Slowing down a brushless DC motor in a VCR

[jla2125]

I'm trying to modify a VCR so that the big drum that scans the tape spins slower (for an art project...). It's a brushless DC motor that uses a special VCR-specific IC to control it, I believe. The chip is a sanyo LB11885 (http://www.semiconductor-sanyo.com/ds_e/EN7884A_d.pdf" datasheet). VCR repair manuals I've read also indicate that there's a control loop regulating the speed of the motor.

Is there any way to bypass that, so that I can use a potentiometer to control the drum's speed, or even just reduce it slightly? I've also looked into brushless DC motor controllers for RC stuff, but they are a bit out of my price range.

 

[Bob S]

Brushless dc motors are synchronous motors, sometimes with the feedback based on signals from a Hall Effect sensor. Look carefully at the input circuits for the LB11885 to determine which L or C component(s) to change for slowing the tape speed down..

Bob S

 

[jla2125]

I'm not trying to slow the tape down, but rather the read drum itself.

Pin 32 of the LB11885 chip is the drum speed control pin, the description of which is as follows: "Control is the constant current control to which current return is applied from DRS." DRS isn't explained anywhere else in the document, but is it safe to assume it stands for Drum Rotation Sensor?

Also, apparently it wants 0V to VCC as an input (assuming it's an input pin. It isn't clear to me if it is or isn't) so maybe if I vary that voltage, would that affect the speed?

 

[Mike_In_Plano]

Hello Jla,

Pin 32 controls the motor's current, which in turn controls it's torque. While the sheet says it can take up to Vcc, 1 volt is a more likely the operating point.
I'd cut the trace going to 32 and route it to a pot that swings from ground to pin 43. Then vary the pot from 0 volts up.

Note - VCRs are too smart for you good and will fight you based on "logic." You may have to trick it to keep it spinning.

- Mike

 

[jla2125]

Hey, thanks for the reply! This helps a lot! Just to verify, when wiring up the pot, the edge pins (1 and 3) would go to ground and pin 43 respectivly, and the middle wiper pin would go to pin 32, correct?

As for tricking the VCR, I only need to slow it down slightly. The VCR that I'm using seems to only get "confused" and stop operation if the drum motor is reduced to around half speed.

 

[Mike_In_Plano]

Yes, the processor in these things is fairly smart and will get in the way of much fun by shutting things down.

Note that by slowing the drum down, you won't get the picture to slow down - that's the function of the capstan motor.

- Mike

 

[jla2125]

Yeah, my goal is to generally wreck havoc on the image, as opposed to slow it down. But, am I going about using the pot in the right way?

 

[AskTheGenius]

you could use a fast pulse from a 555 astable to the power of the motor so when the output is high the motor speeds up but while it is speeding up the output goes low and the motor turns off and it continues. instead of a resistor, put a preset so you can vary the pulse find it on this link http://www.kpsec.freeuk.com/555timer.htm#astable"

 

[jla2125]

I'm not sure I'm following you. Are you talking about connecting the pulse from a 555 timer to the motor itself? It's a 3-phase motor, so there's nowhere to connect that. It's driven by the LB11885 chip.

 

[Mike_In_Plano]

If you get the drum out of sync, that should give you a bunch of tracking lines through the picture without slowing it down.

 

[jla2125]

There seems to be this kind of rather small zone of speed where it just generates really colorful patterns, and that's what I'm trying to go after.

 

[AskTheGenius]

You need to connect the output of the astable to the power supply of the motor

or you can decrease the voltage going to pin 2 (i is spinning so fast because there is an amplifier in the chip)