VT BASS DI

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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tommymariot
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Post by tommymariot »

Hi guys,
I found a VT BASS DI with some issues and before repair it I decided to trace it.
Actually I removed all thru hole components in order to see better the tracks.
Here you can find the first gut shots.

In the next days I will start write down the schematic
All the VR are 100kB, all IC are TLC2262 except a 4013 and the module is CH34-9.
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Rear.jpg
Front.jpg

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karul
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Post by karul »

There is a thread for Tech21 Character Series, with some schematics and table, but the thread is too long, I can't find quickly where is the one for the VT bass (no DI), if there's one. Nevertheless, this is the thread and the table with some values.
viewtopic.php?p=237567#p237567

* there's no info for CH34-9 module in that thread

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Post by tommymariot »

karul wrote: 04 Jan 2024, 22:56 There is a thread for Tech21 Character Series, with some schematics and table, but the thread is too long, I can't find quickly where is the one for the VT bass (no DI), if there's one. Nevertheless, this is the thread and the table with some values.
viewtopic.php?p=237567#p237567

* there's no info for CH34-9 module in that thread
Yes I know. I opened a specific thread so it's easier to find info on the VT BASS DI.
I think is a mix between the Sansamp Bass Driver late v1,v2 and the VT Bass.
DI section is the same of BDDI V2 and the module CH34-9 is the same found in the late version of BDDI V1.

The only thing I don't understand is why the non inverted DI section is connected to the - pin of XLR and the inverting DI section is connected to the + pin of XLR.
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09_285_05_tech21_sansamp_bass_driver_di_v2_schematic.png
09_284_05_tech21_sansamp_bass_driver_di_v1l_schematic.png
Last edited by tommymariot on 06 Jan 2024, 01:26, edited 1 time in total.

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Post by tommymariot »

OK this is the first draw of the pedal.

Not sure about bite section, switch section and clean section.

Tomorrow I will check some connections directly on the pcb.


----------------EDIT------------------------------------------------------------


Checked all connections and corrected the schematics and picture with ref name.

In the next days I will desolder all the capacitors and I will measure them with my LCR45.
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VT Bass DI.pdf
(771.1 KiB) Downloaded 100 times
Trace.jpg

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Post by tommymariot »

Hi guys, finally I found the time for desolder the capacitors on the signal section. I have to check the value on the switch circuit but I think it's less interesting.
Enjoy with the schematics. Now I will resolder all parts and I will start to repair it.
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VT Bass DI Rev2 Final.pdf
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tuck
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Post by tuck »

I'm trying to understand the bypass switching. Can some one explain to me why they put the one transistor for the off position on U5B and not before on U5A?
u5b.PNG

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Post by tuck »

If I want to remove the CMOS switching, would this be the way to do it?
Blue = new connection
vtbassdi.png

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Post by tuck »

My pcb is made from schematic rev2 (tommymariot). The XLR jack is not connected yet and the switching seems to be kind of messed up as it is always on. But other than that it sounds :-)

I'm kind of stuck with the switching. Ok I added a toggle switch instead of a momentary switch but this shouldn't be a problem.
The pedal is always on. When I close the switch for a short period of time the led turns on.
When I close it again the LED stays on and the pedal stays on.
I added J111 instead of MMBF4393 for the switching circuit.
SO I need to wrap my head around the switching first.
vtbass.pdf
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vtbass_pcb.pdf
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IMG_8519.jpg
IMG_8520.jpg

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Post by skizoide »

OMG. What trace width is this?

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tuck
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Post by tuck »

KiCad standard 0,25mm :-) I know… I try to use 0,5mm for power traces but otherwise it wouldn’t be possible in this dense layout

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Post by 287m »

you nuts masochist, Tuck!

i was think, is that mrblack work, because so fuckin crazy layout. Nice!

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Post by tuck »

I don't have access to my pcb right now but I simulated the switching circuit.
What is the purpose of C32? It keeps the clock signal of CD40133 Pin11 at 9V. If I remove the capacitor it works (in the simulation).
The reset wiring for power on = bypass works.
c32.PNG

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Post by tuck »

Update: Pin 9&11 to ground like in the older pedals.

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Post by tommymariot »

Hi guys, I have an issue with my original VT BASS. My unit now have low volume with the FX engaged. If I put a preset suggested by Tech21 in the manual I got lower volume than bypassed. I started to follow the signal (blend at max) with my oscilloscope and I found I lose volume in the level section. I forgot to check the volume in that section with blend signal at minimum. Tomorrow I will check again if I lose signal before the blend.
Any suggestion?

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Post by tommymariot »

Today I checked the clean volume in the blend section. Everything si ok. I Found I lose signal in the LEVEL section but I cannot understand why. At pin 1 of U5 I have a normal signal, after R44 I have half signal and after U5B I lose again signal. I reach the unity volume with LEVEL at 15:00 but in the videos I found the unity is around 11:00-12:00.
Any suggestions?
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After Pin 7 U5B.jpeg

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Post by tuck »

Is there maybe a problem with Q3?

Out of curiosity what is Q3 VGSoff in the original unit? I used J111 but the min VGSoff is -3V for the J111 instead of the -0,5V for MMBF4393. I changed to J113 with also -0,5V and the switching works.
This not related to your problem but I am really interested in the original values.

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Post by tommymariot »

I have to remove it and I will test it with my DCA75. I will try to do it these days.

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Post by tuck »

Sorry I made a copy&paste error. I mean what is VGS in the circuit of the unit not VGSoff of the transistor.

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Post by tommymariot »

tuck wrote: 07 Jun 2024, 21:52 Sorry I made a copy&paste error. I mean what is VGS in the circuit of the unit not VGSoff of the transistor.
You need the value of VCOM (see my schematics)? I think 3.85V more or less. I will confirm tomorrow.

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Post by tommymariot »

tuck wrote: 07 Jun 2024, 17:48 Is there maybe a problem with Q3?
Removed and didn't solved the issue.
Changed Level Pot and didn't solved the issue.

I cannot understand what happens. Is the opamp?

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