Working on dynaco pots

Hello !

I manage to renew many pots of dynaco amps/preamps, that is to say treble, bass, volume, balance...
Let's imagine I love working on very little things, and I bought some stereo alps stepped pots.




I open them (nothing more easy !! 2 philips screws...).
I extract the "retainer" of the axe, and then unmount the 2 boards with smd resistors...

I unsold the resistors, clean the boards.

I want to create a 750k pot (linear). I apply then the table above :



I need 866ohm and 845ohm smd 0805 resistors. I sold (using soldering pasta to glue smd on the board, and mini hot air or electric oven...).
I obtain a linear 750k pot...

What do you think about that my friends ? do you think it is possible ? Then it would be possible to recreate ALL the pots for dynaco machines no ?

That's why I need your help for this project...
If someone could give me the characteristics of each pots on PAT4, PAS3, PAS3.X, SCA35 and st35, I would be very greatfull !

And this work conduct me to another question about pas3X pots... It seems that treble and/or bass pots are special for the 3X version...
With my stepped pots, do you think we can re-create this pot ? I explain...If we conect a resistor between the 11th and the 13rd position, and nothing on the 12th position, we could obtain a pot like the pas3x one no ? Am I true ?

Another and a last question...
Here, i'm talking about pots with resistors. In the soft to calculate the resistors values, there is a place to enter the dB per step, in which I enter 0.
Am I right or wrong ?




Many many thanks for your help in my project !
Best regards
JL

I built an R2R stepped attenuator with relays and arduino for a linear pot replacement. I used this calculator https://www.amb.org/audio/delta1/

1db per step for 100k pot equivalent and 6 relays per side yielded a range from -64db to 0db. Arduino powered. Works well, clacks a lot… The last resistor sets input impedance of next stage, 270k for vta driver board.


I imagine you could create a bass or balance pot this way. Haven’t given it much thought.





To

And a new question... Someone have a calculator of resistors in the case of a log pot ?
thanks !

You guys got me thinking.  I am working on a project mixing digital and analog components for my on synthesizer and have located digital potentiometers for part of my project.  There are these very tiny processors that I am using that have Wifi and Bluetooth built in.  I could create a very tiny circuit board with these components and create remote controls for volume and even preamp tone and balance controls.  The parts for each control is less than $20 and multiple controls could go on one board with volume, balance, bass, treble and relay input selectors that would cost less than $30 the parts.  I would just have to prototype to get all of the caps and wiring correct and do the software which will be very easy.  The end product might sell for $60 to $100.

Anybody interested?

It's a good idea and perhaps usefull for someone.
In my case, I want to stay closer to the original machine reproducing pots with the same impedance...
And my subject do not interest many people...
This afternoon, I will mesure my original SCA-35 pots desoldering them and test the different impedances... I hope they will be good because I don't know, if noone help me, how to reproduce them with my idea described above...

First pot tested... the 500k one (treble control). Front at 520kohm, rear at 595kohm... I think it is LOG... not very good...
Second pot : the bass pot at 2Meg Ohm... Front at 2,02Mohm, rear at... 1,65Mohm !!  LOG POT...  Do you think I can replace with a linear pot ?
Third pot : 750k, the balance pot... Front at 760k, rear at 650k...


front channel is inverse LOG, rear channel is LOG...
And that's here I wanted to modify an alps pot... resistors for linear response from step 1 to 10 for the front channel (from 0 to 750k) and resistors from step 10/11 to 22 for the rear channel from 0 to 750K...
What do you think ?

I mentioned this before. Don't know if you looked. Not exactly what you're trying to do but explains how the bass, treble, and volume work. This is on the update my dynaco PAT4 page about 1/3 down. Hope it helps...

https://www.updatemydynaco.com/pat4.html

PS, that's all I know about it.

So this post got me thinking about digital potentiometers. So I bought two of the xc9104 digital pot chips. They are linear, I think someone makes audio taper versions. 100 step 100k range. I bypassed the wiper with a 15k to perhaps taper it a bit. So here is an Arduino powered remote volume based on the chip. It works. A useable range of around 30 steps. It’s next to the relay resistor ladder por. 64db range, 1db per step. The nano is a great device for simple audio control, even measures voltage.

Hi guys !
I've worked on my sca-35 pots...
I have measured, every 10 degrees of rotation of each pot 'treble/bass/balance/volume' the resistance obtained on each channel. I modelised the slopes and I obtained the resistance of each step for stepped potentiometers (21 or 23 steps)
I obtained an excel table like this one :

Here are the graphics of original pots response (TREBLE) and modelised stepped potentiometer :


Now I have 2 types of stepped potentiometers but I haven't decided which one I'm going to use yet...
The first one seems interesting but the rotation of the shaft is "difficult" with a big clic... It has 23 steps...

The others for which my heart swings are the ALPS blue... The clics between steps are smoother, less noisy..  they are easy to open and disassemble. I've found on my google researches people that create their own pots with ALPS !


I have ALL the resistors in MF25 type 1% precision metal film 1/4w to solder...
Now I just have to decide on which of the 2 potentiometers I will use...