots show the pretrim value and which two pins to trim right on the silkscreen (illustrated by the little line and value above it). There are a couple that need to be trimmed as well though that don't have the value indicated, so just to go down the list:
VR1L/R (Unity Gain): Pretrim to 2K05 (Trimmed between Wiper and Pin 3) [Illustrated on Board]
VR2L/R (Magic): No pretrimming required
VR3L/R (Distortion Trim): No pretrimming required
VR4L/R (Sidechain): Pretrim to 2K (Trimmed between Pin 1 and Wiper) [Illustrated on Board]
VR201 (Meter): Pretrim to 1K (Trimmed between Pin 1 and Wiper) [Illustrated on Board]
VR203 (+Gain): Pretrim to 10K (Trimmed between Pin 1 and Wiper)
VR204 (-Gain): Pretrim to 2K28 (Trimmed between Wiper and Pin 3)
VR205 (Ratio): Pretrim to 9K (Trimmed between Pin 1 and Wiper) [Illustrated on Board]

 

[canidoit]

Thanks for that SR1200!

I was wondering, is it possible if the lunchbox is not giving the power evenly, could that lead to this volume level issue? I bought the 6 slot lunchbox used from an unreliable seller who only used one slot and it looked like it has been pulled apart before.

I just don't understand how the SB4001 was working fine one moment, and then all of a sudden the volume levels are off when I installed it in this used lunchbox after a few weeks use.

Il try installing it at another lunchbox, maybe it's the lunchbox.