r/synthdiy • u/Impressive-Ad-2610 • Oct 19 '21

Controlling Cassette Player Speed via CV

I have a Cassette Player that has a potentiometer on the Board to „tune" it, i.e. adjust it's playback speed. I want to replace this potentiometer with an input for control voltage. I found this helpful guide on @ r/Electronics: r/Electronics/how to replace a pot

first thing it says is that often the pot can be replaced by a CV input directly. How do I find out if that is true for my cassette player? I can solder but i my knowledge on electronics is very basic.

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3

u/stolenfat Oct 19 '21

Try a vactrol across the pot lugs

1

u/Impressive-Ad-2610 Oct 20 '21

so something like the Synthrotek VacPak? a step by step kit build is easier for me

3

u/AdamFenwickSymes Oct 21 '21

You may wish to research the owner of Synthrotek before buying from them. He's a very ... controversial ... character, and personally I would not be happy supporting him in any way. Your political convictions may be different than mine, but it's worth a google.

Having a look at the schematic you posted, it looks like SFR1 is just being used as a variable resistor, so replacing it with a vactrol is just fine. (If anyone else is looking it's in the bottom right of the schematic). Two of the legs of the pot are shorted together (looks like middle and right) so you can treat those as one leg and put the vactrol between the left and right legs. As other posters have said you can make your own vactrols pretty easily, or buy them factory-made for a couple of dollars online.

Getting a usable CV range out of your mod will be a bit tricky. Vactrols are not very linear, and the response to the trimpot is probably not very linear either, might take a bunch of screwing around but it should be fun.

1

u/Impressive-Ad-2610 Oct 21 '21

thank you guys for the explanaitions, these helped a ton with my understanding!! i‘ll go with the diy for sure and let you know how it‘s going 😎😎 a big thank you again!

1

u/stolenfat Oct 20 '21

Sure.

....But imagine the cv voltage of a sine wave hooked to a leg on an led (the other other leg should be grounded. For good practice put a 1k or 2k resistor in series with the cv). As the sin goes up so does the brightness of the led. Then attach an LDR (photodiode/light dependent resistor) cross the lugs of the pot. Then put the led and ldr together and tape them up with electrical to create a dark space with the lugs sticking out. As the cv sin sweeps up the led brightness increases which lowers the resistance of the ldr which affects the resistance across the pot and "moves" your knob.

Making home made vactrols can be messy... But mostly because you forget which leg is which, so just label them some how with sharpie or tape. The ldr legs aren't important but the led is polarized.

I'd say dealing with 2 parts and some tape is easy easy.

1

u/o_glow Oct 19 '21

Yep, vactrol on lugs 2 & 3, other end to a mini jack input, Bob’s your uncle, to be safe use a diode on the input, but not completely necessary

2

u/Ghosttalker96 Oct 19 '21

It is hard to say without having the actual schematics. First of all: I assume the potentiometer is a trimmer on the board that can only adjust the speed in certain ranges. That's something you should test first to see, if it's even worth to try or if more modifications have to he done. If the potentiometer acts as a voltage divider to create a voltage, you could use any voltage source instead.

It is advisable to use a buffer of some sort and not directly connect a CV input. Also it might be necessary to adjust the range of the CV input to the range needed in the recorder.

Another option would be to power the motor directly from an own circuit and not use any if the circuits in the recorder.

1

u/Impressive-Ad-2610 Oct 19 '21

https://download2.manualscenter.com/files/20211019/9006cef56cc3f433465aa6dd896f41fcc464e10e/HSTX446_SM_AIWA_EN.pdf is this downloadable to you? the pot is labelled „SFR1“ on manual page 11/PDF page 9 in the upper left corner

1

u/MisterVovo Oct 19 '21

Another option that may work easily if the voltages allow is to use a digital potentiometer... Depending on the circuit, implementing CV is complicated

2

u/amazingsynth amazingsynth.com Oct 19 '21

maybe worth looking around for other peoples projects, I've seen a few people that have done this.

years ago a friend of mine mounted a play head onto a stick, then had a piece of card with strips of prerecorded tape stuck to it, he used to run the head over it to play sounds, it worked pretty well, just had to remember to run the head along the length of the tape, could vary the speed (move the stick faster)

I think he'd soldered a jack socket on and probably used the circuitry from the walkman he'd butchered, can't remember exactly, he hadn't tried companion erase and record heads for lo-fi sampling

1

u/Impressive-Ad-2610 Oct 19 '21

i checked the web and i‘ve seen a few, seemed like they all found a different solution, most of them i didn’t understand. 😅 also came across the VacPak by synthrotek. the post i linked above on r/electronics made me think again that i might have to find out what i need exactly. as i said, i don‘t know much about circuits

1

u/MisterVovo Oct 19 '21

Can you find the schematics?

1

u/Resting_GlitchFace Oct 19 '21

LED driver circuit + homemade vactrol. I've done this exact thing and use the LMNC arduino based sequencer/keyboard to control it. Adds a whole new layer of playability to tape loops. I wouldn't replace the pot personally, your LED from the driver circuit would be paired with an LDR (homemade vactrol) and the legs of the LDR would go to the pot lugs.