Sound Skulptor forums

Build help and troubleshooting => EQ531T Tube Equalizer => Topic started by: Lud on June 30, 2025, 10:54:43 AM

Title: EQ531T build
Post by: Lud on June 30, 2025, 10:54:43 AM

Hi everyone, I have started building the 531T and there's three 470 condensers and one 470 KAK listed. I have four KAK. Is that an Issue?

Cheers.

Title: Re: EQ531T build
Post by: JPK on June 30, 2025, 03:06:04 PM

No issue there.
We have supplied all the 47pF in 250V to avoid mistakes.

Title: Re: EQ531T build
Post by: Lud on June 30, 2025, 03:21:06 PM

Ok, thanks a lot.

Title: Re: EQ531T build
Post by: Lud on July 02, 2025, 11:06:56 AM

I've done the build and i have strong dirty noises parasites in, unit off or on, around -30/-25 dbfs. Everything was good when testing exept with TR1 that i can't match the 1vAC, it only goes to 0,88v.

Have you got an idea what the first thing i've got to check?

Cheers.

Title: Re: EQ531T build
Post by: JPK on July 02, 2025, 05:17:32 PM

I added a Try to follow the signal section in this thread: https://www.soundskulptor.com/forum/index.php?topic=7346.0
Try it!

Title: Re: EQ531T build
Post by: Lud on July 03, 2025, 10:45:39 AM

Ok, so this is what i got :

TP1 : 0,75
TP2 : 0,7
TP3 : 0,7
TP4 : 0,65
TP5 : 0,65
TP6 : 0,65

Those cracklings are actually always there, even when the Lunch box is off.

I've removed the face, the IC's and there is still that noises crackling. I remade some solder joints, I checked the caps polarities. It seems to me that it cames from a ground issue and/or a cap but i can't see from where. I also wonder if Q2 is well placed?

(https://i.imgur.com/mHJTtPV.jpeg)(https://i.imgur.com/XkdDDmi.jpeg)

Title: Re: EQ531T build
Post by: JPK on July 03, 2025, 05:22:20 PM

Check the output transformer connections: You should read around 35ohms between the output XLR pins 2 & 3, Power on, EQ on, no input signal.

Title: Re: EQ531T build
Post by: Lud on July 03, 2025, 05:28:50 PM

I have 36 ohms.

Title: Re: EQ531T build
Post by: Lud on July 05, 2025, 09:36:37 AM

Unit is working now.

 I've tested another module in the lunch box i use for testing and there was the same noises so i replace a full working module in my wokflow chain and no noises at all. Everything seems fine. I have to know now whats wrong with the lunch box i usualy work with for building modules.

Title: Re: EQ531T build
Post by: Lud on July 05, 2025, 10:12:05 AM

After playing with it frequencies seems right but the HPF does not work correctely, it attenuates pretty much all the frequencies, it act like kind of a fader or attenuator?

Title: Re: EQ531T build
Post by: JPK on July 07, 2025, 08:52:40 AM

Check the solder joints and value of pot P1, capacitors C8, C9 and resisstors R5, R6.

Title: Re: EQ531T build
Post by: Lud on July 07, 2025, 11:36:43 AM

R5 & R6 are 3K3 and C8 & C9 are labelled 0,15 > 150n.

I have remake the joints but still P1 act like a whole spectrum attenuator.

I also remake the joints of J1a & J1b.

Title: Re: EQ531T build
Post by: Lud on July 07, 2025, 12:04:19 PM

Could it be C8 & C9 that are faulty? I have some wima 150n in stock so i can replace them.

Title: Re: EQ531T build
Post by: JPK on July 07, 2025, 03:43:46 PM

Maybe one of them? Check that they are not shorted.

Title: Re: EQ531T build
Post by: Lud on July 08, 2025, 08:15:50 AM

So i relaced the caps but it does not change anything. How do i check they are not shorted?

Here is a video of the issue where i compare how acts the HPF against the low band.

https://www.youtube.com/watch?v=m53ydKHYfiY&ab_channel=AppartBayonne

Title: Re: EQ531T build
Post by: JPK on July 08, 2025, 09:34:52 AM

Check that there is continuity between pin 1 of U1 and the center pin of pot P1 furthest from the front panel.
Check that there is continuity between 0V test pin and the center pin of pot P1 closest from the front panel.

Title: Re: EQ531T build
Post by: Lud on July 08, 2025, 10:34:08 AM

There is continuity between U1 pin 1 and pot1 middle back pin and the one below, and there is continuity between 0V and Pot1 middle front pin and the one below.

Title: Re: EQ531T build
Post by: JPK on July 08, 2025, 10:56:10 AM

What is the attenuation of a 1kHz sine signal when the HPF is turned fully clockwise. It should not be more than 0.5dB.

Title: Re: EQ531T build
Post by: Lud on July 08, 2025, 11:42:09 AM

First the unit has a slighty lower level than the original signal and it comes up close to the original when i engage the unit. When then i engage the HPF there is a small drop and when i turn the pot fully clockwise i go from around -11dbfs to -28dfs, a massive drop.

Edit: sorry i should have mesured it properly with my multimeter. I'll do it tomorow.

Title: Re: EQ531T build
Post by: Lud on July 09, 2025, 08:04:27 AM

1VAC : Unit on > 1,11
           HPF on > 0,95
           HPF fully clockwise > 0,15

Title: Re: EQ531T build
Post by: JPK on July 09, 2025, 03:49:11 PM

The problem must be between U1, C8, C9, R5, R6, P1, J1a, J1b. There are no other components involved.
Check the values, the solder joints, the min/max value of P1, swap U1 with another OPA2134.

Title: Re: EQ531T build
Post by: Lud on July 09, 2025, 05:18:48 PM

It is all fine!!!

I swapped u1 with another IC and it works. I've done that before with a Burr Brown i have in stock and it did not worked, so i thought it wasnt that.

Huge thanks for your time and patience!

Lud.

Title: Re: EQ531T build
Post by: JPK on July 10, 2025, 09:14:56 AM

(https://fonts.gstatic.com/s/e/notoemoji/16.0/1f44d/32.png)