How to change a degree of volume pot rotation by remote control?

[kotofei]

Hello everyone,

Hope that I'm posting to the right forum.

I have a Nakamichi CA-7A preamplifier with the remotely-controlled volume pot. I can rotate the volume control manually to a very small angle, such as 1-2 degrees. However, if I press up or down button at the remote, the pot is rotated to a significantly larger angle, such as 4-5 degrees. This is very inconvenient at low listening levels.

So I'm looking for an advice how to decrease a degree to which the volume pot is rotated upon pressing the "up" or "down" buttons at the remote. I HOPE it's possible by changing some passive component in the remote control circuit. If it's possible, I'll post a schematics.

Thank you.

 

[AndrewT]

a complicated way would be to change the gearing between the motor and the wiper.

 

[Mooly]

The correct "feel" in the way a motorised pot operates is really important and many manufacturers don't get it right.

You could try something as simple as adding a resistor in series to the pot motor. Trial and error to get the speed correct. That would allow a brief tap of the up/down buttons to move the motor less turns.

 

[Max Headroom]

Don't Add Motor Series Resistor....

NAKAMICHI CA-7-A-E CA-70 SM Service Manual
View attachment LB1645.pdf

Fig 5.5 - Volume Control - Motor PCB Assy.....ZD604, 10V.
Replace ZD604 with lower value to reduce motor speed...8.2V, 6.8V, 5.6V...see which one suits....just bridge cross the existing zener diode.

Dan.

 

[kotofei]

Finally got the 5.1 V Zener and soldered it across the existing one.

Now each pressing of up or down button rotates the pot to a considerably smaller degree than before. It really helps at low listening levels.

However, it feels a bit sluggish so I might prefer a slightly higher motor speed i.e. use 6.8V Zener.

THANKS A LOT DAN!

 

[AndrewT]

Could a prolonged press and hold (say longer than 500ms) cut off the 5v6 Zener and revert to the previous "high" speed?
A short press would still give the "low" speed sensitivity.

 

[Max Headroom]

Finally got the 5.1 V Zener and soldered it across the existing one.

Now each pressing of up or down button rotates the pot to a considerably smaller degree than before. It really helps at low listening levels.

However, it feels a bit sluggish so I might prefer a slightly higher motor speed i.e. use 6.8V Zener.

THANKS A LOT DAN!

You are welcome .
You can effectively trim the zener value in 0.6V increments by adding one or more standard IN4001 etc rectifier diodes in series with the new zener diode....might help you to get the exact speed you want.

NB: the rect. diode will need to be installed in the opposite direction to the zener.

Dan.

 

[kotofei]

No. If I press and hold, the pot rotates at a new "slow" speed.

Could a prolonged press and hold (say longer than 500ms) cut off the 5v6 Zener and revert to the previous "high" speed?
A short press would still give the "low" speed sensitivity.

 

[KSTR]

Could a prolonged press and hold (say longer than 500ms) cut off the 5v6 Zener and revert to the previous "high" speed?
A short press would still give the "low" speed sensitivity.

I did something along this lines using a simple fixed supply to a H-bridge feeding the motor but programming the controller smarter than usual, increasing the duty cycle from 20% to 100% within a second for a press&hold. This way the pot can be moved in minute steps (<0.3dB) when required but bigger changes can still be made very quickly with a smooth transition. I even added a small overhang (flywheel effect) to make it feel right and very "analog". Sadly the product did not make it to the market, for non-technical reasons.

 

[nakman]

new problem

I apologise for a quick thread-jack here, but info on CA 7 volume pot is very hard to find, sure hope one of diy gurus here can help me :

Using the remote, my volume pot sometimes used to be intermittent on one way (vol up) but it's minor, my guess is flakey Q617/618 and I always put off checking them. Manually turning the pot by hand always feel normal, a bit heavy /hesitant, as if gear linkages (or tight rubber band) holding the vol pot's shaft - just like most motorized pots.

Yesterday I found remote's up and down did not work at all, and horror, suddenly the pot feel very loose to manual turn. It's like the motor is not there anymore ! Also same loose feel when preamp is off.

Any suggestion of what went wrong and what may have caused this 'damage' in the first place is highly appreciated.

Thanks much in advance,
sm

 

[Stuey]

It sounds like the gear teeth are stripped or otherwise no longer meshing.

 

[Max Headroom]

Rob Peter To Pay Paul.....

Post pics of the existing motorised volume control that you have.
Maybe you can change mechanical section according to the advice that we give.

Dan.

 

[nakman]

thanks

It sounds like the gear teeth are stripped or otherwise no longer meshing.

Hi Stuey, yes it indeed felt that way, however since remote's <up vol> sometimes acting up, every now and then I turn it by hand, and never encountered "pre-damage symptom" before. This happened suddenly so I am still in a bit of denial that something inside is actually broken.

Post pics of the existing motorised volume control that you have.
Maybe you can change mechanical section according to the advice that we give.

Dan.

Hello Dan, thanks for your help. Do you mean pics of this nak ca7's volume pot ? I intend to open the preamp but it will take quite some time In the meantime could you please advise whether problem within its circuitry can possibly resulted in very loose volume pot ? perhaps burnt or shorted zener mentioned before.

cheers,
salim sm